In this article we will discuss about:- 1. Introduction to Milk Microbiology 2. Growth of Micro-Organisms 3. Destruction of Micro-Organisms.
Introduction to Milk Microbiology:
Nearly all the changes which take place in the flavour and appearance of milk, after it is drawn from the cow, are the result of the activities of micro-organisms. Of these, the most important in dairying are bacteria, mould, yeast and virus—the first one predominating. Micro-organisms are visible only with the aid of a microscope.
A few are desirable, while most cause undesirable changes; a relatively small proportion are disease-producing types, and are called ‘pathogens’. In the dairy industry, considerable effort is expended in controlling micro-organisms which cause spoilage. The greater the bacterial count in milk, i.e., the greater the number of bacteria per ml of milk, the lower is its bacteriological quality.
Bacteria are microscopic, unicellular fungi (plants without chlorophyll) which occur principally in the form of spherical, cylindrical or spiral cells and which reproduce by transverse fission. In milk and its products, the spherical and cylindrical forms are predominant.
Most bacteria vary from 1 to 5 microns in size. Although individual bacterial cells are invisible to the naked eye, they form bacterial ‘colonies’ (consisting of a large number of individual cells) which are visible. Bacteria are found nearly everywhere in nature. They are found in large numbers in the soil, sewage, decaying plants or animals; and are also present in air, water, etc.
Under favourable conditions, bacteria multiply very rapidly and may double their number in 15 minutes or less. Some bacteria also form ‘spores’, which are tough resistant bodies within the bacterial cell. Spores, when placed in an environment favourable to growth, form new vegetative cells. Spore-forming bacteria cause trouble in the dairy industry because of their resistance to pasteurization and sanitization procedures.
Moulds are multi-cellular, differing greatly in most respects from bacteria. Although the individual cells are not visible to the naked eye, at maturity they may be observed readily as ‘Mycelium’. They are found in soil, feeds, manure and poorly-washed utensils. Most spores of moulds are destroyed by pasteurization. They are of considerable importance in cheese-making and are responsible for some defects in butter and other milk products.
Yeasts are unicellular but are somewhat larger in size than bacteria. Spores of yeasts are readily destroyed during pasteurization.
Viruses include all ultra-microscopic forms of life. In the dairy industry, only those viruses that are parasitic on lactic acid bacteria and known as Starter Bacteriophage (or simply Phage) are of special importance. The viruses range in size from 0.22 to 0.23 microns. The lactic phages are usually not destroyed by normal pasteurization of milk employed for cheese and cultured buttermilk, but they can be destroyed by higher heat-treatment.
Growth of Micro-Organisms:
In microbiology, growth refers to increase in numbers. Milk drawn from a healthy cow already contains some bacteria. Their numbers multiply during production and handling, depending on the cleanliness of these operations. Subsequently, their numbers may grow still further (either substantially or only slightly) depending on storage conditions.
The changes which take place in the physico-chemical properties of milk are the result of the activities of the individual microbial cells during their period of growth (development and reproduction) or of substances produced during such activity.
Stages of Growth:
The growth of micro-organisms normally takes place in the following stages:
(i) Initial stationary phase;
(ii) Lag phase (phase of adjustment);
(iii) Accelerated growth phase (log phase);
(iv) Maximum stationary phase; and
(v) Phase of accelerated death.
Factors Influencing Growth:
The growth of micro-organisms is influenced by the following factors:
(i) Food Supply:
Milk and most dairy products furnish all the food requirements of microorganisms.
(ii) Moisture:
Milk contains adequate moisture (dry products, due to their low moisture content, keep longer),
(iii) Air:
Supplies oxygen to aerobic types of bacteria and to moulds in general,
(iv) Acidity or pH:
Most common types prefer a pH from 5.6 to 7.5.
(v) Preservatives:
These check or destroy growth, depending on their concentration,
(vi) Light:
Ii is more or less harmful.
(vii) Concentration:
High sucrose or salt content in product checks growth, and
(viii) Temperature:
An important means for controlling growth. Each species of micro-organism has its optimum, maximum and minimum temperatures of growth.
According to their optimum growth temperatures, bacteria can be classified into:
a. Psychotropic:
It can grow at refrigeration temperatures (5-7°C).
b. Mesophilic:
It can grow at temperatures ranging between 20°C and 40°C.
c. Thermophilic (Heat Loving):
It can grow at temperatures above 50°C.
Note:
In general, low temperatures (below 10°C) retard microbial growth. Hence milk and dairy products should be adequately refrigerated to prevent rapid spoilage. The optimum growth rate of the majority of micro-organisms will be found at temperatures between 15 and 38°C. Most microorganisms are destroyed at high temperatures, viz., 60°C or above.
Products of Microbial Growth:
These are- (i) enzymes; (ii) decomposition products (of fats, proteins, sugars, etc.); (iii) pigments; (iv) toxins; and (v) miscellaneous changes.
Results of Microbial Growth in Milk:
These are:
(i) Souring:
It is most common. Caused by transformation of lactose into lactic and other volatile acids and compounds, principally by lactic acid bacteria. (The development of a sour acid flavour is not due to lactic acid, which is odourless, but due to volatile acids and compounds.)
(ii) Souring and Gassiness:
Caused by Coli group, which are commonly found in soil, manure, feed, etc., and therefore indicate contamination of milk and dairy products.
(iii) Aroma Production:
Due to production of desirable flavour compounds such as diacetyl (in ripened-cream-butter).
(iv) Proteolysis:
Protein decomposition leading to unpleasant odours. (Sometimes desirable flavours may develop, as in cheese curing.)
(v) Ropiness:
Long threads of milk are formed while pouring.
(vi) Sweet Curdling:
Due to production of a rennin-like enzyme, which curdles milk without souring.
Destruction of Micro-Organisms:
This may be done by means of:
(i) Heat:
It is most widely used. Different types of heat-treatment (temperature-time combination) are employed, such as pasteurization, sterilization, etc.
(ii) Ionizing Radiation:
Ionizing radiation such as ultra-violet rays, etc.
(iii) High Frequency Sound Waves:
This is such as supersonic and ultrasonic.
(iv) Electricity:
Here, the micro-organisms are destroyed, actually, by the heat generated.
(v) Pressure:
Should be about 600 times greater than atmospheric pressure, and
(vi) Chemicals:
These include acids, alkalis, hydrogen peroxide, halogens, etc.