2.1) Microbiology of Milk

Milk drawn from a healthy milk animal already contains some bacteria.  Most of the changes which take place in the flavor and appearance of milk, after it is drawn from udder are the results of the activities of microbes.  These microbes are of two types i.e. favourable–which bring favourable changes in flavour & appearance while pathogenic-which may cause diseases.  The favourable are carefully propagated while pathogenic (unfavourable) are destroyed to make the milk & its products safe for human consumption.

2.1.1) Following are the important microbes found in milk:

A).Bacteria: Bacteria are microscopic, unicellular, occurs in the form of spherical, cylindrical or spiral cells.  Size of bacteria is the range of 1-5m.  Spore forming bacteria produce trouble in dairy industry because of their resistance to pasteurization & sanitization produces.

B).Moulds:  Multi-cellular, at maturity are as Mycelium. Useful in cheese making which is responsible for defects in butter and other milk products.  Most spores of moulds are destroyed by pasteurization.

C).Yeast: Unicellular, larger than bacteria.  Destroyed during pasteurization.

D).Viruses: viruses are ultra-microscopic forms of like can be destroyed by pasteurization or higher heat treatment.

 2.1.2) Growth of micro-organisms

Bacteria multiply during production and holding of milk, depending on storage time and conditions.  The changes take place in the Physico-chemical properties of milk are result of the activities of the individual microbial cells during their period of growth and reproduction or of substances produced during such activity.

 2.1.2. A) Stages of growth:

i. Initial stationary phase

ii. Lag phase (Phase of adjustment)

iii. Accelerated growth phase (log phase)

iv. Maximum stationary phase

v. Phase of accelerated death.

 2.1.2. B) Factors Influencing Growth:

  • i. Food supply – Milk and its products are good food source, provides all food requirements.
  • ii. Moisture – Milk contains adequate moisture to development.
  • iii. Air – Supplies O2 to aerobic bacteria and moulds.
  • iv. Acidity or pH – Preferably range 5.6 to 7.5.
  • v. Preservatives – Check growth depending upon concentration.
  • vi. Light – More or less harmful.
  • vii. Concentration – High sucrose or salt content check growth.
  • viii. Temperature – Important means for controlling growth. 

According to their optimum growth temperature, bacteria can be classified into:

  • Psychotropic – can grow at refrigeration temperature 5-70C.
  • Mesophilic – can grow at temperature 20-400C.
  • Thermophilic – can grow at temperature above 500C.

 2.1.2 C) Products of Microbial Growth:

  • i. Decomposition products (fats, proteins, sugars).
  • ii. Pigments
  • iii. Toxins

 

2.1.2 D) Results of Microbial Growth in Milk:

  • i. Souring: Most common, due to transformation of lactose into lactic acid & other volatile acids & compounds, principally by lactic acid bacteria.
  • ii. Souring & gassiness: Caused by coil group, indicates contamination of milk and its products.
  • iii. Aroma production:  Due to production of desirable flavour compounds.
  • iv. Proteolysis: Protein decomposition leading to unpleasant odour.
  • v. Ropiness:  Long threads of milk are formed while pouring.
  • vi. Sweet curdling: Due to production of a renin like enzyme curdles milk without souring.

 

2.1.2 E) Destruction of Micro-organisms:

May be done by following means:

  • i. Heat – Most widely Pasteurization & sterilization used.
  • ii. Ionizing radiation – Such as ultraviolet rays etc.
  • iii. High frequency sound waves – Supersonic and ultrasonic.
  • iv. Electricity – Microbes are destroyed actually by heat generated.
  • v. Pressure – Should be about 600 times greater than atmospheric pressure.
  • vi. Chemicals – Includes acids, alkalis, hydrogen peroxide, halogens etc.

2.1.3) Action of Microbes on Milk:

        Microbes                  Action                                 Result

1.     Streptococcus lactis

Souring

Lactose-lactic acid casein precipitation

2.     St. bulgaricus

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3.     Lactobacillus casei

Cheese ripening

Controls intestinaltermentation.

4. E coil

Souring & gasiness

Lactic acid & gases Affects cheese ripening.

5. Bacillus substallis

Proteolysis

off flavors.

6.     Alkaligenes viscus

Ropiness

Ropi milk

7.     St. liquifiecence

Bitter Flavour

Bitter flavour to cream &butter.

8. B. substallis

Sweet curdling

Curd formation

9.     St. paracitrovorus

Attacks citric acid

Flavors curd.

 

 

2.2) Microbiology of Butter

Butter is made as a means of extracting and preserving milk fat. It can be made directly from milk or by separation of milk and subsequent churning of the cream. Butter contains around 15% water, 81%fat and generally less than 0.5% carbohydrate and protein.

 

2.2.1) Sources of contamination

In addition to bacteria present in the milk other sources of bacteria in butter are (1) equipment, (2) wash water, (3) air contamination, (4) packing materials, and (5) personnel.

 

2.2.1. A) Equipment

In smallholder butter-making, bacterial contamination can come from unclean surfaces, the butter maker and wash water. Packaging materials, cups and leaves are also sources of contaminants. Washing and smoking the churn reduces bacterial numbers. But traditional equipment is often porous and is therefore a reservoir for many organisms.

When butter is made on a larger processing scale, bacterial contamination can come from holding-tank surfaces, the churn and butter-handling equipment.

A wooden churn can be a source of serious bacterial, yeast and mould contamination since these organisms can penetrate the wood, where they can be destroyed only by extreme heat. However, if care is taken in cleaning a wooden churn this source of contamination can be controlled.

2.2.1. B) Wash water

Wash water can be a source of contamination with both coliform bacteria and bacteria associated with defects in butter. Polluted water supplies can also be a source of pathogens.

2.2.1. C) Air

Contamination from the air can introduce spoilage organisms: mould spores, bacteria and yeasts can fall on the butter if it is left exposed to the air. Moulds grow rapidly on butter exposed to air.

2.2.1. D) Packaging

Care is required in the storage and preparation of packaging material. Careless handling of packaging material can be a source of mould contamination.

2.2.1. E) Personnel

Personnel pass organisms to butter via the hands, mouth, nasal passage and clothing. Suitable arrangements for disinfecting hands should be provided, and clean working garments should not have contact with other clothes.

 

2.2.2) Types of spoilage in butter:

  • i. Surface taint or Putridity: This condition is caused by “Pseudomonas putrefaciens” as a result of its growth on the surface of finished butter. It develops at temperature with in the range 4 to 7oC and may become apparent within 7 to 10 days.
  • ii. Rancidity: This condition is caused by the hydrolysis of butterfat with the liberation of free fatty acids. The causative organism is “Pseudomonas fragi”, although “Pseudomonas fluorescens” is sometimes found.
  • iii. Malty flavour: This flavour reported to be due to the growth of “Lactococcus lactis”.
  • iv. Skunklike: odor is reported to be caused by “Pseudomonas mephitica”.
  • v. Discoloration: Black discolorations of butter have been reported to be caused by Pseudomonas nigrifaciens.
  • vi. Fungal spoilage: Butter undergoes fungal spoilage commonly by species of Cladosporium, Alternaria, Aspergillus, Mucor, Rhizopus, Penicillium and Geotrichum. These organisms can be seen growing on the surface, where they produce coloration referable to their particular spore colors.

 

2.2.3) Control of micro-organisms in butter

Salting effectively controls bacterial growth in butter. The salt must be evenly dispersed and worked in well. Salt concentration of 2% adequately dispersed in butter of 16% moisture will result in a 12.5% salt solution throughout the water-in-oil emulsion.

Washing butter does little to reduce microbiological counts. It may be desirable not to wash butter, since washing reduces yield. The acid pH of serum in butter made from ripened cream or sour milk may control the growth of acid-sensitive organisms.

Microbiological analysis of butter usually includes some of the following tests: total bacterial count, yeasts and moulds, coliform estimation and estimation of lipolytic bacteria.

Yeast, mould and coliform estimations are useful for evaluating sanitary practices. The presence of defect producing types can be indicated by estimating the presence of lipolytic organisms.

All butter contains some micro-organisms. However, proper control at every stage of the process can minimize the harmful effects of these organisms.

            

2.3) Microbiology of Cheese:

A) Cottage cheese: This is a type of cheese and undergoes spoilage by bacteria, yeast and molds. The most common spoilage pattern displayed by bacteria is a condition known as “slimy curd”. Alcaligenes spp. have been reported to be among the most frequent causative organisms, although Pseudomonas, Proteus, Enterobactor, and Acinetobactor spp have been implicated. Penicillium, Mucor, Alternaria, and Geotrichum all grows well on cottage cheese, to which they impart stale, musty, moldy and yeasty flavors.

B). Ripened cheese: This is also type of cheese and has low moisture content make them insusceptible to spoilage by most organisms, although molds can and do grow on these products. Some ripened cheese has sufficiently low oxidation reduction potential to support the growth of anaerobes. Anaerobic bacteria sometimes cause the spoilage of cheese when water activity permits growth to occur. Clostridium species especially C. pasteurianum, C. butyricum and C. sporogenes have been reported to cause gassiness of cheese.

     One aerobic spore former, Bacillus polymyxa, has been reported to cause gassiness. All the organisms utilize lactic acid with the production of CO2, which is responsible for gassy condition in these milk products.

 

2.4) Microbiology of Ice-cream:

In frozen desserts include ice-cream, the ingredients may be

  • i. Various combination of milk,
  • ii. Cream,
  • iii. Evaporated milk,
  • iv. Condensed milk,
  • v. Dried milk,
  • vi. Coloring material,
  • vii. Flavors,
  • viii. Fruits,
  • ix. Nuts,
  • x. Sweetening agents,
  • xi. Egg and egg products and
  • xii. Stabilizers.

Any of these may contribute microorganisms to the product and affect the quality of dessert as judged by its bacterial content or its content of specific kind of bacteria such as coliforms. The desserts are not ordinarily subject to spoilage, however, as long as they kept frozen. The only important type of spoilage takes place in the ingredients before they are mixed or in the mix before it is frozen. So, mix is pasteurized before frozen. pe of cheese