Module 5. Microbes in spoilage of milk
Fermentation is a process in which a change is brought in milk as a result of the activity of one or more species of microbes. As, milk leaves the udder, it is normally good medium for the growth of microbes and, accordingly, undergoes a series of biochemical changes; the complete processes producing different changes constitute milk fermentation. Many of fermentation that occurs in milk also occur in various milk products.
Spoilage of milk and its products resulting from the growth of acid-producing fermentative bacteria occurs, when storage temperature is sufficiently high to outgrow psychrotrophs, or when product composition is inhibitory to Gram-negative aerobic micro-organisms. Fermented dairy foods though made using lactic acid bacteria can be spoiled by the growth of ‘wild’microbial strains that produce unwanted gas, off-flavours, or appearance defects. The other non-spore-forming bacteria responsible for fermentative spoilage of dairy products are mostly fromeither the lactic acid-producer or coliforms.
23.2 Fermentation of Milk
As said earlier also fermentation in generalis a chemical process where biochemical changes are brought about on an organic substrate like carbohydrate, fat or protein by the action of enzymes produced by natural or added microbes in milk
23.2.1 Normal fermentation cycles
The common changes thatoccurs in milk is development of lactic acid with resulting coagulation of casein and the process is designated lactic as souring of milk. Because of regularity with which it occurs under the usual conditions, it is called normal fermentation of milk.
If raw milk is kept under ambient environment, the immediate effect is souring followed by curdling. This is mainly due to the production of acid (usually lactic acid) by certain group of bacteria present in raw milk. Souring of milk is most commonly encountered spoilage under tropical climate. Holding of raw milk under ambient environment without refrigeration is therefore leads to spoilage. The increased acidity in raw milk, makes it unfit for subsequent processing into fluid market milk and other milk products. Sometimes to mask the developed acidity,milk producers neutralize the milk with caustic soda but this a malpractice and should be strictly avoided.
188.8.131.52 Mixed fermentations
Often two or more fermentations occur in milk simultaneously; for example production of acid may be accompanied by gas formation or a color change, development of ropiness may occur along with a change in flavor,etc.These mixed fermentations are due to the activity of more than one species; but some microbes even in pure culture, produce two or more biochemical changes in milk at a time. The Escherichia-Enterobacter group produces gas, off-flavors, and acid and ropiness; so that it should be considered under at least four fermentations. This group is an example where, there are many species that produce more than one type of change in milk.
The stages of milk fermentation resulting as a sequence of microbial growth are depicted as placed below:
Fig. 23.1 Fermentation cycles-stages
Phase from ‘A to B’ is lag phase during which the microbes will adjust to the new environment and will not multiply. Lactocccus spp. naturally present in raw milk, grows first and produce acid up to approximately 0.9% lactic acid (i.e. phase B to C ). Eventually, acid restricts the growth of Lactocccus spp. and then Lactobacillus spp. also normally present in milk becomes predominant (i.e. phase C to D). These Lactobacillus spp. are more acid tolerant than Lactocccus spp. and hence,are able to survive acidity already produced and grow slowly producing more acidity, up to 2.0 % lactic acid. The Lactobacillus spp. like Streptococcus spp. cannot survive under high acidic milk. At this step, the typical acid tolerant yeasts and moulds start growing (i.e. phase D to E) using acid and producing several of basic compounds that tend to neutralize the acidity. Geotricum candidum may grow on the surface of coagulated milk and oxidize lactic acid to H2O and CO2. A change from a very high acid product to one that is basic then takes place. During phase E to F, proteolytic microbes predominate. When lactose is exhausted the proteolytic and spore formers (Bacillus spp.) develop and decompose the casein. Finally, water and little fat remain. Proteolytic bacteria like moulds produce basic products; as a result the milk becomes extremely alkaline with a pH ranging from 8 to 10. The organic matter of milk is reduced to simple inorganic substances like carbon dioxide, ammonia and hydrogen sulphide.
23.2.2 Desirable fermentations
It is the type of fermentation that is brought about by the addition of desirable microorganisms or starter cultures (i.e. lactic acid bacteria) with the intention of making fermented milk product.
The growth of desirable starter cultures in milk resulting in the conversion of lactose into exclusive end product of lactic acid is called homo fermentation. In homo-fermentative lactic acid bacteria the lactose transport across the cell membrane involves the Phospho-enol-pyruvate dependent phospho-transferase system wherein the lactose is phosphorylated to lactose-P (glycosyl β galactoside-6-Phosphate) during its translocation.
In this system the lactose-P is hydrolyzed by β-phospho-galactosidase (β-D phosphogalactoside-galactohydrolyse) to D-glucose and galactose-6 phosphate. Glucose is metabolized to pyruvate via Embden Meyerhof (EM) Pathway. In the metabolism of galactose it is first converted to glyceraldehydes -3 phosphate via D-tagatose-6 phosphate pathway.
So two pathways are involved
· EMP for the metabolism of glucose
· Tagatose 6-phophate pathway for metabolism of galactose
Lactose in Streptococcus salivarius ssp. Thermophilus and Lactobacillus delbruckeii ssp. Bulgaricus is transported into the cell by the enzyme permease and or PEP: PTS system. These microorganisms posses, β-D galactosidase (β-gal) enzyme, which hydrolyses lactose into β-D galactose and D-glucose. Glucose is converted into lactic acid via EM pathway.
Transformation of lactose to lactic acid as follows:
The first step is the hydrolysis of lactose to glucose and galactose.
Phosphorylation of the sugar molecule probably plays a role in the dissimilation.
The transformation of lactose to lactic acid is not quantitative and the fermentation yields other products, the so-called secondary products, which may include various acids, aldehydes, and alcohols. The nature and quantity of the secondary products vary greatly with the different organisms; the products are very important since they greatly influence the flavor of the fermented milk.
Lactic acid bacteria producing large quantities of ethanol, CO2 as well as lactic acid when grown on lactose or glucose are considered as heterofermentative microorganisms.
· CO2 is produced by the oxidative decarboxylation of 6-phosphogluconate
· Ethanol is produced by reduction of acetaldehyde
· Lactic acid is produced by EM pathway