Fermented Milk: Introduction, Types, Quality and Defects

In this article we will discuss about:- 1. Introduction to Fermented Milk 2. Types of Fermented Milk 3. Starter Propagation, Quality and Defects.

Introduction to Fermented Milk:

Fermented milks refer to those milks which have been made by employing selected micro-organisms to develop the characteristic flavour and/or body and texture.

Fermentation has been defined as the metabolic process in which chemical changes are brought about on an organic sub­stratum, whether protein, carbohydrate, or fat, through the action of enzymes liberated by specific living micro-organisms. (In dairy­ing, the most important fermentation is the lactic acid fermenta­tion or souring of milk.)

Types of Fermented Milk:

The fermented milks, which will be discussed here, are- Natural buttermilk, cultured buttermilk, Acidophilus milk, Bulgarian buttermilk, Kumiss, Kefir and Yoghurt.

(i) Natural Buttermilk:

This is a by-product of churning cream for butter making. Ripened cream which has undergone a clean, lactic fermentation is usually preferred. Also included, under Indian conditions, is country buttermilk obtained as a by-product of churning whole milk curd for production of country butter.

(ii) Cultured Buttermilk:

This is obtained by inoculation and incubation of pasteurized skim milk with lactic starter.

(iii) Acidophilus Milk:

This type of fermented milk is produced by development in milk of a culture of Lactobacillus acidophilus. It is claimed that acido­philus milk has therapeutic and health-promoting properties. It is also claimed that the growth of Lact. acidophilus under the conditions existing in the intestinal tract will replace undesirable putrefactive fermentations with a beneficial lactic fermentation.

(iv) Bulgarian Buttermilk:

A pure single strain starter of Lact. bulgaricus is propagated for product manufacture. The incubation temperature ranges from 38 to 43°C (100-110°F). The finished product is characterized by a higher acidity (1.2-1.5 per cent).

(v) Kumiss:

This is a lactic acid-alcohol fermented milk (formerly mare’s, now cow’s milk), originated in Russia. The culture consists of Lact. acidophilus or bulgaricus. The first incubation period is followed by the addition of yeast and a second incubation period. The finished product may contain upto 2.5 per cent alcohol.

(vi) Kefir:

This has its origin in the Caucasian Mountains. It is a self-carbo­nated beverage containing 1 per cent lactic acid and 1 per cent alcohol. It is made with a fermenting agent called ‘Kefir grains’, which contains Str. lactis, Betabacterium caucasicum, Kefir bacilli and lactose fermenting yeasts.

(vii) Yoghurt:

Turkish yoghurt, Egyptian leben, Armenian matzoon and Indian dahi are all similar products. Originally yoghurt was made from boiled concentrated whole milk, but most modern methods of manufacture use whole or partly defatted milk containing small amounts of skim milk powder or concen­trate. The fat content in yoghurt may vary from 0 to 5 per cent and the solids content from 9 to 20 per cent.

Two micro-organisms, Lact. bulgaricus and Str. thermophilus growing together symbiotically, are responsible for the lactic fermentation of yoghurt. In some countries, it contains lactose fermenting yeast. Leuconostoc strains, Str. diacetilactis and Laci. acidophilus are also added to improve the flavour of the yoghurt.

The method of culture control is very important in yoghurt manufacture, and for this reason stock (mother) cultures are best maintained individually, rather than mixed. The optimum pH and temperature for growth of Str. Thermophilus is 6.8 and 38°C (100°F); Str. thermophilus cultures normally attain acidities of 0.85 to 0.95 per cent, whereas Lact. bulgaricus reaches acidities of 1.20 to 1.5 per cent. The extreme sensitivity of both these micro-organisms to Penicillin makes it essential to select antibiotic-free milk for yoghurt manufacture and all starter propagation.

Mother starter is prepared as follows:

Fresh or reconstituted skim milk is autoclaved at 15 psi for 10-15 minutes, cooled to 41°C (105°F) and inoculated with 0.2 to 1 per cent inoculum. Cultures of Str. thermophilus are incubated at 38°C (100°F) and Lact. bulgaricus at 43°C (110°F).

Normal coagulation occurs in 12-18 hours, at which time the cultures are cooled to 5°C (40°F). Trans­fers should preferably be made daily. A commercial yoghurt cul­ture, which contains both micro-organisms should be incubated between 41 to 43°C (105 to 110°F), or according to the supplier’s instructions.

Bulk starters are carried in stainless steel vessels or vats with sufficient capacity to contain 2 parts of starter for every 100 parts of yoghurt made. The necessary quantity of skim milk is heated to 85-88°C (185-190°F) for 30-45 minutes, cooled to 43°C (110°F) and inoculated preferably with 1 per cent each of Str. thermo­philus and Lact. bulgaricus, which in a starter or product have a marked effect on its flavour and odour.

Starters are mixed in the milk and the latter incubated at 41-43°C (105-110°F) until coagu­lation occurs. The bulk starter is cooled to 10°C (50°F) and stored at 5°C (40°F) if not required for immediate use.

Method of Preparation of Yoghurt (Firmbodied):

Flow Diagram of Manufacture:

Details of Manufacture:

Skim milk powder/concentrate is added to whole or partly defatted milk to increase the solids- not-fat content by 2-3 per cent to a total of approximately 12 per cent. The mix is pre-heated to 60°C (140°F) and homogenized single-stage at 2000-2500 psi. Stabilizers are frequently added to the mix but are not essential and excessive amounts are to be avoided.

The mix is heated in a vat to 85°C (185°F) for 30 minutes and then cooled to 43-44°C (110-112°F), inoculated with 2 per cent bulk starter and stirred briefly to ensure proper mixing. The mix is then packaged, care being taken that the tempera­ture does not fall below 41°C (106°F) during the filling opera­tion.

The time interval between inoculation and filling should not exceed 45 minutes. Yoghurt is incubated in the package without further agitation at 41-42°C (106-108°F) for about 3 hours, till a titratable acidity of 0.75 per cent is reached. It is then placed under refrigeration to cool to 5-7°C (40-45°F); approximately 8 hours are required, at which time the product is ready for distribution. A final acidity of 0.9 per cent is desired in the product.

Method of Preparation of Flavoured Yoghurt:

Flavoured yoghurt has become popular in recent years. Flavours are of three types, viz., synthetic or artificial, natural with synthetics added, and natural fruits.

These flavours may be added to yoghurt in one of the following ways:

(i) By Mix Blending:

The flavouring material, sugar and colour are added to the mix prior to fermentation.

(ii) Fruit on Bottom:

Preserves or fruits are added to the package prior to filling or setting.

(iii) Bulk Mixing:

Yoghurt is prepared, bulk chilled and mixed with a fruit dressing and packaged. Fresh fruit, preserves, fla­vours, sugar and colour are blended in after fermentation.

Note:

Flavoured yoghurt has a definite advantage over plain yoghurt in that harsh acidity in the product is less pro­nounced, the incidence of objectionable off-flavours is reduced and much of the need for concentrating the milk is eliminated.

Merits:

(i) Much more palatable than milk;

(ii) Nutritive value usually increased;

(iii) More easily assimilated by the human system than milk;

(iv) May contain beneficial antibiotics;

(v) May possess therapeutic properties;

(vi) Regular consumption of some types claimed to increase longevity of human beings.

Starter Propagation, Quality and Defects of Fermented Milk:

The term ‘starter’ refers to an active bacterial culture that has been propagated for use in the manufacture of fermented milks (or milk products). Lactic starter is usually con­sidered to indicate a culture containing lactic streptococci. A number of different kinds of starters or cultures are commercially available for use in the dairy industry. Micro-organisms commonly found in fermented milks belong to the germs Streptococcus.

They may or may not contain associative organisms of the genus Leuconostoc. Starter species most commonly used in the industry are- Str. lactis, Str. cremoris, Str. diacetilactis, Leuc. citrovorum and Leuc. dextranicum. A starter may contain a single lactic strain or a mixture of lactic streptococci with or without Leuconostoc species.

An active starter performs three important functions in the manufacture of a fermented product:

(i) Acid production;

(ii) Desired, characteristic flavour production;

(iii) Prevention of growth of undesirable micro-organisms that may have survived pasteurization or contaminated the product. (Inhibition is caused principally by the production of lactic acid which establishes a pH unfavourable to growth.)

Each fermented milk requires a select starter culture in its manu­facture. Commercial starter cultures are prepared by commercial laboratories, usually from carefully selected species of bacteria, and are available in liquid, tablet or powder forms; they may be freeze-dried as well.

‘Mother starter/culture’ refers to stock lactic cultures prepared routinely in the laboratory or dairy plant from day to day for bulk propagation. ‘Bulk starter/culture’ is made in large amounts to be actually added to the milk, etc., whenever required for the manufacture of fermented milks or milk products.

Mother Starter:

The preparation of starters/cultures in­volves the following basic considerations:

(i) Selection of Milk:

The milk (preferably skim milk) should be of high quality: low count milk from healthy animals is desir­able; presence of antibiotics and inhibitory substances in milk should be avoided. (Spray dried skim milk re-constituted with distilled water to give 10-11 per cent milk solids may be used for mother starter.)

(ii) Heat Treatment:

It should preferably be not less than 82°C (180 °F) for not less than 1 hour. After heating, the milk should be slowly cooled to 21-22°C (70-72°F) for inoculation. Severe milk heating (distinct browning) should be avoided as it may adversely affect starter activity.

(iii) Containers:

Wide mouth, screw cap, heat-resistant glass bottles preferred for mother starter. Both bottles and cans should be thoroughly cleaned and sanitized before use.

(iv) Inoculation:

Usually 1 per cent inoculation is performed with a sterile pipette from mother stock under aseptic conditions. Mixing of inoculation is done by rotating the bottle after capping.

(v) Incubation:

The inoculated milk is placed in a controlled incubator at 22°C (72°F), and incubated without further agita­tion for 14 to 16 hours until a titratable acidity of 0.75 to 0.85 per cent is developed.

(vi) Cooling:

It is then removed and quickly cooled to 5°C (40°F) and stored in the refrigerator. It is extremely important that ‘mother’ and ‘bulk’ starters should be cooled rapidly as soon as the desired acidity is reached, as otherwise rapid losses in cul­ture activity result. Stirring or shaking for cooling should be avoided.

Bulk Starter:

The preparation of starter cultures in bulk or larger volumes presents many difficulties. It is considerably less difficult to maintain sterility in bottles (flasks) than in cans (vats). Bulk starter milk is usually heated between 85 to 93°C (185 to 200°F) for 45 to 60 minutes and cooled rapidly to an inocula­tion temperature of 22°C (72°F).

The bulk milk is inoculated immediately with a 1 per cent inoculation from a fresh mother starter/culture which has demonstrated the proper degree of acti­vity in laboratory tests. The inoculated starter milk is stirred briefly (under aseptic conditions) to allow proper mixing of the inoculum, and then allowed to ripen at 22°C (72°F) for 14 to 16 hours. As soon as a titratable acidity of 0.75 to 0.85 per cent is reached, the starter is rapidly cooled to 5°C (40°F) and held at that temperature for use.

Flow Diagram of Starter Propagation:

Starter Qualities:

(i) Flavour:

Mild, pleasant, ‘nutty’ smell; clean, acid taste.

(ii) Body:

Soft and firm; curd should give a clean and smooth break; should have no gas holes or whey pockets.

(iii) Texture:

Smooth, free from lumps.

(iv) T.A.:

0.75 to 0.85 per cent.

Note:

The colour will be yellowish for (whole) cow and whitish for (whole) buffalo milk.

Starter Defects:

Insufficient flavour development; insuffi­cient acidity development; hard and lumpy curd; excessive develop­ment of acidity; gas holes: whey pockets; ropiness; bitterness; pre­sence of bacteriophage; etc.