Microbial spoilage in foods

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MICROBIAL SPOILAGE

•Principles•Spoilage of Meat & its products Cereal & Bakery products Milk & its products

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PRINCIPLES OF SPOILAGE• Quality of the edibility becomes reduced.• Due to one or more of : (1) MO : onion-black mold rot- A.niger (2) Insect damage : eat, leave wastes (3) Enzymes : browning of apples-PPO (4) Chemical reactions by light, oxygen,etc : rancidity (5) Physical injury by drying, burning, freezing• Factors affecting the development of MO in foods : (1) Intrinsic (2) extrinsic (3) Processing : irradiation, slicing,pasteurization

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1. Meats 75%water,18% protein, 3% fat, 1%minerals, CHO

Aerobic condition Aerobic bacteria 1. Surface slime2. Change in colour of meat

pigments3. Fat decomposition4. Colour changes5. Off-odors % tastes

Yeasts 1. Sliminess2. Lipolysis3. Discolourations4. Off-odors & taste

Molds 1. Stickiness2. Whiskers3. Disolourations4. Off-odours & tastes

Anaerobic condition Anaerobic bacteria 1. Souring putrefaction2. taints

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DIFFERENT KINDS OF MEATS1) Fresh productsa. Fresh beef :1. Colour changes2. Sliminess3. Stickiness4. Whiskiness5. souring

b. Fresh Ham Sourness

c. Fresh pork Sausage1. Souring2. Sliming3. colouring

2) Dried productsa. Dried beef1. Spongy2. Gas

b. Sausages1. Sliming2. Greening

3) Refrigerated packaged meats

1. Off-flavours2. Slime3. Putrefaction

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Pseudomonas Acanthophylia syncynea deshayesiana

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2. Cereal & bakery products

Spoilage occurs in1. Bread2. Cake & other bakery pdts3. Macroni & tapioca4. Breakfast cereals & other cereal snacks

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1. BreadA. Mold • Killed during baking• Come from air during cooling, handling and

wrapping1. Rhizopous solonifer - Bread mold 2. P . Expansum - Green spored 3. A.niger - greeninsh/purplish brown 4. Monilia - pink

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Aspergillus on bread crust

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P.expansum R.solonifer, Aspergillus

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B. Rope : • B. subtilis, B. licheniforms – withstand upto

100°C and under favourable conditions • Characterisitics : yellow to brown colour, softy

and sticky. When pulled apart, slimy material can be drawn into long threads

C. Red Bread : • Serratia marscens • Monilia -> pink to red bread• Geotrichum aurantiacum -> red crumb

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Red bread by Serratia marscens

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2. Cakes & cereal pdts• Cakes -> molds• Other bakery pdts : obaking -> no m.oo Frustings, fillied with custard, creamsoNo spoilage due to higher sugar contentsoUpon storage due to molds or yeasts

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3.Macroni & tapiocaa) Macroni• Moist macroni : swelling -> gas production

by Enterobacter colacea • Drying on paper : purple streaks -> Moniliab) Tapioca • If moistened -> orange pigmented bacteria

-> starch hydrolysing

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3. Dairy products Milk : white emulsion containing protein;CHO : lactose,

glucose, galactose;Ca, K,Mg,Na,Cl; Vit A, B6,b12, C,D,E,K, thiamine,biotin,riboflavin,folates -> food for m.o.

Dairy products include :• Milk and cream• Butter• Cheese• Fermented milk• Condensed & dried milk• Frozen deserts Spoilage occurs if methods of preservation are inadequate

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SPOILAGE

• Gas production• Proteolysis• Ropiness• Changes in milk fat • Alkali production• Flavour changes• Colour changes

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1. Flavor changesa. Sour or acid flavour :• Clean, aromatic (desired) and sharp (undesired)• Clean: Streptococcus lactis & other lactis• Aromatic: S.lactis and Leuconostoc sp• Sharp: Clostridium sp (volatile FA

formic,acetic,butyric)b. Bitter flavours:• Proteolysis,Coliform ,asporogeneous yeasts• Some cocci – very bitter• Actinomyces- bitter musty

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c. Burnt or caramel flavour :• Resembling cooked flavour of over heated milk• Caused by Streptococcus lactis

2. Colour changesa. Blue milk: • Pseudomonas syncyanea – blusin grey to brown

– pure culture• With S.lactis- deep blueb. Yellow milk:• Pseudomonas synxantha – in cream layer of milk• Flavobacterium

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c. Red milk:• Brevibacterium erythrogenes • Micrococcus roseus – sour milkd. Brown milk:• Pseudomonas putrefaciens• P.fluorescens – enzymatic oxidation of tyrosine

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3. Ropiness

Nonbacterial Bacterial

1. Nonbacterial• Stringiness -mastitis-fibrin,leucocytes

- thin films of casein or lactalbumin• Sliminess from thickness of cream2. Bacterial • Surface : Alcaligens viscolactis• Throughout : -Enterobacter aerogens, E.cloacea

-LAB -> S.lactis, L.bulgaricus -Micrococcus

* Source of ropiness bacteria wre water,manure,utensils. Adequate pasteurization destroys these bacteria

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4. Proreolysis Hydrolysis of milk proteins- peptides- bitter flavour -

> at low temperature• Spore forming : bacillus and Clostridium• Non-spore forming : Alcaligens, Pseudomonas,

Proteus, Flavobacterium

5. Gas productionGas production accompanied by acid formation• H₂, CO₂ Costridium sp& Bacillus sp• CO₂ yeats, heterofermentive lactisAt or above pasteurization temp, acid formers are

killed. Only spore formers remain

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5. Alkali production• Ammonia – (non protein source)-> urea, carbohydrates,

organic acids such as citric acid• Can survive pasteurization• Pseudomonas fluroscens, Alcaligens viscolactis

6. Changes in fat• [o] of c=c FA aldehydes,ketoneztallowy odors.

Favoured by meatls,sunlight,oxidizing m.o.• Hydrolysis of butterfat to FA & glycerol by lipase (milk

or m.o.Pseudomonas,Proteus,Alcaligens. Pseudomonas fragi and Staphylococcus aures survive pasteurization.)

• Combined [o] & hydrolysis

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MILK PRODUCTS1. Condensed milk :• Thermoduric bacteria – tolerate increased solutes2. Evaporated milk:• Selling in cans-Clostridium• Coagulation- B.subtilis, B.cereus3. Sweetened condensed milk:• Due to entry of m.o after heat treatment if air is present• Gas forming- sucrose fermenting yeasts• Thickening-Micrococci• “Buttons”-mold colonies-size depends on amt of air-

A.repens and Penicillum