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MILK, FERMENTATION, ANDFERMENTED AND NON-FERMENTEDDAIRY PRODUCTS
Ch Chapter 7
Food Microbiology
By
1
ByM. Majdinasab
FERMENTATION Numerous food products owe their production p p
and characteristics to the fermentative activities of microorganisms.Ri d h i kl k t d f t d Ripened cheeses, pickles, sauerkraut, and fermented sausages.
Due to fermentation: Shelf life is extended
A d fl i i d Aroma and flavor is increased vitamin content of the fermented food is increased Digestibility of the raw materials is increasedg y Increase or decrease the toxicity of some foods 2
THE LACTIC ACID BACTERIA This group is composed of 13 genera of Gram-g p p g
positive bacteria at this time:
Carnobacterium Oenococcus Enterococcus Pediococcus
L t P l t b ill Lactococcus Paralactobacillus Lactobacillus Streptococcus Lactosphaera Tetragenococcus Lactosphaera Tetragenococcus Leuconostoc Vagococcus Weissella
3
Generalized pathways for the production of some fermentation products from glucose by various organisms. (A) Homofermentative lactics; (B) heterofermentative lactics; (C) and (D) Propionibacterium; (E) Saccharomyces spp.; (F) Acetobacter spp.; and (G) Acetobacter overoxidizers.cetobacte ove oxidize s.
CO2
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DAIRY PRODUCTS Milk Compositionp
Water 87.0Average Chemical Composition (%) of Whole Bovine Milk
Protein 3.5Fat 3.9Carbohydrate 4 9
The higher structural protein content of red meats enables
Carbohydrate 4.9Ash 0.7
The higher structural protein content of red meats enables these products to exist as solids.
Water content near the surface of fresh meats of ca. 75.5% is lower than the average of 87% for milkis lower than the average of 87% for milk
The aw of both products is near 1.05
CONTI B vitamins with pantothenic acid and riboflavin p Vitamins A and D are added for human
consumption
It is an ideal growth medium for heterotrophic microorganisms, including the nutritionally fastidious
Gram positi e lactic acid bacteriaGram-positive lactic acid bacteria.
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CONTI The pH of fresh whole milk is around 6.6 but it may reach
ca. 6.8 from a cow that has mastitis.
Mastitis is most often caused by: Mastitis is most often caused by: Streptococcus agalactiae S. uberis sometimes by: Staphylococcus aureus Streptococcus dysgalactiaep y g
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PROCESSING Milk is processed in a number of ways to produce a p y p
variety of products such as cream, cheese, and butter. Skim milk (0.5% fat)
R d d f t ilk ( t 2 0% f t) Reduced fat milk (up to 2.0% fat) Evaporated milk: removal of about 60% water from
whole milk. lactose content about 11.5%. Sweetened condensed milk: addition of sucrose or
glucose before evaporation. sugar content about 54% or >64% in solution.or 64% in solution.
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CONTI In the United States, grade A raw milk that is to be
pasteurized should not have an: APC that exceeds 300,000 cfu/ml for blended milk should not exceed 100,000/ml for milk from an individual should not exceed 100,000/ml for milk from an individual
producer. After pasteurization, the APC should not exceed 20,000
cfu/ml and the coliform count should not exceed 10/mlcfu/ml, and the coliform count should not exceed 10/ml.
Raw milk should not be held longer than 5 days at 4.4C.
Chocolate milk is processed at a slightly higher temperature than unflavored milk (75C for 15 sec rather than 72C). 9
PASTEURIZATION Destruction of all disease-causing microorganismsg g Increase shelf life Has no effect on nutritional value
Endospores of pathogens such as: Cl t idi b t li Clostridium botulinum
and spoilage organisms such as: Clostridium tyrobutyricum C. Sporogenes Bacillus cereus are not destroyedare not destroyed. 10
CONTI Low temperature-long time (LTLT): 63C, 30 minp g ( ) , High temperature-short time (HTST): 72C, 15 sec
The basis for the heating time and temperature is the thermal death time (TDT) of the most heat-resistant non-spore-forming milk-borne pathogens.
Prior to 1950:TDT of Mycobacterium tuberculosis After the discovery of the Q fever agenty Q gTDT of Coxiella burnetti
I l t i d ilk th t ll i In properly pasteurized milk, the naturally occurring enzyme alkaline phosphatase is destroyed
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STERILIZATION UHT (ultra-high temperature):( g p ) Destroys non-sporeforming pathogens in milk, but in
addition some sporeformers are severally reduced in numbersnumbers.
135140C for a few sec (the minimum treatment is 130 C for 1 sec)
UHT-treated milk is commercially sterile with a shelf life of 4045 days at 40F when aseptically packaged in sterile containers.
UHT-treated whole milk is said to be more flavorful, due apparently to formation of some Maillard products.
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CONTICONTI Although pasteurized milk is free of non-sporeforming
pathogens, it is not sterile. pathogens, it is not sterile.
The efficacy of either LTLT or HTST to destroy the mycobacterial subspecies that is associated with:mycobacterial subspecies that is associated with:
Crohns disease
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CONTI Most if not all Gram-negative bacteria (especially g ( p y
psychrotrophs) are destroyed along with many Gram positives.
Thermoduric Gram positives belonging to the genera Enterococcus, Streptococcus (especially Streptococcus
li i b th hil ) Mi b t i salivarius subsp. thermophilus), Microbacterium, Lactobacillus, Mycobacterium, Corynebacterium, and most if not all sporeformers survive.
Among the survivors are a number of psychrotrophic species of the genus Bacillusspecies of the genus Bacillus
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GENERAL MICROBIOTA OF MILK Milk constitutes an excellent medium for the growth of
microorganisms. Freshly drawn milk from healthy animals contains a small
number of harmless microorganisms.
Nearly all the changes that take place in the flavor and appearance of the milk after it is drawn form the cow are appearance of the milk after it is drawn form the cow, are the result of the activities of microorganisms.
Th f it i ti l t t l th Therefore, it is very essential to control these microorganisms
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MILK-BORNE PATHOGENS Some of the most obvious are the animal diseases
below to which humans are susceptible and which may occur in milk of cows:
Brucellosis Anthrax Tuberculosis Listeriosis Salmonellosis Q fever Campylobacteriosis Crohns disease (?)
E t h h i liti St h /St M titi Enterohemorrhagic colitis Staph./Strep. Mastitis
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SPOILAGE As the only natural source of the disaccharide lactose, y ,
milk undergoes microbial spoilage in a way that is unique.
Lactic acid bacteria (LAB) Lactic acid bacteria (LAB) The coliform bacteria are the most conspicuous utilizers
of lactose among Gram-negative bacteria.
Thus:The bacterial spoilage of either ra or pasteuri ed The bacterial spoilage of either raw or pasteurized milk is conspicuous by the production of lactic acid
by lactose users17
CONTI The thermoduric Streptococcus salivarius subsp. p p
thermophilus strains preferentially use the glucose moiety of lactose.
The spoilage of UHT milk is caused by Bacillus spp that The spoilage of UHT milk is caused by Bacillus spp. that survive the UHT process.
B B. cereus B. licheniformis B. badius
Anaerobic spores appear not to be a problem
B. sporothermodurans Paenibacillus spp.
because of the relatively high Eh of milk
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CONTI During cold storage of pasteurized milk: Psychrotrophic Bacillus weihenstephanensis sweet curdling Production of proteases and peptidasesp p p
The spoilage of UHT-treated milk: Heat resistant proteases & lipases that are produced by Heat-resistant proteases & lipases that are produced by
some psychrotrophs in raw milk
Ropiness in raw milk Ropiness in raw milk Alcaligenes viscolactis low-temperature maintenance of raw milk for several days
St i i t Stringy consistency 19
CONTI Souring Lactose fermentation LAB Souring & gassiness coliforms acid & gas Aroma production starter culture diacetyl Proteolysis unpleasant odors, undesirable. controlledy p ,
desirable cheese production. Ropiness- Milk drawn into long threads
(Alkaligenes viscolactis) ( g ) Sweet curdling- due to production of rennin like
enzyme which curdles without souring Stormy fermentation- Rapid fermentation by y p y
Clostridium perfringens Color changes P. syncyanea (blue); P. synxantha (yellow);
Serratia marcescens (red)( )20
CONTI Lactococci L. delbrueckii sub sps. lactis (Str.Lactis) L. Lactis sub sps. Cremoris (Str. Cremoris) Lactobacilli L. Casei L. delbrueckii sub sps. lactis (L. Lactis). L. delbrueckii sub sps. bulgaricus
(L bulgaricus)(L. bulgaricus) Leuconostoc
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CONTIColiform: Facultative anaerobes Optimum growth @ 37 C Indicator organism and are closely associated the Indicator organism and are closely associated the
presence of pathogens but not necessary pathogens themselves.
They ferment lactose with production of acid and They ferment lactose with production of acid and gas
Cause rapid spoilage of milk Th kill d b i i They are killed by pasteurization
Their presence is an indication of PP contamination of milk
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RESULT OF MICROBIAL GROWTH IN MILK(SPOILAGE)o Principal cause are Psychrotrophsp y po Most of these are destroyed by pasteurizationo Some may survive e.g. Pseudomonas fluorescens,
P d f giPseudomonas fragio Other species and strains that survive pasteurization
and grow at refrigeration tem. produce heat stable proteolytic & lipolytic enzymes and cause spoilage:
o Bacillus ClostridiumCornebacterium Arthrobactero Cornebacterium Arthrobacter
o Lactobacillus Microbacteriumo Micococcus Streptococcus
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Stages of milk decayStages of milk decayRancid
(on the turn)-Milk consumableCurdlingg
Separation of curd and whey milk still consumable
Coagulationperiod of aromatic decay with mould
growth milk beyond usegrowth milk beyond useDry
d h d ti h d d h lk dehydration ensues- hard and chalky milk beyond use
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MICROBIAL PATHOGENS
Bacillus cereus Listeria monocytogens Yersinea entrocolitica Salmonella spp. E.coli O157:H7 Compylobacter jejuni Coxeilla burnetii Mold (Aspergill s F sari Pe icilli ) g o Molds(Aspergillus, Fusarium, Penicillium) grow
in milk and milk products & produce potentially hazardous mycotoxins.
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STARTER CULTURES, FERMENTEDPRODUCTS
Dairy products require the use of an appropriate y p q pp pstarter culture
Lactic starters are also used for preparing butter, cultured buttermilk cottage cheese and cultured sour cultured buttermilk, cottage cheese, and cultured sour cream and are often referred to by product.
Lactic starters always include bacteria that convert lactose to lactic acid:
L. lactis subsp. LactisL. lactis subsp. Lactis L. lactis subsp. Cremoris L. lactis subsp. lactis biovar diacetylactis
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How Does Milk Turn Into Yogurt?
LactoseLactic acid Bacteria-------------------------> Lactic Acid
(Milk sugar)
Acid causes casein (milk t i ) t d t d
pH: 3.654.40
protein) to denature and hold water into a semi-solid
gel = yogurtgel yogurt
YOGURT (YOGHURT) Starter: A mixed culture of S. salivarius subsp. thermophilus
& Lactobacillus delbrueckii subsp. bulgaricus in a 1:1 ratioratio.
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Commercial YogurtContains 2 species of bacteria specialized to grow well in milk (but cant surviveContains 2 species of bacteria specialized to grow well in milk (but cant survive inside the human body):
First, Next,Streptococcus thermophilus is more active, then slows down when
Next, Lactobacillus bulgaricus is more acid tolerant and
slows down when acidity reaches 0.5%
takes over until acidity >1%
These bacteria work in symbiosis. Each bacterium stimulates the growth of the other => acidifies the milk more rapidly than either partner on its own.
Acetaldehyde: the chief volatile flavor component of yogurt
CONTI Freshly produced yogurt typically contains around y p y g yp y
109 organisms/g During storage, numbers may decrease to 106/g,
especially when stored at 5C for up to 60 daysespecially when stored at 5 C for up to 60 days. The International Dairy Federation norm for yogurt
is 107/g or above. The antimicrobial qualities of yogurt, buttermilk, sour
cream, and cottage cheese have been examined by inoculating Enterobacter aerogenes and Escherichia coli.
A sharp decline of both coliforms was noted in yogurt and buttermilk after 24 hours.
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KEFIR Kefir is prepared by the use of kefir grains.f p p y gBacterial species of the genera Acetobacter Lactobacillus Lactococcus Leuconostoc Leuconostocyeast species of the genera Candida Kluyveromyces Saccharomyces
31These symbionts are held together by coagulated protein
CONTI The important Lactobacillus spp. in kefir are:p pp f L. kefiri L. parakefiri L. kefiranofaciens subsp. kefiranofaciens L. kefiranofaciens subsp. kefirgranum
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BUTTERComposition:p 15% water 81% fat Less than 0.5% carbohydrate and protein
Although it is not a highly perishable product it does Although it is not a highly perishable product, it does undergo spoilage by bacteria and molds.
The main source of microorganisms
Salted butter may contain up to 2% salt
The main source of microorganisms for butter is cream
Salted butter may contain up to 2% salt 33
CONTI Bacteria cause two principal types of spoilage in p p yp p g
butter:1. Surface taint or putridity: Pseudomonas
putrefaciensputrefaciens 47C within 710 days The odor of this condition is apparently due to
certain organic acids, especially isovaleric acid.
Surface taint along with an apple odor is caused Surface taint along with an apple odor is caused also by Chryseobacterium joostei.
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CONTI2. Rancidityy hydrolysis of butterfat with the liberation of free
fatty acids.
Th ti i i P d f i
Lipase
The causative organism is Pseudomonas fragi, although P. fluorescens is sometimes found.
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CONTI3. Malty flavor : Lactococcus lactis var. maltigenes. y g
4. Skunklike odor: Pseudomonas mephitica
5. Black discolorations: P. nigrifaciens
Fungal spoilage of butter:Cladosporium AlternariaAspergillus MucorRhizopus PenicilliumG t i h i ll G didGeotrichum, especially G. candidum 36
CONTI Discolorations on butter: Black yeasts of the y
genus Torula
The generally high lipid content and low water content make butter more susceptible to spoilage by molds than by bacteria.
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CHEESE Cheeses result from a lactic fermentation of milk. Milk is prepared and inoculated with an appropriate
lactic starter.Th t t f h d ti The starter for cheese production:
S. salivarius subsp. thermophilus is employed for acid production in cooked curds (up to 60C)
A combination of S. salivarius subsp. thermophilus &L lactis subsp lactis for curds that receive an L. lactis subsp. lactis for curds that receive an intermediate cook
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CONTI The low moisture content of hard and semihard
ripened cheeses makes them insusceptible to spoilage by most organisms Molds
Some ripened cheeses have sufficiently low oxidation-reduction potentials to support the growth of anaerobes.
Anaerobic bacteria sometimes cause the spoilage of these Anaerobic bacteria sometimes cause the spoilage of these products when aw permits growth to occur.
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CONTI Gassiness of cheeses Clostridium spp., especially C. pasteurianum C. butyricum C. sporogenes C. tyrobutyricum
C. tyrobutyricum is well established as the cause of a butyric acid fermentation or the late-blowing defect in hcheeses.
An aerobic sporeformer, Paenibacillus polymyxa: Gassiness
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DISEASES CAUSED BY LACTIC ACID BACTERIA Although the beneficial aspects of the lactic acid g p
bacteria to human and animal health are unquestioned, some of these bacteria are associated with human illness.
Lactobacilli Several species of leuconostocs
P di i Pediococci Enterococci: nosocomial (hospital acquired) infections, with
E. faecalis and E. faecium
LAB are opportunists that are not capable of initiating infection in normal healthy individuals.y
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SOME FERMENTED MILK PRODUCTS
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