Module 3. Microorganisms associated with milk

Lesson 12

MICROORGANISMS ASSOCIATED WITH RAW MILK AND THEIR SIGNIFICANCE – II

12.1 Mesophiles

Mesophiles are microorganism that grows best in moderate temperature, optimally between 25 and 40°C. These microorganisms have important applications in food preparation, especially in cheese and yogurt making. The important mesophilic microorganisms are shown in Table 12.1 and standards in Table 12.2, 12.3, 12.4.

Table 12.1 Microbial load of raw milk

Group	Genera
Micrococci	M. varians, M. luteus, M. roseus
Streptococci	S. agalactiae, S. dysgalactiae, S. uberis, S. faecalis, S. zymogenes, etc.
Lactococci	L. lactis ssp. lactis, L. lactis ssp cremoris
Gram positive rods	L. casei, L. plantarum, Corynebacterium spp., Microbacterium spp., Arthrobacter spp., Bacillus spp.
Gram negative rods	Pseudomonas spp., Alcaligenes spp., Coliforms, Flavobacterium etc.

Table 12.2 Quality of raw of milk as per BIS standards  

Table 12.3 Microbiological standards for raw milk (BIS Standards)

Direct Microscopic Clump Counts/ml	Grading of milk
<5,00,000	Good
5,00,001 - 4,000,000	Fair
4,000,001 - 20,000,000	Poor

12.2 Thermoduric Microbes

Microorganisms that can survive pasteurization but cannot grow at high temperatures are called thermoduric. Hence, these are carried into the pasteurized product, where these may cause defect and promote health hazard risks when standard growth conditions are prevalent.

Source: Thermoduric microbes are widely distributed in animal feeds, bedding, soil and feces. These contaminate animal’s teats and milking equipments. Therefore, it is difficult to completely eradicate these microbes from the milk processing system. The only effective measure control these, is to control their number in milk. For example, Micrococcus, Microbacterium, Bacillus and Coryneform group comes under thermodurics.

Control measures:

1. Implementation of hygienic milking practices

2. Use of clean and hygienic milk equipments

3. Proper fecal disposal system

4. Maintenance of clean environment

Table 12.4 Thermoduric counts (BIS standards)

Counts cfu/ ml	Remarks
Below 10,000	Good
10,000 to 30, 000	Fair
Above 30,000	Poor

12.3 Coliforms

These are ‘Gram negative, rod shaped, non-spore former that ferment lactose to produce acid and gas at 35-37°C/ 24-48 h. Their presence in food indicates the sanitary quality of production. Therefore, sometimes these are also called as an ‘index of direct fecal contamination’ in product. These microbes produce different toxins in foods that create major health hazard issues for consumers. A toxin producing strain has been identified as E. coli O157: H7 that presents serious health issues, especially in children.

Sources: Feces of healthy livestock including dairy cattle, contaminated water, and unhygienic milk production practices, animal suffering with coliforms mastitis, handlers and contaminated utensils are major sources of coliforms. The example includes: E. coli, Salmonella, Enterobacter aerogenes. Coliforms count for raw milk should not be more than 10/ml.

12.4  Significance of  Coliforms in Raw Milk

Presence of coliforms in milk indicates a possible fecal contamination, especially when these are handled under unsanitary conditions. The presence of these micro-organisms is considered undesirable because they produce acid, gas and objectionable taints in milk products. Generally, coliforms are destroyed during pasteurization and hence, their presence in pasteurized milk indicates a post pasteurization contamination. Absence of coliforms in 1:100 dilution (<100 per ml) in raw milk and in 1:10 dilution (<10 per ml) of pasteurized milk is accepted as a criterion of satisfactory quality. The presence of fecal E. coli is a proof that contamination from excreta has taken place.

12.5 Significance of Thermophilic Bacteria in Milk

12.5.1 Thermophiles

As such there are no natural limits for the growth temperature of these groups of microbes. The obligate thermophiles have a temperature range as high as 85°C, while some facultative thermophiles grow as low as 20°C and as high as 60°C. The thermo-resistant bacteria, although do not form spores, survive pasteurization and should not be confused with thermophiles. Thermophiles are a heterogeneous group of microbes including representatives of many unrelated species. These should be regarded as variants that may occur in any species and usually differ from the normal strains, in their ability to grow at high temperatures. Among the thermophiles, there are representatives of lactobacilli, coliforms, anaerobic, and especially aerobic spore-forming rods. The latter group of aerobic spore formers is most commonly found in pasteurized milk.

 12.5.2  Sources of thermophiles in milk

Thermophiles occur naturally under conditions that make these easy to account for their presence in milk, aside from, fermenting manure and other vegetable materials. That is why; it is possible to isolate thermophiles from almost any sample of ruminants manure. From these sources, these become widely distributed in nature and found in soil, dust, on the leaves of plants etc. Under certain conditions, the surface of soil may reach a temperature high enough to favor the growth of thermophiles, and it is possible that under such circumstances these may grow in the soil relatively at lower temperature.

It is unlikely that thermophiles ever live in the udder of an animal. Milk drawn directly from the udder into a test tube and held at 50 or 55°C sometimes reduces the litmus, but it does not necessarily mean that this milk is inoculated with thermophiles from the ducts of udders. For this, very rigid precautions are necessary to positively exclude the outside contamination of thermophiles. There are number of conditions that either singly or jointly led to high thermophilic count in milk; first is an exceptionally high initial contamination prior to pasteurization.

The milk may be inoculated from substandard equipments, such as leaks that may permit milk to contaminate with thermophiles, and to seep out into pasteurizer. Milk may also become infected from the foam and residue left from the previous batch of milk. Thermophiles may also multiply in pasteurizer, when milk is not maintained at the proper temperature. The reduction of the pasteurization temperature even a few degrees bring the milk within the growth range of many microbes that cannot grow at 62.8°C. This may usually happen in the dead ends of pipes, possibly in the corners of vats not reached by the agitators, especially in foam. The foam is much lower in temperature than milk under it and not only the more resistant bacteria carried by the foam survive, but it acts as an incubator for many types of thermophiles. The most common cause of higher thermophilic counts is the building up in numbers, when successive batches are pasteurized in the same vat without proper and sufficient cleaning between the two different runs. The foam and milk left in the bottom of the vat serve as inoculums for the following run, so that if 5 to 6 batches are pasteurized in a single vat, the later ones are likely to have higher counts.

The question arises that what does the thermophilic count indicate? The answer is that it indicates one or more of the conditions that exists in pasteurizing plant. The plant may be receiving an unusual contamination; it may be re-pasteurizing the milk brought back from the wagons; the walls of the pasteurizer may have a thin layer of cooked milk where thermophiles may grow; the construction of the pasteurizer may be defective so that some of the milk fails to reach the required temperature; there may be an excessive amount of foam that is not heated properly; or the vats may not be cleaned between the successive runs. There are no reasons that the thermophiles are more harmful in milk than microbes growing at lower temperatures. In fact, most of these grow slowly or not at all.

12.6 Pathogenic Microbes Present in Raw Milk

Microbial pathogens may get entry into raw milk either from the surroundings (i.e. contaminated utensils, feces, water, handlers) or from the infected udders/ tissues of the producing animals. The most important pathogens present in raw milk are given in Table 12.5.

Table 12.5 Different pathogenic microbes and disease associated

Pathogenic microbial agent	Disease associated with microbe
Campylobacter jejuni	Campylobacterosis
Corynebacterium diphtheria	Diphtheria
Coxiella burnetii	‘Q’ fever
Escherichia coli O157: H7	EHEC - Enterohaemorrhagic E. coli
Hepatitis A	Jaundice
Listeria monocytogenes	Listeriosis
Mycobacterium tuberculosis	Tuberculosis
Salmonella typhi Salmonella spp.	Typhoid fever Salmonellosis
Shigella dysenteriae	Shigellosis
Yesinia enterocolitica	Yersinosis