Rural Dairy Technology

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Rural

Dairy

Technology

C.B.O'Connor

International Livestock

Research

Institute

Addis Ababa,Ethiopia

January 1995


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5.3

Y

easts

.......................................................

23

5.4

V

iruses ......................................................

23

5.5 Milk m icrobiology

.............................................

24

5.5.1

Pasteurisation

......................................... 25

5.5.2

Sterilisation ..........................................

25

5.6

M

icrobiology of

butter .........................................

26

5.6.1

Sources

ofcontamination

...............................

26

5.6.2

Control

of

micro-organisms

it.butter

......................

26

6.

Clean

m

ilk

production

................................................

29

6.1 Sourcesofcontamination

.......................................

29

6.1.1

The interior of

the

udder

................................

29

6.1.2

The cow

as

a

source

of

pathogens

.........................

29

6.1.3

Milking utensils

.......................................

30

6.1.4 Miscellaneous sources

ofbacteria in

milk ..................

30

6.2 Cooling

m

ilk

.................................................

30

7. Milkreception, dairy

accounting and record

keeping ........................

33

7.1

Reception

....................................................

33

7.1.1

Q

uality

..............................................

33

7.1.2 Quantity .............................................

33

7.1.3

Composition

of

m ilk

...................................

33

7.1.4 Adulteration of m

ilk

...................................

33

7.2. Dairyaccounting

and recordkeeping

.............................

34

7.2.1

M

ilk

quantityand

quality ................................

34

7.2.2

Processingrecords .....................................

34

7.2.3

Records ofproduct

quality and

sales

.......................

34

7.2.4 Suggested

formats for recordsof

milkintake,

payment

and

utilisation

.........................................

35

8.

M

ilk

processing

..................................................... 37

8.1

M

ilk

separation ...............................................

37

8.1.1

G

ravity

separation

.....................................

37

8.1.2

Centrifugal separation ..................................

38

8.1.3

Hand

separator

........................................

40

8.1.4

Separator

maintenance ..................................

42

8.1.5 Calculations

..........................................

42

8.1.6

Standardisation

ofmilkandcream

........................

44

8.2

Buttermaking

with

fresh milk

or

cream

............................

46

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8.2.1 Buttermaking

theory

....................................

47

8.2.2 Theory

ofthe

mechanism of

churning

......................

47

8.2.3 Chum

preparation ......................................

47

8.2.4

Churning

temperature ...................................

47

8.2.5

W

ashing the

butter

.....................................

48

8.2.6 Salting, working andpacking

the

butter

....................

48

8.2.7

Washing thechurn

and

buttermaking

equipment after

use

48

8.2.8

Overrun

and

produce in buttermaking

......................

49

8.2.9 Butterquality

.........................................

50

8.3 Buttermaking

with

sour

whole

milk ...............................

51

8.3.1

Buttermaking .............................................

51

8.4 Ghee,

butteroiland dry butterfat

..................................

53

8.5

Cheesemaking

using fresh milk ...................................

53

8.5.1 Rennet

coagulation theory

... ...........................

54

8.5.2

Cheese

varieties

.......................................

54

8.5.3

Cheese yield ..........................................

67

8.6 Cheesemaking

with sour

skim

milk

................................

67

8.7

M

ilk

ferm entations .............................................

68

8.7.1 Ferm

ented

m ilks .......................................

69

8.7.2 Concentrated

fermented milk .............................

71

9. Cleaning,sanitising

and sterilisingdairy

equipment

......................... 73

9.1

Dairy

water

supplies ............................................

73

9.1.1

Bacteriological

quality ..................................

73

9.1.2

Chem ical quality .......................................

73

9.1.3

W

ater softeners ........................................

74

9.2

Chemicals used for

cleaning .....................................

74

9.2.1

Sterilisers

............................................ 75

9.3

Cleaning

procedure ............................................

75

10. Sampling and

analysis of

milk,

milkproducts

and

water

...................... 77

10.1 Sampling

.....................................................

77

10.2

Milk

pH

.....................................................

77

10.2.1

Measuring pH

using indicator ............................

77

10.2.2

Electrometric

measurement

of

pH .........................

77

10.3 Titratable

aciditytest

...........................................

78

10.3.1

Using

N10

sodium hydroxide

...........................

79

10.3.2

Using

N/9

sodiumhydroxide

(milk) .......................

79

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10.3.3 Using N/9sodium

hydroxide

(cream)

...................... 80

10.4 A lcohol

test

.................................................. 80

10.5 Clot-on-boiling

test

............................................ 80

10.6

Fatdetermination

.................................

81

10.6.1

M

ilk ................................................ 81

10.6.2

Skimmilk,

buttermilkand

whey

..........................

84

10.6.3

Cream

............................................... 84

10.6.4 Cheese ..............................................

85

10.7 Specific gravityof

milk

......................................... F5

10.8 Total solids(TS) inmilk ........................................ 87

10.8.1 Lactometermethod ....................................

87

10.8.2

Oven-drying method

...................................

87

10.9

Formaldehyde in milk ..........................................

88

10.10 Methylene

blue

reduction

test

.................................... 88

10.11 Resazurin

10-minute test

........................................ 90

10.12 Sediment orvisible dirt test ...................................... 91

10.13 M

oisture content of

butter ....................................... 91

10.14

Saltcontent

ofbutter ...........................................

92

10.15

Protein content

of

milkby

formaldehyde

titration

....................

92

10.16

Estimationofhardness

in

water .................................. 93

10.16.1 Temporaryhardness ...................................

93

10.16.2 Permanent hardness .................................... 93

10.16.3 W

ater-testing tablets ...................................

94

11.

Dairybuilding design and

construction

...................................

95

11.1 Site selection 95

................................................

11.1.1

W

atersupply .........................................

95

11.1.2 Land

................................................

95

11.1.3 Other

buildings

....................................... 95

I1.LIA Proximity to the road ...................................

95

11.1.5 Effluent

disposal

...................................... 95

11.2 Type ofbuilding ..............................................

96

11.2.I Construction

materials

..................................

96

11.3 Arrangement

and

installation of

equipment

.........................

96

11.3.1 Arrangem

ent

.........................................

96

11.3.2 Installation

........................................... 96

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List

ofAppendices

Appendix

1.

Dairycalculations

........................................ 99

Appendix 11. Glossary

ofterms

.......................................

103

Appendix III.

Composition of

somefoods

...............................

10'

Appendix

IV. Temperature

conversion

..................................

106

Appendix

V.

Titratable

acidity expressed

in

differentways .................

107

Appendix

VI.

Volumeunitconversions

.................................

108

Appendix

VII. Tableof

atomic weights

..................................

109

Appendix

VIII.

Somestandard solutions

for volumetric

analysis

...............

110

Appendix

IX.

Approximate

strengths of

somecommercial

laboratory

reagents .

.III

Appendix

X. Indicators forvolumetric

analysis

...........................

112

Appendix

XI.

Length

and area

units

....................................

113

Appendix

XII.

The improved

traditional chum for

buttermaking

..............

114

Appendix

XIII. Useful

references, names and

addresses ......................

118

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Listof

Tables

Table

I.

Composition

(%)

of

milk

of

somespecies

of

mammal

..................

3

Table

2.

Average

composition (%)of

cow

milk................................

7

Table 3.Composition

of

cow

milk..........................................

II

Table 4.

Composition of lipids inwhole

bovine

milk

...........................

II

Table

5.

Principal

fatly

acids

found in milk

triglycerides

........................

13

Table 6.

Distribution ofmilk saltsbetween the

soluble

and

colloidal phases ......... 18

Table

7.Stability

olf

itam ins..............................................

18

Table 8.Bacterial

types

commonlyassociated

with

milk

........................

24

-

oftemperattire

different

conditions..................................

............

3

I

Table 9. flect on the

growth

of

bacteriainmilk produced

under

Table 10. Manufacluring procedures

for yoghurt,acidophilus milkandkefir.

.......

70

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List

of

Figures

Figure 1.Flow chart illustrating

the incorporation

of

the

majormilk

solid fractions

in m ilk products . ................................................. 5

Figure 2.

Changes in

the concentrations offat, protein and lactose

over

a

lactation of

Figure

4.

Structural formulaeof

four

18-carbon fattyacids varying in degree of

acow. .......................................................... 8

Figure 3.Fat globules

in m

ilk

.

............................................. II

saturation

....................................................... 12

Figure 5.

Milk-protein

fractions............................................. 15

Figure

6.

Struciure

of

a

lactose molecule

.............................. 17

Figure 7. Rod-shaped

(bacilli)

andspherical (cocci) bacteria...................... 19

Figure 8.

Schematic

illustrationofbacterial stucture............................ 20

Figure

9.

The fourphases of

bacterial growth

..................................

21

Figure

10. Structure

ofmoulds

.

............................................. 23

Figure

11. Structureofa

yeast

cell. ..........................................

23

Figure 12.

Batch

separation ofmilkby

gravity

.................................. 38

Figure

13.

Cutaway diagrams

of

(a)

hand-operated milkseparatorand

(b)

thebowl

showing

thepaths

ofmilk and cream fractions......................... 39

Figure 14. Products andby-productsofbuttermaking

from

sourwhole

milk

......... 52

Figure 15.

Adding

lemonjuiceand stirring the

milk.............................. 55

Figure 16.Stirring

the

curds andwhey........................................ 56

Figure

!7.

Separating

thecurds fromthewhey using

a

muslin cloth...............

56

Figure 18. Adding curd

to

a muslin-lined mould................................. 57

Figure 19. Pressing cheese.................................................. 58

Figure20. Cuttingthecurd mass

.............................................

59

Figure 21. Floatingcheese

pieces

. ........................................... 59

Figure 22.

Cheesz

pieces folded

over

aftersalting............................... 60

Figure 23. Ladlingcurds andwhey into a cheese mould....................... 62

Figure 24. Whey draining

and,

in the foreground,

cheesepieces incool water ........

62

Figure 25.

Checking

the coagulum before cutting................................ 63

Figure

26.

Cutting thecoagulum with avertical knife............................

64

Figure

27.

Cutting the cheese

curd

............................................

64

Figure 28. Covering thecurd with

cheese

clo'h tokeep it

warm

duringcheddaring

..... 65

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Figure

29. Putting

thecheese

intoamuslin-lined

mould

..........................

65

Figure

30.Cheese

ready for

pressing .........................................

66

Figure 31.

Outline offour

important

lactose fermentations

compared

to a

76%fat

recovery whenusing

the

clay

pot fittedwith

the

internalwooden

agitator...

68

Figure 32.

Flow diagram of

fermented milk manufacture

.........................

70

Figure

33. pH

meter .......................................................

78

Figure

34. Apparatus

for the

Gerber test .......................................

82

Figure

35. Transferring

milk

to the butyrometer

and reading

the fatresult

83

Figure

36. Lactom eter.....................................................

86

Figure 37. Improved

traditional

churn

. ......................................

114

Figure38.

Componentsof

the

improved traditional chum

........................

115

Figure

39.

Components

of

the internal agitator

................................

116

x


10/123

Acknowledgements

The production

of this

training manual

has

been

made possible

through the efforts

and

collaboration ofseveral

people.

Sincerethanks are

due

toProfessor

PA

Morrissey,

University

College,

Cork,Ireland and

to

Mrs

PABorland,

DR

and

SS,

Harare,

Zimbabwe

for

their

verymany

helpful

comments

and

suggestions.

The

interestandencouragement

ofDrM

E

Smalley,

Director

ofTraining

and

Information,

in

thepreparation

of

the manual

isvery

much

appreciated.

Many

thanks

are

due

to

PaulNeate,

Head

of Publications,

and

to

Anne Nyamu, Scienc.

Writer/Editor,

for their

constructive

comments,

patient,

critical

andcarefulediting

and

overall

layout

andpresentation

of

themanual.

To

thestaff

of

the

Publications

Department

fortheir

skill

and

care

inprinting the

manual

and

toFantu

Yimer

for typingthe

script I

offersincere thanks.

Charles

B.O'Connor

xi


11/123

Foreword

This training manual

is

based largely on ILCA Manual No.

4

written by the

late

Frank

O'Mahony.

As a result of

experiences

and

suggestions

obtained

particularly

from

participants

in our

Rural Dairy

Processing

training

courses

it

was

considered necessary

to

provide

a more

comprehensive

manual

withmore

up

to

date technical

information.

Emphasis

has

been

given

to clean

milk

production

as milk

is our raw

material

for

processing and

preservation.

In

recognition

ofthe need

tocomply

with

hygienic

and

compositional

requirements

ofmilk and

milk

products

the

chapter

on

analytical

methods

has

been

expanded.

To

recognise

the

importance

of

water

in milk

processing,

water-quality

standards are

discussed

and methods

for the

determination

of

itschemical

quality

are detailed.

Milk production

and

processing

is

an

important activity

of

smallholders throughout

the

worldand

withthe

helpof

this

manual itis

hoped

thatdairytechnologists

andextensionworkers

willfurtherassist

andpromote

thedevelopment

ofmilk

processing

particularly

incountries

with

a

developing

dairy

industry.

Michael

E. Smalley

Director ofTraining

and Information

Previous

Page

Blank

xiii


12/123

1.Introduction

Milk

and milkproducts

have been

used

by

mansince

prehistoric

times.

There

is evidence

thatbutter was

made

as far

hack

as

2000

BC.

It

is

thought that cheesemaking was

discovered

accidentally

and

initially

developed

in

Iraqcirca7000-6000

BCand

spread

withthe

migrationof populations

dueto famines,

conflicts

and invasions.

Examples

of

these migrations

are

the development

ofSwiss cheeses

by

the

Hclveti

tribe

in

Switzerland

and the introduction

ofcheesemaking

into

England

by the Romans.

Cheese

varieties peculiar

to

each region

developed

because

ofthe different

agricultural

conditions

prevailing

in eachcountry.

There

are,at present,

almost

2000 recognised

varieties of

cheese.

Fermented

milks have

been

prepared for

more

than

2000years.

Allowing

milk to

ferment

naturally

gives

an

acidic

product

that does not

putrefy.

Fermented

milks are wholesome

and readily digestible;

examples

of

suchproducts

are yoghurt,

kefir,

koumiss and

acidophilus

milk.

Thedevelopment

of

the

milk

separator

in the

19

'hcentury

made

centralised milkprocessing

possible.

Initially,

cream

was

separated

and

retained for

buttermaking

and the

fresh skim

milkwas

returned to

the

milk producers.

As the nutritional

importance

of the

non-fat component

(skim milk)

became

recognised,

processes

weredeveloped

toconserve

milk

solids-itot-fat

(SNF).Cascin

andcasein

products as

wellaslactose

and

dried

milk were prepared. Today,

up to

60%

of

the

milk produced

in the

world is converted

into

dehydrated

milk

products and

foods containing

a large

proponion

of

milk solids. In

countries

with

commercial

dairying

these processes

are carried

out in large-capacity

processing

plants.

In Africa,

milk is produced

in most

agricultural

production

systems. Itiseither

sold fresh, consumed

as

fermented milk or

manufactured

into products

such

a.

butter, ghce

and cheese. Sour

milk

is !he

most

common

product,

and

vnilk

is

usually

soured

before further

processing.

While there

are several

milk

processing

plants

in Africa,

much

of

the

milkproduced

by

rural

smallholders

is processed

on-farm

using

traditional technologic:.

It

is important,

therefore,

to

consider these processes

and

look

to

possible

technological interventions

at

this level

when considering

dairy

development

in

the

rural

sector.

Farmers in many

Africancountries

produce

sourmilk,

butterand

cottage

cheese for

home

consumption

and

sale.The

Maasaii

in Kenya

makeghee from sourmilk.

Fernented

milks

aremade

throughout

sub-Saharan

Africa,

andconce:itratcd

fermented

milks

aremade

in sonic

parts

of

the continent.

While theprocesses

used

have

not

been

subject to extensive

scientific

investigation,

they

appear to

be effective

methods

of

converting

milk into

stablemarketable products

and

have long

been used forprocessing

,urplus

milk.

Milk is

processedprimarily

to convert it into

a

more stable

product,e.g fermented

milk

canbe

stored

forabout

20days

compared

with

less

than oneday

for fresh

milk. Milk

products

are morestable

than

fresh

milk

because they

are more acidic

and/orcontain

less moisture. Preservatives,

e.g.

salt may

also beadded

tom ilk

products.Thus,

by increasing

the acidity and

reducing the

moisture

content, the

storagestability

of

milk

can

be

increased.

This manual deals with

milkprocessing

in a

rural and

small-scale environment.

It

concentrates

on

traditional

productsor

on

products

that

are

easily

made,

need little specialised

equipment

and

can

beeasily

adapted

to the rural

processing

plant. Some

background

information

in

the areas

of

milk

chemistry, dairy

microbiology

andmilk and milk-product

analysis

is also

gi,,en.


13/123

2.

Milk

as afood

Milk is

secretedbythemammary

glandsofmammals

tofeedtheiryoung.Cow

milk

-

awhite fluid

oflow

viscosity

and slightly sweet

taste

-

is most

commonly used

as human food. There are, however,

wide

variations

in

the

chemical and

physical propertiesof

themilk ofvarious

mammalian

species

as

shown

in

Table

1.The

tablegives the

average

gross composition

values ofmilk

from

somecommon

species.

Table 1. Composition

( ) of milk

of

some species

of

mammal.

Species

Total solids Fat

Protein

Lactose Ash

Human

12.4

3.8 1.0

7.0

0.2

Cow

12.7 3.7

3.4

4.8

0.7

Goat

12.3

4.5

2.9 4.1 0.8

Sheep

19.3

7.4

5.5

4.8

1.0

Horse

11.2

1.9

2.5

6.2 0.5

Donkey

11.7

1.4 2.0

7.4

0.5

Domestic ranbit

32.8

18.3 13.9

2.1 1.8

Camel

12.9

4.2 3.7

4.1

0.9

Milk is

the sole

sourceofnutrients

for mostyoung

mammals for lengths oftime

which vary with

the

species.

Overall, milk serves the following broad functions for both young

and

old:

(a)

growth,

(b)

reproduction,

(c) supply of

energy, (d) maintenance

and repairs and

(e) appetite satisfaction.

The

requirements of

these

categories vary with

the individual, andin

some instances

not

all

the

statedfunctions

ofthe

food

need to

be

served,

e.g.

adults

do

not require

food

for growth whereas infants

do.To

fulfil

its

functions as a food

milk contains various

nutritionally important

components, namely

proteins,

carbohydrates,

lipids, minerals,

vitamins and

water.

The gross energy

supplied

by

milk

can

be calculated

from

its

lactose,

protein and

fat contents.

The

metabolically available

energy is approximately

4.0,4.1 and8.9

kcal/g(16.8, 17.0 and37.0

kJ/g) forlactose,

protein

andfat,

respectively.

On thebasis

of

the

data

inTable

I

human

andcowmilk

contain

670-720

kcallkg

(2.8-3.0

MJ/kg).

The

chief

function

of

lactose in

milk

is to

supplyenergy formuscular

activity and maintenance

ofbody

temperature.

Like

other

disaccharides,

lactose must

be

hydrolysed

to

its monosaccharide

components,

glucose and

galactose, before it is

absorbed across

the

intestinal membrane

into

the

bloodstream.

Some

peoplecannot

toleratelactose because

they lack

the

enzyme

(lactase)

whichis required

to

hydrolyse

it.

Lack

of

lactase may

result inabdominal

cramps,bloating anddiarrhoea

ondrinkingmilk.Whenlactose is removed

from

milk,

or

converted into lactic

acid

during

cheese

manufacture,

milk products

can be consumed

by

lactose-intolerant people.

Lactose has certain

therapeutic properties

and is known to enhance

the

intestinal

absorption

of

calciumand

phosphorus.

Itspresence inthe

intestine favoursan

acid-type

fermentation

which

may

prevent

intestinal

disorders.

Fermented

milks

may

be

preferable

to

fresh milk

because

theyprevent the

propagation of

infectiousdiseases.

Proteins

are

essential

for

thegrowth and maintenanceofall

cells inthebody.

Thevalue

of

milkproteins

depends

primarily

on their

content

of

some nine essential

aminoacids

whichcannot be

synthesised

by the

body. Cow

and

human milk

contain

all

the essential aminoacids required

for

human infants.

Fortunately,

both

cow

and human milkareeasily

digestedand theamino

acids arereadily absorbed.

3

kPreVioUS.

kage.b~ara


14/123

Cow milk

(3.4%

protein) forms

a rather

frm curd

in

the stomach

and

digestion

is slower

than

with

human

milk,

whichcontains

about

1%

protein.

Diluting

cow

milk with

wateror

high heat

treatment

softens

the curd.

Lipids

supply

the

body

withaconcentrated

source

ofenergy

and

are

also

important

contributors

to

both

desirable

and

undesirable

flavours

in milk

and

milk

products. Certain

fatty

acids

are

not

synthesised

by the

animal.

They

include

the

polyunsaturated

acids,

linoleic

(Ct8:

2

)acid

and probably

linolenic

(C18:3) aced.

It

is

considered

that

2-4%

of

the total energy

of

the diet should

he

supplied

by

polyunsaturated

acids.

The

linoleic

acid contentin

human

milk

fat accounts

for

approximately

5%

of

the

energy

in

milk. This

is

much

higher

than for

cow milk fat

which

accounts

for only

about 1%of

the

total

energy.

Human

and

cow

milkare excellent

sources of

vitamins.

Vitamins

A, D,

E

and K

occur in

the fat phase

and

theothers

in the

aqueous

phaseof

milk.Milk isa

major

source of

someof

thevitamins

needed

by

infants

and

adults. It

is reltively

rich in

vitamins

Aand

E,

thiamin,

riboflavin,

folic

acidand

vitamin

B

12. However,

large

variationsoccur

between

humanand

cow milk.

Humanmilk

contains

only

about

35%as much

thiamin,

25%

as

much riboflavin

and5%

as much

Bt2 as

cow

milk.On

the other

hand,

human

rmilkcontains

about

I0

times

as much

vitamin

Eand

2.5

timesas

much

ascorbic

acidas

cowmilk.In

many

countries

milk

is

fortified

with

vitamins

A and

D. Vitamin

A

is central

to

the visual

process

as aconstituent

of

the

visual pigment

rhodopsin.

Vitamin

D

is

essential

for

the

calcification

processes

in

the body,

including

bone

and teeth

formation.

Milk is

alsoanexcellent

sourceof

many

minerals

and

supplies virtually

allof

the

minerals

required

by

humans.

Cow milk

furnishes

a major

portion

of

the

total calcium

consumed

in many countries.

The

high

levels of

calcium

and

phosphorus

in

milk are

important

in

bone and

tooth

formation

in young

children;

both

these elements

play

a

significant

role in preventing

osteoporosis

in elderly

people.

Milk also

contains

high

levels of

magnesium,

zinc

and

iodine. Ilowever,

milk is apoor

source of

ironand

neither

human

nor

cow

milk

supply

enough

forhuman

infants.

Fortunately,

infants

havea

store of

iron

in the

liverwhich

is sufficient

to

meetthe

needs of

the

body during

thefirst

six

months.

The nutritive

value

ofmilk

may

be

considerably

altered by processes

such as

separation,

concentration

of

the components,

addition

of non-milk

constituents

and heat

treatment.

For

example,

duringbuttermaking

thefat

and fat-soluble

vitamins

are

retained in

thebutter

while

the

protein, lactose,

minerals

and B

vitamins

remain

in thebuttermilk.

'art

of

the fatin

butter

can

be replaced

by vegetable

oiltogive

better

spreadability.

Diluting

cow

milk

with water

or

severe

heat treatment

greatly

softens

the casein

curd

and allows

for

easy

digestion.

When

mother's

milk

is not available

milk

formulations

for

babies are

prepared

by mixing

cow

milk,

cream,

whey

proteins,

lactose

and water.

The

ratio of casin

to

whey, protein,

the

lactose content

and

salts in milk

formulations

are similar

to

those ofhuman

milk.

Mild

heat

treatment

uch aspasteurisation

or ultra

high temperature

(UtlT)processing

cause

very

little

change

in nutritive

value.

Severe

heat

treatment

results

in

some loss

of

available

lysine,

but this

has little

effect

on

the nutritional

quality

because

milkproteins

are rich

in lysine.

The interaction

between

lysine

and

lactose

during

heating

results information

of

a

brown

pigment

(Maillard

browning)

that causes

off-flavours

to

develop

during

storage

of

milk products.

Figure

I

shows

the major

milk

constituents

and a

range of

products

that

can be manufactured

from

these

constituents.

4


15/123

FRlprel.

Flowchartillustrating

the incorporation

ofthe major milk-solidfractions

in milk products.

S

Whole milk

solids

en

a.t [Pr"i I

L

actos.

milk

Precipitate

pH46

Supernatant

IFCasein

Wheyprotein

B

t

r

l

fermen-f

team

35%

tat

tto

SChum

cream

Whey ,

Lactic

ac

I

utter

82%

fat

Acetic ai

1Propionic

ai

Aldehyde

Remove

Rmoture

Ketone

mosue.

Hard

C

I

ottage

cheese

cheese

uttr

B


16/123

3.

The

composition

of

milk

Milk

composition

isaffected

byanunber

offactors

includinggenetic

andenvironmental

factors.

3.1

Genetic

factors

3.1.1

Breedand

Individuality

of

thecow

Both

milkyieldand

composition

varyconsiderably

among

breeds

of dairy

cattle.

Jersey

andGuernsey breeds

give

milk

withabout5%fatwhile

the

milk

of

ShorthornsandFriesians

containsabout

3.5%

fat.Zebu

cows

cangive

milk

containing

up

to7%

fat.

Table2gives

the

averagecomposition

ofmilk

from different

breedsofcow.

Table2.

Average composition

( )

of

cow milk

Breed

Fat

Protein Lactose

Ash

Zebu

5.6

3.1

4.6

0.71

Ayrshire

3.8

3.4

4.8

0.70

Fresian

3.4

3.2

4.6

0.74

Guernsey 4.9

3.8 4.8 0.75

Jers,y

5.1 3.8 4.9

0.75

Shorthorn

3.6

3.4

4.8

0.70

Milk

of

individualcows within

a

breed

varies

over

a

widerange

bothinyield

and

in

thecontent

ofthe

various

constituens.

The

potential

fatcontentof

milk

from anindividual

cow is

determined

genetically,

as areprotein

and

lactose

levels.

Thus

selection

forbreeding

on

thebasis ofindividualperformance

iseffective

in

improving

milkcompositional

quality.

Herdrecording

oftotalmilk

yieldsand

fatandsolids-not-fat

(SNF) percentages

willindicate

themost

productivecows,

and

replacement

stock

should

be

bred

from

these.

3.2

Environmental factors

3.2.1

Interval

between milkings

Thefatcontent

of milkvaries

considerablybetweenthemorning

and

evening

milking

because

there isusually

a

much

shorter

interval

between

morning and

evening

milking

thanbetween

eveningandmorning

milking.

If

cows

were

milkedat t2-hour

intervalsthevariation in

fat

content

between

milkings

would

be

negligible,

butthis

isnotpracticable

onmostfarms.Normally,

SNFcontent

doesnotvarywith

thelength

of

timebetween

milkings.

3.2.2

Stage

oflactation

Thefat, lactose

and protin

contents

ofmilkvary according

to

stage

oflactation. Solids-not-fat

content

is

usually highest

during the

first two to three

weeks,after

which it

decreases

slightly. Fat

content

is high

immediately

aftercalving butsoon begins

to

fall,

and

continues todo

so for

10to

12

weeks,

afterwhich

it

tends

torise

again

untilthe

end

of

the

lactation.

The

highprotein

content

of

earlylactationmilk

is

due

mainly

to thehighglobulin

content.

The

variation

in milk

constituents

throughout

lactation

is

shownin

Figure

2.

7

')J*V1OUS

Pa~ge

Blaznk


17/123

Figure2.

Changes in the

concentrations

offa, protein

and lactose

over

a lactation

of

a cow.

Solids

concentration

(g/litre)

55.

I

Fat

5

.........--

Lactose

45

I

Protein*..*

40

\

35.

30.,

, 40

80

120 160 200

240 280

320

360

Parturition

Stage

of lactation

(days)

Dry

period

3.2.3 Age

and health

As

cows growolder

the fatcontentoftheirmilk

decreases byabout

0.02

percentage unitsperlactationwhile

the

fallin

SNFcontent

isabout0.04percentage

units.Bothfat

and

SNF

contentscan

be reducedby disease,

particularly

mastitis.

3.2.4

Feedingregime

Underfeeding

reduces both the fat

and theSNF

content

of

milk,although

SNF

content

isthemore

sensitive

to feeding

level.Fatcontent

and fatcomposition

are influenced

more by

roughage(fibre) intake.

The SNFcontent

may

fall

if thecow

is fedalow-energy

diet,but isnotgreatly

influenced

by

protein

deficiency,

unless

the deficiency

isacute.

3.2.5 Completeness

ofmilking

The

first

milk

drawn from

theuddercontains

about1.4% fatwhilethe

last

milk

(or

strippings)

contains

about

8.7%

at.

Thus, it

isessential

to

milk

the

cow

completelyand

thoroughlymix all

the milkremovedbefore

taking asample

foranalysis.

The fat left in theudder

at theend

of

a

milking is

usually

picked

up

during

subsequent

milkings,

sothere isnonet loss

of

fat.


18/123

4.

Milkchemistry

4.1

Physical

status

of

milk

About

87%

ofmilk

is

water, in

which

the

otherconstituents are distributed invariousforms.

Several

kinds

ofdistribution

aredistinguished

according

tothetypeand

sizeofparticle

presentin

theliquid.

Kind

of

solution Particlediameter(nm)

Ionicsolution 0.01-1

Molecular

solution 0.1-1

Colloid

(fine

dispersion)

1-100

Coarse

dispersion

(suspension

or

emulsion) 50-100

Inmilk,

examples

of

emulsions,

colloids,

molecularand

ionic

solutions

are found.

4.1.1 Ionicsolutions

Anionic solution

isobtained whenthe forcesthathold the ions together

in

a

solidsaltare overcome.The

dissolved

salt

breaks

up

into

ions which float freely

in

the solvent.

Thus

when

common salt

-

sodium

chloride -

isdissolved

in

wateritbecomes

an

ionicsolution

of

freesodium

andchlorideions.Ionicsolutions

are composed

largely

of

inorganic compounds.

4.1.2

Molecular

solutions

In

a molecularsolution

the

moleculesareonlypartly,

if

at

all,

dissociated

into

ions.

The

degree

of

dissociation

represents anequilibriumwhich isinfluenced

by othersubstances

in

thesolutionandby thepH

(or

hydrogen

ion

concentration) ofthe

solution.

Molecular solutionsare

usuallycomposed

oforganic

compounds.

4.1.3

Colloids

In

a colloid, one substance isdispersed in

another ina

finer

state thanan emulsion but the

particle

size

is

larger

than

that

ina truesolution.Colloidal

systemsare

classifiedaccording to

thephysicalstate

ofthe two

phases. Inacolloid,solidparticles consisting

of

groupsofmolecules

float

freely.

Theparticles

in

a colloid

aremuch smaller

than

those in

a

suspensionand

a

colloid is

much morestable.

4.1.4Emulsions

An emulsionconsists ofone

immiscible

liquid

dispersedinanother

in

theform ofdroplets

- the

disperse

phase.

The

otherphase

is

referred to

as

the

continuous

phase.The

systems

haveminimal

stabilityand

require

a surface-activeoremulsifying agent,

e.g. lecithin inmilk, forstability.In

foods,

emulsionsusually contain

oilandwater. If

water

is

thr-continuous

phase

andoil

the

disperse

phase, it

isanoil-in-water (o/w)emulsion,

e.g.milkorcream. In the reverseoasethe emulsion is

awater-in-oil (w/o)type,e.g. butter.

4.1.5 Dispersions

A dispersioa is

obtained

when particles of

a

substance

are

dispersed

in aliquid. A

suspension

consists

of

solid particlesdispersed in

a

liquid,and the force

of

gravity

cancause themtosink

to

the

bottom

orfloat

to

the top.

For

example,

fine

sand,

dispersedin

water,

soonsettlesout.

9


19/123

4.2pH

and

acidity

An acid

isasubstance

which

dissociates

toproducehydrogen

ions

in solution.

A

base

(alkaline)

isasubstance

whichproduces

hydroxyl

ions

in

solution.

It

can

equallybe

stated

that

an

acid

isasubstance

whichdonates

a

proton

and

abase

is

a

substance which

acceptsaproton.

The symbol

pH

isused to

denoteacidity; it

is

inversely

related

to hydrogen ion

concentration.

On a

scaleof

0-14:

Neutrality

=

pH7

Acidity

is

pH7

Fresh milkhas

apHof

6.7

andis therefore

slightly

acidic.

When

an acid

is

r.ixed

with

a

base,

neutralisation

takes

place;

similarly

a

base will

be

neutralised

by

an acid.

4.2.1

Buffer

solutions

Buffersare

defined

as materials

that

resistchange

in

pH

on

addition

of

acid

or

alkali.

Characteristically

they

consist of

aweak

acidor

a

weak

base

and its

salt. Milk

contains

alarge

number

of these

substances and

consequently

behaves

as

a

buffersolution.

Fresh cow

milkhas

apH

of

between

6.5and

6.7.

Values

higher

than

6.7 indicate mastitic

milk

and values below

pH

6.5 indicate

tie presence of

colostrum

or bacterial

deterioration.

Because

milk is

abuffer solution,

considerable

acid

development

may occur

before

thepH

changes.

A

pH

lower

than6.5 therefore

indicates

that

considerable

aciddevelopment

has

taken place.

This

is

normally

dueto

bacterial

activity.

Litmus

test

papers,

which

indicate

pH,

are

used

to

test

milk

acidity;

pH

measurements

are

often

used

as acceptancetests

for

milk.

Milk

acidity

is

an important

indicator

of

milk

quality.Acidity

measurementsare

also

used

to

monitor

processes suchas

making

cheese

and

yoghu t.

Thetitratable

acidity

oftmilk

isexpressed

in

termsofpercentage

lactic

acid

- theprincipal

acid

produced

by

fermentation

after

milk

is

drawn

from the

udder. Fresh

milk

contains

onlytracesof

lactic

acid.

However,

C_e tothe

buffering

capacity

oftheproteins

andmilk

salts

fresh

milk,

in

which

no

lactic

acid has been

produced,

normally

exhibitsan initial

acidity

of0.14

to 0.16%

when

titrated

using

sodiumhydroxide

toaphenolphthalein

end-point.

4.3

Milk

constituents

The

quantities

ofthe

main

milk

constituents

can

vary

considerably

depending

on the individual

animal,

its

breed,

stage of

lactation,

ageand

health status. Herd

management

practices

and

environmental

conditions

also

influence

milk

composition.

The

averagecomposition

ofcow

milkis

shownin

Table3.

Water

is themain

constituent

of

milkandmilk

processing

is usually

designed

to

remove

water

from

milkor

reduce the moisture

content

oftheproduct.

4.3.1

Fat

If

milk

is left

tostand,

alayerof

cream

formson

thesurface.

Thecream

differs

considerably

in appearance

from the

lowerlayer

of

skim

milk.

10


20/123

Table

3. Composition of cow

milk

Main

constituent

Water

Totalsolids

Fat

Proteins

Lactose

Minerals

Cream consists

of

a

Range(%)

Mean(%)

85.5-89.5

87.0

10.5-14.5

13.0

2.5-6.0

4.0

2.9-5.0

3.4

3.6-5.5

4.8

0.6-0.9

0.8

large

number

of

spherical

microscopic

globules

ofvarying sizes

floating

in

the

milk.Each

globule is surroundedby

athin

skin

-

the fat

globulemembrane-

which

actsas the

emulsifying

agent for

the

fat suspended

in

milk

(Figure

3).

The

membrane

protects

the

fat fromenzymes

and prevents

the

globules

coalescing into

buttergrains.

The

fat is present

as

anoil-in-water

emulsion

that

can

be

broken

by

mechanical

action

such

as shaking.

Figure

3. Fat

globules in milk.

Fat

globule

0

E0

Serum

Fats are partly

solid at

room

temperature. The

term

oil is reserved

for

fats

that arecompletely liquid

at

room

temperature. Fats and

oilsare

soluble

in non-polar

solvents, e.g. ether.

The lipidcontent

of

milk

is

usually

defined

as

the fraction

which

is

extracted

by

organic

solvents. Table4

gives themain

lipid

classes

of

milk fat.

Table

4.

Composition

of lipids

in whole

bovine

milk.

Lipid

Carotenoids

+vitamin A

Cholesterol esters

Triglycerides

Diglycerides

Monoglycerides

Free fattyacids

Cholesterol

Phospholipids

il

Weight (%)

trace

0.02

98.3

0.3

0.03

0.1

0.20-0.40

0.20-1.0

http:///reader/full/0.20-0.40http:///reader/full/0.20-0.40


21/123

About

98%

of

milk

fat is a mixture

oftriacyl

glycerides.

The

partial glycerides

(diglyceridesand

monoglycerides)

andfree

fattyacids

are probably

partly

leftoverfrom

the

biosynthesis

process. Also

present

are fat soluble

vitamins

A,

D,E and K

and

pigments,

e.g.

carotene

which

gives

butter its natural

yellow

colour.

Examples

ofeach

type offatty

acid

are shown

in Figure

4. The main variables

arcas follows:

I. Chain

length.

Fatty

acids

vary

in

chain

length from

4

carbon

atoms,

as

in butyricacid,

to20 carbon

atoms,

as

in arachidic

acid. Nearly

all

thefatty

acids

in

milk

contain

an

even

number

of

carbonatoms.

Milk

fat

contains

significant

levels ofshort

and

medium chain

fattyacids.

Butyric

acid

(C4) is specific for

milk

fatof

ruminantspecies.

2. Number

ofdouble

bonds.

A fatty-acid

molecule comprises

a hydrocarbon

chain

and

a carboxyl

group

(-COOH).

In

saturated

fatty

acids the

carbon

atomsare

linked

in achain

by single

bonds

(e.g.

stearic acid,

C18:0, in

Figure 4).

Unsaturated

fatty

acids

have one double

bond, e.g.

oleic acid, CI8:t,

while

polyunsaturated

fatty

acids

havemore

thanonedouble

bond, e.g.

linoleic

acid,

C18:2

(two

double

bonds),

and

linolenic

acid, Ct8:3 (three

double

bonds).

It is the

double

bonds

in

thecarbon

chain

that

make

the

fatty acid unsaturated.

Twohydrogenscan

be

added

perdoublebond

at high

temperature

witha

suitable

catalyst.

This

process

is called hydrogenation

and has the effect

of

converting

a

soft fat to a hard

fat at

room

temperature.

Figure

4.

Structuralformulae

offourl8.carbonfittt.acidthmra.ingintdegreeofsatiuraItion.

Stearic

acidC18:0

H-

H

I

C-

H

HHHHHHH

I I I

I

I I

I I

C-C-C-C--C-C-C-C-C-C-C-C-C-

H

I

H

I

H H

I

H

I

H

H H

I

I I

C-C-C

,0

SI

I I

I

I I

I I I I

I I

I I

I

H H

H H H

H H H

H H H H

H H H

H H OH

18

15

13

11

9

7

5

3 1

Oleic

acid

C18:1

H

I

H

I

H

I

H

I

H

I

H

I

H

I

H

I

H

I

H

I

H

I

H H

I

H

I

H

I

H

I

H

I

H-C-

C-

I

I

CC-

I

C-

I

-C-

C-C

I 1

i

C-C-

C-

C-C-

C-C-C-

I

I I I

I I

I I

H H

H H H H

H H H

H H H H

H H OH

18

16

14

12 10

9

7 5

3 1

Unoleicacid C18:2

H

H H

H H

H

H H

H H

H H

H

H H

H H

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

1

A

H-C-CIC-C-C-

C C-C-C-C-

C-

C- C-

C- C-

C-

-

I I

I I I

I I

I I

I I

I I I

I

II

H

H H

H

H HHH

H H

H

OH

18

13

12 11 10 9

7 5

3 1

Linoleic

acidC18:3

H

H H

H H

H H

H H

H H

H

H H

HHH

I

I I

I I

I

I I

I I I

t I

t

t I

A

H-C-C-C=C-C-CC-CCC=C-C-

C-

C-C-C--C-

H H H

H H

H H H H H H

OH

18

16

14

11

9

7 5

3

1

12


22/123

3.

Positionofdoublebond. The double

bond

can occur

in

many

positions (called isomers).

Olcicacid has

thedoublebond

atthe

ninthpositionwhich may

be indicatedas

follows:

CI8:t

9. Linoleicacidhas

two

double

bonds

at the

ninthandtwelfth

positionswhich

may

be

indicated

as

follows:

C18:2

9, 12.

4.

The

proportion

of

saturated

fatty

acidspresent

in

milk

fatisabout

63%.

5.

Oleicacid is

the

mostabundant

of

the

unsaturated

fatty

acids.

Table5. Principalfatty

acids found

in milk triglycerides.

Average

Melting

Fattyacid

Molecularformula

amount

in milk Chainlength point

fat

(%)

(No.ofcarbons)

'C)

Butyric

CH3(CH

2

)

2

COOH

3.7 4

-8

Caproic CH3(CH2)

4

COOH

2.0 6

-2

Caprylic

Ct12(Cl12)

6

COOH

1.6 8

16

Capric Cl13(CH2)8COOIl

2.6

10

31.5

Lauric

CHt(CH2)IoCOOH

3.3

12 44

Myristic

CI3(Ctl2)12COOH

8.7 14

58

Palmitic CH3(CHt2)I4COOI

27.0 16

64

Stearic

CH3(CH

2

)t6COOH

10.0

18 70

Oleic

Ct4I(CH2)7CH=Ctt(CH2)7COOH

35.0

18 13

Linoleic

CH3(CI12)4(CH=CH.CH2)2(CH2)6COOH

4.5

18 -6

Linolenic

C113.CH12(CH=CH.CH2)3(CI

2

)6COOH

0.6

18

-13

Arichidic

CttI(CIl))IsCOOH

1.0

20 77

The

most

importantfatty

acids

found inmilk

triglyceridesare

shown

inTable5.

Fatty

acids

areesterified

with

glycerol

as

follows:

H

2

_C-OH HOOC-RI

H

2

_C-OOCRI

Ht-C-OH

+

HOOC-R2 >

H-C-OOCR

2

+

3H

2

0

H2-C--OH IIOOC-R

3

H2-C-OOCR

3

Glycerol

+

fatty acids-

.

triglyceride

(fat) +

water

The

melting point and

hardness ofthe fattyacid isaffected

by

the length

ofthe

carbon

chainand

the

degreeof

unsaturation.

As chain

length

increases, meltingpoint

increases.

As

the

degree

of

unsaturation

increases,

the

melting pointdecreases.

Fatscomposed

ofshort-chain

or

unsaturated

fatty acids

have lowmelting

pointsand

are

liquid

at

room

temperature,

i.e.oils. Fats

high

in

long-chain

saturated

fatty

acids

have highmelting

pointsandare

solid

at

room

temperature.

Butterfat

is

a

mixture

of

fatty

acidswithdifferent

meltingpoints and therefore

does

not

have

a

distinct melting

point. Sincebutterfat

melts gradually

over

atemperature

range

of

0-40'C,

someof

the

fat is

liquid

and some solidat temperatures

between16 and 25"C.

The ratio

ofsolidto liquid fat at

the

time

of

churning

influences

the

rate of

chuming and the

yieldandquality

ofbutter.

Fats readilyabsorb

flavours,

e.g.

buttermade

in

asmokedgourd

has

a

smoky flavour.

Lipids in foods

are

subject

to two

forms

of

deterioration

that

affect the

flavour

offoodproducts:

Hydrolytic

rancidity

Lipolysis,

which

isthe breakingdown

of

milk

fat intocomponent fatty

acids, increases

theconcentrationof

free

fattyacids.Lipolysis

is

inducedby

theaction ofnatu~ally

occurring lipase

in milkwhich

hydrolyses

the

13


23/123


24/123


25/123

Wheyproteins

When

milk

isbrought

to

pH

4.6,

the

caseins

precipitate.

Thesupernatant

contains

fourprincipal

proteins

in

the whey

fraction,B-lactoglobulin,

a-lactalbumin,

blood

serum

albumin,

immunoglobulins

and

a

number

of

minor

proteins, e.g. lactoferrin

and enzymes.

Most ofthe whey

proteins

are

denatured

byheat,

i.e. they

become

less

soluble

ifmilk

isheated.

B-Iactoglobulin

is the

principal

whey

protein

of the

cow,

goat

and

sheep,

although

there

are slight interspecies

differences.

B-lactoglobulin

accounts

forabout

50%

of

the

total

wheyproteins

or

about

11%

of

the Iotal

protein in

milk.

Relatedbut

substantiallydifferent proteins

occur

in

porcine milk.

No

13-lactoglobulin

has

been identified

inhuman,camel

or horsemilk

in

which a-lactalbumin

is the principal

whey protein.

Denaturation

ofwhey

proteins

andB3-lactoglohulin,

in

particular,is

of

majortechnological

significance.

0-lactoglobulin

interactswith

k-casein

duringheating

and this

reduces the

heatstability

ofmilk,

slows

down

rennet

clotting

duringcheese

manufacture and gives

asoft

curdwhich

tends

to retain water.

(x-lactalbumin

represents

about

20% of

theprotein

of

bovinewhey

(3.5% of

thetotal milk

protein)

and

isa

relativelyminor

proteinin

terms

of

quantity.

Itfunctions

as part

of

theenzyme

system

involved

in lactose

synthesis.

The

immunoglobulins

are

antibodies

which

are

present

in

high

concentrations

in

colostrum.

Infants

and

mammals

areborn

withoutcirculating

antibodies

andthe

main wayin

which

theyacquire

these

isbyingestion

of

colostrum.

Minorproteinconstituents

About

50

enzymes

have

been detected in

bovine

milk.The

concentration

of

milk

enzymes

varies

greatly

among

species.

Some milkenzymes

act

on

substrates present

as normal

constituentsofmilk

andmay

play

eitherbeneficialor

deleterious roles

duringmilk

processing.

Catalase.

This

enzymecatalyses

thedecompositionof

hydrogen

peroxide

(1-1202) to

1-120

and

02.Its

activity

ishigher

in mastitic

milk

andcolostruin

than in

normalmilk and

increases

with

increase

in

bacterial

numbers.

latoperoxidase.

This

enzyme catalyses

oxidation

ofa

range

of

substrates

by

11202.

Theenzymecatalyses

oxidation

of

thiocyanatc

to

products

that

inhibit

certain

bacteria.

It is

relativelyheat

stable: it

isnot inactivated

by

pasteurisation

(72'C x 15

seconds) but

is destroyed

when

milk

is heated above

80'C. The

absence of

lactoperoxidase

in milk

indicates

that the

milk

has

beenheated

toat

least 80'C.The

test for

the

presence

of

lactoperoxidase

isbased

on the

oxidationof

the

substrate para-phenylenediarnine

in the

presenceof

11202.

Phosphatase.

Phosphatase

enzymes

catalyse

the hydrolysis

of

phosphate

esters.

Milkcontains

an acid

and

alkalinephosphatase.

Alkaline

phosphatase

hasap-I

optimum

near

9 and

is inactivated

byheating milk

to

72"C for 15

seconds. Its

absence

indicates

that milk has

been properlypasteurised.

Ifmilk

is inadequately

pasteurised,

the

residual

enzyme

willcatalyse

thehydrolysis

of

added

disodiumpara-nitro-phenol

phosphate

liberating

para-nitro-phenol

which is

yellow

in

alkalinesolution.Acid

phosphatase

which

has apH

optimum

of 4,

ismore heat stable

than

alkaline

phosphatase.

Other

milk

enzymes.

Milk

alsocontains

lipases

(discussed

earlierunder hydrolytic

rancidity)

proteases,

amylases,

xanthine

oxidase,

carbonic

anhydrase and lysozyme.

4.3.3

Carbohydrates

Lactose

is

tie

majorcarbohydrate

fraction in milk.

It is

a

disaccharide

composed

of

two sugars,glucose

and

galactose

(F;gure

6).

The

average

lactosecontentof

milk

varies

between

4.7

and

4.9%,although

milk

from

individual

cowsmay

vary more.

Mvastitis

reduces

lactose

secretion.

Lactose

is

a

source of

energy for

the young

calfand

provides

4calories/g

of

lactose

metabolised,

It

is

less

soluble

in

waterthan

sucrose

and

is

also

lesssweet.It can

be broken

down

to

glucose

and

galactose

by

bacteria

that have

the

enzyme

LB-galactosidase.

Theglucose

aidgalactose

can then be

fermented to

lactic

acid,

Thisoccurswhen milk

goessour.Under

controlled

conditions

theycanalso

befermented

to

otheracids

to

give adesired

flavour,

such

as

propionic

acidfermentation

in Swiss-cieese

manufacture.

16


26/123


27/123


28/123

5.

Microbiology

Micro-organism

is

the term applied

to

all microscopic

living

organisms.

Micro-organisms

tend

to

be

associated

with disease; those

that cause

disease

are

called

pathogens.

However,

few

micro-organisms

are

pathogens

and

micro-organisms play

a

crucial

part

in

life

on ourplanet.Forexample theyprovide

food

for

fish,they occur

in soil

where they

provide

nutrients

forplants

andtheyplay

animportant

role

in

ruminant

digestion.

In dairying

some

micro-organisms

are

harmful,

e.g.

spoilage

organisms

andpathogens

while

othersare

beneficial,

e.g.

cheese and yoghurt

starters and

yeasts and

moulds

used

in

controlled

fermentations and

cheesemaking.

The micro-organisms

principally

encountered

inthe dairy

industry

are bacteria,

yeasts, moulds

and

viruses.

5.1

Bacteria

Bacteria

are

microscopic single-celled organisms

that are

present

inair,

water

and

onmostsolidmaterials.

When

observed

under a

microscope

the

cells

can be

seen to

differ

inshape

and

in

conformation

ofgroups

of cells.

Cells

are either

spherical orrod-shaped

(Figure

7);

spherical

bacteriaare called

cocciwhile

those

that

are rod-shaped are called

bacilli. Thisis the

firstbasis fordifferentiating

betweenbacterial

cells.

Figure

7. Rod-shaped

bacilli)and

spherical

(cocci)

bacteria.

6

9

~

0

0

0

&Q

00

Diplococci Micrococci

With

endospores Staphylococci

Streptococci

With flagella

:01

Rodbacteria

(bacilli)

and

spiralbacteria

Spherical

bacteria(cocci)

Bacteria

are alsoclassified

according

tocell-cluster

formation:

*

Diplococci: paired

cocci

cells.

*

Staphylococci: a

number

ofcells

clustered

together.

*

Streptococci:

a

number

ofcellsarranged

ina

chain.

Somebacteria

aremotile.

They move

using

flagellac-long,

hair-like

appendages growing

outof the

cell. Some

rod-shaped

bacteria

may

formspores

whenthe

cells

are

facedwith

adverse

conditions

such

as

high

temperature.

Once

suitableconditions

are

re-established

the spores

germinate

to

formnewcells.

19


29/123

Close

examination

of

the

simplecell revealsthat

it iscomposed

of thefollowing

components

(Figure

8):

* cell wall

which

gives thecell

its shape and

retains

the

constituents

*

cell

membrane

for filtering

in foodconstituents

anddischarging

waste products

* nucleus

where

the

geneticmaterial

of

the

cell

is

stored.

The

cytoplasm

is a

semi-liquid

proteinaccous

substance which

contains

starch,

fat

andenzymes.

Figure

8. Schematic illustration

of

bacterial structure.

Nucleus

Cytoplasm

Cytoplasmic

membrane

Cell wall

-Capsule

The

cell

membrane is

semi-permeable

andallows the

cell to

feed

by osmosis,

i.e.

the

exchange of

nutrientsbetweenthe

cytoplasm

ofalivingcell and

thesurrounding

aqueous

material.Only small molecules

can

pass

in

and out of

the

cell, e.g. with

asugar

solution

on

one

side

of a

semi-pemeable

membrane and

water

onthe

other,water

will

diffuse

in,

diluting

the

sugar

solution. The

sugarmolecules

cannot

passout

so

a hydrostatic

pressure,

knownas

osmoticpressure, develops.

Bacteria

feedby

selective intake

of

nutrients

dissolved

in water.They

can also

take in

nutrientsagainst

the

normal

osmotic

flow,

aprocess called active transport.

5.1.1 Bacterial

growth

Bacterial

growth

refers to

an increase

in

cell numbers

rather

than an

increase

in cell size. The

process

by

which

bacterial

cells

di,,

ide

to reproduce

themselves

is

known

as binary

transverse fission. The

time

taken

from

cell formation

tocell

division

is

calledthe generation

time

whichcan

he definedas the

time taken for

the cell

count todouble.

Figure

9 shows the

phases

of bacterial

growth following

inoculation

of

bacteria into

a new growth

medium.

20


30/123

Figure

9. e fourphases of

bacterial

growth.

Number

ofbacteria

(log)

d

a

b

a

Lagphase c

Stationaryphase

bLogphase d

Death

phase

Thefollowing

phases can

be

identified:

I. Lag phase: There isusually some

delay ingrowthafter

inoculation of

bacteria

intoanew

medium.During

this time

the bacteria adapt to the medium

and enzymes neededynthesise '.he

to break down the

sub.t

,nces in

it.

2. Log phase: Or'-e the bacteria

haveadapted to thenew medium they

start toreproduce quicklyandtheir

numbers multiply

evenly

foreach increment oftime.Plotting

the

lognumber

ofcells

against

time

gives

a linearrelationship; this is

therefore

called

the

logphase.

The

cells

are

attheir greatest

activityin

this

phase. Transferring cultures to

a fresh medium

at

regular intervals

can maintain thecells in an active

state. An active culture

can

rapidly

dominate any

new

environment.

Thisphase can

be

prolonged

by

removing

toxicwaste,

addingmorenutrients

or

both.

3. Stationary phase:

As the

bacteria

dominate

the growth

medium

they deplete

theavailable nutrients

and

toxicwaste productsaccumulate,

slowing the rateofreproduction.

Atthe

sametime,

cellsare dyingoff.

A state

of

equilibrium

is reached between the

death ofoldcells andformation

ofnewones resulting

in

no netchange

incell

numbers.

4. Death phase:

In

this

phase

the formation

of

new

cellsceasesand the existing cells

graduallydieoff.

5.1.2 Factorsaffecting

bacte..al growth

Bacterial growth

isaffected

by temperature,

nutrient

availability,

water

supply,

oxygen supply,and acidity

of

the

medium.

Temperature

Theoretically,bacteria

can

grow

atall

temperaturesbetween

thefreezing

pointofwater

and

the

temperature

at which

protein or protoplasm

coagulates.

Somewhere

between

these

maximumand

minimum

points

lies

theoptimum

temperature at

which

the bacteria

grov best.

Temperatures

below

the minimum

stopbacterial

growth

butdo not

killthe

organism,

however, ifthe

temperature

is

raised above

the

maximum,

bacteria

are

soon

killed.M ostcells

die

after

exposure

to

heat

treatments of70'C

for

15 seconds,

although spore-forimngorganisms require moresevere

heattreatment,

e.g.

livesteamat 120'C

for30 minutes.

Bacteria

can be

classified

according

to

temperature

preference. Psychrotrophic

bacteria

grow

at

temperatures

below 16"C, mesophilic

bacteria

grow

best

at

temperatures

between

16 and

40"C,

and

thermophilic bacteriagrow

bestat

temperatures

above

40'C.

21


31/123

Nutrients

Bacteria

neednutrients

fortheirgrowth

and

some

need

more nutrients

thanothers.

Lactobacilli live

in milk

andhave losttheir

abilitytosynthesise

many

compounds,

while

Pseudomonas

can

synthesisenutrients

from

verybasicingredients.

Bacteria normally feed on organic

matter

which

contains

both material

for

cell

formation

and

the

necessary

energy.

The

organic

mattermust

be soluble

inwater

andof

low molecular

weight to

be able to

pass

through

the

cell

membrane.

Bacteria

therefore need

waterto

transport

nutrients

intothe

cell.

If

the nutrient

material

isnotsufficiently

broken

down, the

micro-organismcan

produceexo-enzymes

which

split the nutrients

into smaller,

simpler

components

so

they can

enter

the

cell.

Inside the cell

the

nutrients

arebroken

down further

byother

enzymes,

releasing

energy

which

isusedby the

cell.

Water

Bactcria

cannot

grow

without

water.

Many

bacteriaarequickly

killed

bydryconditions,

although

otherscan

tolerate

such conditions

for

months;

bacterial

spores can survive

dry conditions

for

years.

Water

activity

(Aw) isusedas

an

indicator

of

theavailability o.

water

for

bacterial

growth.

Distilledwater

has

anAwof

1.

Addition

of

solute, e.g.salt reduces the

availabilityof

water tothe cell

and the Aw

drops;

at Aw less

than

0.8

cell

growthisreduced.

Cells

that

cangrowat low

Aw

arecalledosmophiles.

Oxygen

Animals

require oxygen

tosurvivebut

bacteriadiffer in

theirrequirements

forand

in theirability

to

utilise

oxygen.

Aerobic

bacteria

need

oxygen

for growth,

however,

it is

toxic to

anaerobic

bacteria.

Anaerobic

organisms

are responsilie

for

reactionssuch

as methane

production

inbiogasplants

and

spoilage

incanned

foods

and cheeses.

Some bacteria

can

live

either

with or

without

oxygen

and

are

known

as facultative

anaerobic

bacteria.

Acidity

Theacidity

of

a nutrient

substrate

ismost

simply expressed

as

itspH

value. Sensitivity

to

pH varies

from

onespecies

of

bacteria

toanother.

The terms

pHoptimum

and

pH

maximum

are used.

Mostbacteria

prefer

a

growthenvironment

witha pH

of

about

7,

i.e.

neutrality.

Bacteria

that

can

tolerate

low

pHare referred to

as aciduric.

Lacticacidbacteria

in

milk produce

acid

and

continue

todo

sountilthe

pHofthemilk

falls below4.6,

at which

pointthey

gradually

die

off.

5.1.3

Bacteria

in

milk

Milk

fresh from

ahealthycow

contains

few

bacteria,

butcontamination

during

hAndlingcan

rapidly

increase

bacterial

numbers.

Milk

isan ideal food

andmany

bacteria

growreadily

init.

Some

bacteria

(lactic

acidbacteria)

are

useful

inmilk

processing,causing

milk

to

sournaturally,

leading

to

fermented

products

suchasirgo.

However,

milk

canalsocontain

pathogenic

bacteria,

such

as

Salmonella,

Mycobacteriun

tuberculosis,

Listeria

and

Brucella,

andcan

thus

transmit

disease. Other

bacteria

cancause

spoilage

of

the

milk, andspoilage

and

pooryields

ofproducts.

5.2 Moulds

Moulds

are a heterogeneous

group

of multicelled

organisms which

reproduce

asexually

either

by spore

formation

or

by fragmentation.

They

can growon

.widevariety

ofsubstrates

and

aregenerally

regarded

as

spoilage

organisms. However,

moulds

are

used

inthe

productionof antibiotics

and

in

certain

cheese

varieties.

Moulds

are

aerobic

organisms

and

theirgrowth

onfoods

can

be retarded

by excluding

air

through

careful

packaging.They

can

be

killed

by

jelativelymildheattreatments,

butmould spores

aremoreresistant

toheat.

Thestructure

ofmoulds

isshown

inFigure

10.

22


32/123

Figure 10. Structure

of

moulds.

Aspergillus

Penicillium

5.3

Yeasts

Yeasts

are

unicellular

organisms

which

reproduce

asexually

by

budding. They

are used industrially

to

ferment carbohydrates

to

such

products

as

alcohol

and citric

acid.

Yeasts are not

usually

used

in

milk

processing

and

are

normally

regarded

as

spoilage

organisms

in

dairy products.

The structure

of

yeasts

is

shown

in

Figure

11.

Figure

11. Structure

ofa

yeast

cell.

Fatglobules

Nucleus

Cell wall

Cytoplasmic

Cytoplasm membrane

Vacuole

5.4Viruses

Viruses

are extremely

small

organisms

comprising

a spherical

head containing

thegenetic

material,

and

a

cylindrical tail.

They

must invade

other

cells to

reproduce.

Viruses

that

attackbacterial

cellsare

known

as

bacteriophages.

Bacteriophages

that

attackacid-producing

bacteria

inhibit acid

production

in milk

thereby

causing

problems

in the manufacture

offermented

milks,

yoghurtand

cheese.

23


33/123

5.5

Milk

microbiology

In

addition

to

b

Documents

Rural Dairy Technology