Microorgaisms cause changes in primary characteristics and properties of milk and milk products. The product defects depends on the specific species and number of microorganisms involved in pre and post technological processing. Mostly these changes are related to smell, flavor or consistency.
Milk is an excellent culture medium for many kinds of microorganisms, being high in moisture, nearly neutral in pH and rich in microbial foods.In raw milk at temperature 10 to 37,Streptococcus lactis is most likely tocause the souring with possibly some growth of coliform bacteria, enterococci, lactobacilli and micrococci. At higher temperature e.g from 37 to 50 C, S. thermophilus and S. faecalis may produce about 1 percent acid an be followed by lactobacilli such as Lactobacillus bulgaricus which will produce more acid. Some of the lactobacilli can grow at temperature above 50 C but produce less acid there. Thermophilic bacteria can grow at still higher temperatures e.g. L. thermophilus. Little formation of acid takes place in milk held at temperatures near freezing but proteolysis may take place.
Types of Spoilage
- Gas Production
Gas production by bacteria usually is accompanied by acid formation and with few exceptions is undesirable in milk and milk products. The chief gas formers are coliform bacteria, Clostridium spp, gas forming Bacillus species that yield both hydrogen and carbon dioxide and yeast, propionics and heterofermentative lactic that produce only carbon dioxide and the yeasts, propionics, heterofermantative lactic that produce only carbon dioxide.
The hydrolysis of milk proteins by microorganisms usually is accompanied by the production of a bitter flavor caused by some of the peptides released. Proteolysis is favored by storage at a low temperature by the destruction of lactic and other acid formers by heat and by destruction of formed acid in the milk by molds and film yeasts or the neutralization of acids by products of other organisms.The types of changes produced by proteolytic microorganisms include :
- Acid proteolysis in which acid production and proteolysis occur together. It may be caused by several species of Micrococcus.
- Proteolysis with little acidity or even with alkalinity.
- Sweet curdling, which is caused by rennin like enzymes of the bacteria at an early stage of proteolysis
- Slow proteolysis by intracellular enzymes of bacteria after their autolysis.
- Residual proteolytic activity of heat stable proteinase.
Ropiness and sliminess can occur in milk, cream or whey but are important mostly in market milk and cream. Nonbacterial ropiness or sliminess may be due to:
- Stringiness caused by mastitis and in particular by fibrin and leukocytes from the cow’s blood.
- Sliminess resulting from the thickness of cream e.g. at the top of a bottle.
- Stringiness due to thin films of casein or lactalbumin during cooling, as sometimes is observed on surface coolers. This effect is only temporary.
Ropiness throughout the milk may be caused by any of the following kinds of baterias:
- Enterobacter aerogenes, E cloacae, Klebsiella and rarely Escherichia coli. Ropiness caused by Enterobacter usually is worse near the top of the milk.
- Certain strains of lactic acid bacteria. Lactobacillus casei, L. bulgaricus , and l. plantarum may also produce ropiness. Most of these lactic bacteria can grow in long chains, characteristic that supposedly contributes to the stringy condition of the milk.
- Miscellaneous other bacteria among the alkali formers, micrococci, streptococci and bacilli. Ordinarily these bacteria would be suppressed by acid formers.
Since the sources of the bacteria causing ropiness are water, manure, utensils and feed, the reduction or elimination of contamination from these sources helps prevent ropiness. Adequate pasteurizationof milk readily destroys most of these kinds of bacteria.
Spoilage of Milk Products
Frozen desserts include ice cream, ice milk, frozen custards, sherbets etc. The ingredients may be various combinations of milk, cream, evaporated milk, condensed milk, dried milk, flavors, sweetening agents. Any of these may contribute microorganisms to the product and affect the quality of the dessert as judged by its bacterial content or its content of specific kinds of bacteria such as the coliforms. The desserts are not ordinarily subject to spoilage however as long as they are kept frozen. The only important types of spoilage take place in the ingredients before it is frozen no spoilage problems should result unless it is held at temperature above freezing for a considerable time, when souring by acid forming bacteria can take place.
Many of the defects of butter originate in the cream from which it is made especially when the cream has been held for several days on the farm before collection by the creamery. During this time lactic acid bacteria, gas formers and other spoilage organisms may grow and be followed by molds e.g Geotrichum candidum. Lactose fermenting occasionally, yeasts, which are present only occasionally may develop high gas pressures in the can of cream.