Spoilage Of Milk c4n6z

Milk & Milk products Spoilage Milk:

Unique flavor and texture Rejected if off flavor or other defects Spoilage occur by Biochemical reactions of contaminating bacteria (initiated after milking)

Manifestations of spoilage Lactic

acid production/ souring Proteolysis Lipolysis Sweet curdling

Spoilage Process  Fermentation-milk

constituents by

microbes  Normal

fermentation: curdling  Abnormal fermentations: Gassiness, ropiness, proteolysis, sweet curdling, lipolysis  Mixed

fermentation: two or more fermentations occur simultaneously e.g. acid and gas (coliforms)

Associative action Combined

action (two or more-species

or genera) Desirable or undesirable Changes not possible by single microbe Three types Synergism Metabiosis Antibiosis.

Synergism  Changes

brought by two microbes (not single one)  Mixed starter (Streptococcus lactis & Leuconostoc spp.)  Leuconostocs convert citrate to volatile compounds (only at low pH)& produce flavour  Lowering of pH due to lactic acid production S. lactis.

Synergism  Blue

discoloration: Pseudomonas syncyanea only in association with S. lactis  Lactic acid bacteria required for 'yeasty creamy' defect (Candida pseudotropicalis, Torulopsis sphaerica) in cream  Coagulation of milk & foaming for subsequent gas production by yeasts

Metabiosis Food

chain is formed

Metabolic

end products of one are utilized as food by other for producing final change Swiss cheese: lactose to lactic acid (bacteria), utilized by propionibacteria to produce propionic acid (flavour)

Metabiosis  Spoilage

of Raw milk at room temp. Curdling of milk by S. lactis (precipitation of casein) up to 1 % acidity Lactobacilli (L. casei) convert rest of lactose to lactic acid 2% lactic acid  Molds (Geotrichum candidum) growth on surface and reduce acidity by oxidizing lactic acid to C02 and H20.

Cont.. Reduced

acidity, proteolytic spore formers (Bacillus spp.) degrade casein fraction Sub-sequently lipolytic bacteria develop and utilize fat fraction Decomposed mass-water, inorganic substances, C02, NH3, H2S etc.

Antibiosis  One

organism inhibits/suppresses growth of the others  Lactic acid -bacteria causes the inhibition of proteolytic organisms (spore formers)  Starter cultures do not propagate well in reconstituted milk-certain preformed substances inhibitory to starter bacteria are elaborated in milk and get carried over to the product during subsequent drying

Natural souring/curdling Raw milk held at ambient conditions Immediate effect is souring followed by curdling (due to acidity-lactic acid) by bacteria already present in raw milk Fresh milk normal acidity (0.14 to 0.19%) 

Cont..  Milk

sours (0.20 to 0.25%)  Milk curdles (0.50-0.65%)  COB test positive (0.30 to 0.45%)  Acidity increase even after coagulation of casein till lactic acid producing flora inhibited or till whole of lactose is exhausted (acid tolerant organisms predominate)

Acid coagulation  Interaction

of lactic acid with calcium bound to casein-precipitation of casein-curd (pH range 4.64 to 4.78)  Lactic streptococci - S. lactis, S. cremoris (room temperature) Gets inhibited at 1%  Lactobacilli- L. casei (at room temperature), L. acidophilus and L. bulgaricus (optimally at around 40°C). Gets inhibited beyond 2% level of lactic acid

Cont..  Leuconostocs.

Leuco. dextranicum and Leuco. citrovorum-responsible for flavour development and lower level of lactic acid

Other streptococci  S.

thermophilus at around 45 C though it is produced slowly even at lower temperature range. This organism is also capable of surviving higher heat treatments such as pasteurization.  S. liquefaciens at about 31C. Milk is rapidly coagulated followed by proteolysis (causes the curd to shrink from the walls of the container and separation of whey).

Cont..  Bacillus

coagulalls -aerobic spore formersurvive heating and multiply and produce lactic acid at 31to 55°C.  Coliforms- E. coli and Enterobacter aerogenes Produces acid & gas (37°C)  The coagulum formed by lactic streptococci, S. themrophilus and lactobacilli is smooth and with typical clean sour flavour (used as starter culture for desirable fermentations)

Cont..  S.

liquefacienls, B. coagulans and coliforms produce a coagulum with undesirable flavours due to liberation of certain volatile flavour substances from lactose, proteins and milk  Organisms present in raw milk varies and produce coagulum which varies with the composition of the causative microflora.

Sources of flora  At

the farm level from  Utensils  Coat of the cow  Feed (silage, grains etc.)  Faecal matter  Environment

Malpractices  Neutralization

with caustic soda to mask the developed acidity and escape the rejection of milk

Control measures  Practiced

under hygienic conditions to minimize entry of acid-producing organisms  Immediate chilling of raw milk  Clean/ Sanitized Utensils and equipments involved in milk production, collection and transportation s  Holding of milk needs to be minimized  Adequate pasteurization of milk followed by cooling  Household levels heated immediately and stored in refrigerator.

Gas production  Cream:

production of gas (mainly CO2) by micro-organisms responsible for defect called 'gassiness‘  Foaming as the gas escapes the partially coagulated mass-defect frothiness  Frothiness-associative action of acid producing bacteria & gas producing yeasts.  Gassy cream- accompanied by yeasty odour called yeasty cream

Cont..  Gas

production in canned dairy products-bulging of cans/blowing of cans  Lactose fermenting yeasts-Candida pseudotropicalis, Torulopsis sphaerica produce C02 and ethyl alcohol in milk, cream, whey at or below 31C  Acid tolerant-grow under acidic conditions (as in sour cream)  Coliforms: Escherichia coli, Enterobacter aerogenes ferment lactose into gas and acid  Gas escapes before coagulation of casein-no frothiness unlike that in yeast fermentation.

 Anaerobic

spore forming bacteria Cl. butyricum, Cl. pasteurianum Cl. Sporogenes-produce gas anaerobic

conditions  Grow

well if acid producers are destroyed by heating and the product has anaerobic atmosphere e.g. in canned dairy products

Sources of gas producing organism Soil,

manure, feed or utensils Coliforms mainly associated with faecal contamination Clostridia through feed (silage) and manure

Control measures      

Avoid excessive contamination of milk/cream from potent sources at the farm level Milch animal be cleaned/washed before milking Feed or manure particles should not be allowed to fall in milk Milk - utensils should be cleaned properly Holding of milk and cream under ambient conditions should be minimized The heat treatment given to the product should be adequate (kill heat-sensitive gas producers-coliforms)

Ropiness/sliminess  Growth

of bacteria leading to change in consistency that forms threads or viscous masses when poured  Ropy material may be tough and doughy due to acid-producing bacteria. Causative organisms

Lipolytic/ Hydrolytic rancidity Rancidity from hydrolysis of milkfat Caused by enzyme lipoprotein lipase (LPL) Flavour-short chain fatty acidssuch as

butyric acid LPL indigenous or bacterial Active at the fat/water interface (ineffective unless the fat globule membrane is damagedagitation, foaming, and pumping.

•Homogenized milk-rapid lipolysis unless lipase is destroyed by heating (enzyme denatured at 55-60˚ C) Homogenize milk immediately before or after pasteurization and avoid mixing new milk because it leads to rapid rancidity.

•Some cows can produce spontaneous lipolysis from reacting to something indigenous to the milk. Late lactation, mastitis, hay and grain ration diets (more so than fresh forage or silage), and low yielding cows are more suseptible.

Lipolysis can be detected by measuring the acid degree value which determines the presence of free fatty acids.