Yoghurt

The basic ingredients of yoghurt are milk and microflora. The milk is converted into yoghurt by growing within it specific lactic acid bacteria and souring it under defined conditions. According to FAO/WHO (1977) “Yoghurt is a coagulated milk product obtained by lactic acid fermentation through the action of Lactobacillus bulgaricus and Streptococcus delbrueckii subsp. thermophilus, from milk and milk products(Pasteurized milk or concentrated milk), with or without optional additions (milk powder, skim milk powder, whey powder, etc.). The microorganisms in the final product must be viable and abundant”.

The origin of yoghurt is not clear. According to some sources yoghurt originated in Asia, where the ancient Turks lived as nomads. The first Turkish name appeared in the 8th century as “Yogurut” and was subsequently changed in the 11th century to its present form. A dried type of yoghurt was called “Kurut: and a beverage type “Suvuk yoghurt”. According to still some authors, yoghurt originates from the Blakans. The inhabitants of Thrace, known for breeding large flocks of sheep, used to make soured milks called “Prokish” which later became yoghurt.

Originally yoghurt was made from sheep and buffalo milk and partly from goat and cow milk in containers of wood or argil. Propagation was carried out by using a small quantity of the previously coagulated milk to seed the next batch of boiled milk. Yoghurt was used primarily in the human diet for direct consumption, either as a natural product or fortified with various vegetables and spices, but also for cooking and baking purposes.Ancient physicians of the near and middle East prescribed yoghurt or related soured milks for curing disorders of the stomach, intestines and liver and for stimulating the appetite. In the early part of the 20th century Metchnikoff (1845-1916) in his ‘theory of longevity” noted the beneficial effect of yoghurt in the human diet. Although this theory exaggerated the value of yoghurt, it significantly influenced the spread of the product to many countries of Europe and promoted extensive studies by subsequent workers.

i Definition

Yoghurt is a coagulum obtained by lactic acid fermentation of prescribed milk or milk products by the action of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, the addition of additives is optional. The final product should contain large quantities of the above microorganisms. Yoghurt is characterized by a pungent, fruity small and acid taste. It has a firm body like dahi. Commercial yoghurts are divided into three main categories, i.e.. plain/natural, fruit and flavoured, and these different types of yoghurt are manufactured in many forms such as set, stirred, liquid/drinking, frozen and dried/instant types. The international standards for types of yoghurt are given in table .

                                             The international standard for different types of yoghurt (FAO/

WHO)

iii. Method of Manufacture of Yoghurt

Flow diagram for manufacture of yoghurt

(a) Process lines for yoghurt

The quality of yoghurt depends not only upon the type of raw material used and the manufacturing procedure applied but also upon the good functioning of the process equipment involved and process line.

i) Batch Method: This method was originally used in small-scale production.The manufacture of set yoghurt is carried out as follows:

The clarified milk, adjusted to a specific solids content, is heated in a jacketed vat to 90-95oC for 15-25 min, cooled to 43-45oC, inoculated with 1-3 percent yoghurt culture and thoroughly mixed with the milk and then filled into retail containers, incubated in an incubator at 41-43oC for 2-3 hours and cooling  with in the same incubation chamber or in the cold store. Alternatively, incubation and cooling can be done in water trays.

In the manufacture of stirred yoghurt, heat treatment of the milk, cooling, inoculation and incubation are carried out in a jacketed vat, followed by cooling either in the same vat or in a cooler, with subsequent filling into retail containers.With larger plants it will be desirable to include homogenization of the milk.This can be done using a plate heater or heating the milk to the homogenizing temperature (55-65oC) and then to pasteurize homogenized milk in a jacketed vat.Both types of yoghurt can be produced as natural or plain or flavoured and fruit. Flavouring or fruit are added before incubation when making set yoghurt and after incubation and cooling when making stirred yoghurt. The addition of sugar, particularly in flavoured and fruit types, has the purpose of improving the taste of the product and masks its acidity. If flavorings are not already sweetened, sugar is added before heat treatment of the milk. The sugar concentration should not exceed 12% because of the inhibitory (osmophilic) effect on the cultures.

The main characteristics of the batch method and its process lines are as follows:

•  A number of breaks during the whole process of manufacture, for standardization of the fat content, addition of milk powder, heat treatment of the milk and cooling, inoculation, etc. which involves an incomplete utilization of the processing equipment.
•  A considerable consumption of energy and of time, particularly during heat treatment and cooling of the milk and high labour costs.
•  An increased chance of air borne contamination by yeasts and moulds during the manufacture due to the carrying out of discontinuous process, the use of open vats and the manual additions of cultures, fruit and flavorings.

 ii) Modern Process: Modern process lines for yoghurt are used in medium or large scale production. The following factors contributed to the introduction of modern process lines:

•  Standardization of the fat content in milk by using specially designed equipment.
•  Increasing the solids content in milk, by concentration of the milk using specially designed evaporating units concentration of the milk by 10-15%(raising of dry matter for about 2-3%) normally gives an optimum solids content. This method is usually used in large scale production. Alternatively,1-3% skim milk powder may be added for increasing the solids content in milk.
•  Homogenization at 55-65oC under a pressure of 2000-2500 psi in a homogenizer, usually before heat treatment of the milk.
•  Heat treatment of the milk in pasteurizers at 90oC for 5 min and in UHT sterilizers (direct and indirect heating) at 135-145oC for few seconds, with subsequent cooling to the fermentation temperature.
•  Culturing of milk by the addition of bulk starter (1.5 – 2.0%) with subsequent stirring for few minutes.
•  In manufacturing stirred yoghurt, incubation is carried out in a series of
• insulated tanks working in rotation. After the completed incubation (41- 43oC for 2-3 hrs) the yoghurt is discharged by a positive pump to the plate or tube cooler. Cooled yoghurt is transferred to a pair of buffer tanks which continuously supply it to packaging machines. On the
• packaging line, previously pasteurized flavouring ingredients are continuously added in-line by a positive metering pump and mixed with the yoghurt.
•  In the manufacturing of set yoghurt, the inoculated milk in a pair of mixing tanks is fed to packaging machines for filling into retail containers. On this route, previously pasteurized flavouring or fruit concentrate may be added in line by a metering pump and mixed with the yoghurt milk. The filled containers are transferred to incubation chambers for incubation until a  pH 4.7 is reached. Cooling can be done either within the same chambers or in the cold store.
•  Processing equipment of the plant such as tanks, pumps, heat exchangers,piping and others, being designed for cleaning in-place and sterilization.