Milk products produced by lactic acid fermentation

Yoghurt, Kefir, Filmjolk, Koumiss, ymer.
Generically known as cultured milk as they all derive from the action of bacteria on all or part of the Lactose to produce Lactic acid, carbon dioxide acetic acid, diacetyl, acetaldehyde and several other components that give the products the characteristic fresh taste an smell
The moiro-organisms used to produce Kefir and Koumiss also produce ethyl alcohol, giving these products the characteristic intoxicating effects associated with the consumption of alcohol.

A Legend
legend tells that yoghurt and Kefir were born on the slopes of Mount Elbrus in the Caucasus range of mountains, between the Black and Caspian seas.On the hot southern slopes a pitcher of milk belonging to a Turkish nomad was contaminated by a mixture of organisms that thrived in the warm milk (40 - 45c). The result was what the Turks call "yogurut". The name "yogurut" was supposedly introduced in the 8th century and was changed in the 11th century to the current version "Yoghurt". Legend suggests that Yoghurt can act as a preservative against human ageing, however no scientific evidence actually supports this theory. Undoubtedly a regular intake of the organisms found in yoghurt can have a beneficial affect to the digestive tract.

Similarly Kefir resulted from milk contaminated by organisms that preferred the cooler temperatures (25 - 28c) found on th northern slopes. Kefir as a name originates from the Turkish word "Kef" meaning pleasurable.Kefir contains several organisms the most famous being yeast that produce alcohol, however the maximum alcohol content is in the region of 0.8%.

The production of Lactic Acid from Lactose within the milk has a preservative effect on the milk as the lowered pH of the cultured milk inhibits the growth of putrefactive organisms. This effectively increases the shelf life of the product. On the "down side" the acidified milk provides an ideal growing medium for yeasts and moulds that can produce "off" flavours if they are allowed to infect the product.

Some people suffer from lactose intolerance due to the fact that their digestive system lacks enzymes needed to break down the Lactose to simpler sugars. In many cases cultured milk products where the Lactose has been partially broken down can be acceptable to sufferers of Lactose intolerance.

General requirements for culturing of Milk
The yoghurt manufacturing process is fairy simple and can be broken down into the following steps.

1. Preparation of the milk

The milk may be whole full fat, semi skimmed or low fat skimmed milk depending on the type of yoghurt you intend to make. It is normal in commercial yoghurt production to homogenise the milk prior to its fermentation. The homogenisation helps prevent the cream (fat) rising to the surface during the fermentation.

The addition of stabilisers and gums to the milk to improve viscosity and texture is fairly common in the commercials large scale yoghurt manufacturing plants.
Concentration of the milk by evaporation prior to fermentation is also fairly common, this gives a thicker more full bodied yoghurt. This technique is most common when skimmed milk is used as the base material to produce a low fat yoghurt. The solids level is typically increased by a factor of two.
As energy costs spiral ever higher the cost of concentrating milk is becoming prohibitive and evaporation is being replaced by a technique known as fortification, where dried powdered milk solids are added to the base milk prior to inoculation.

Many commercial yoghurt manufacturers are fortifying the base milk with a cocktail of milk and non milk solids along with starches, gums, gels and stabilisers. These techniques are generally aimed at improving the viscosity, texture "mouthfeel" and PROFIT obtained from a yoghurt.

2. Heat treatment.

The heat treatment of the milk prior to fermentation is generally considered essential in commercial manufacturing. The presence of unknown numbers of unknown organisms in the raw milk would make the fermentation too unreliable and unpredictable for commercial operations.

In order to ensure that the flavour, aroma and texture of the product is optimised the growing conditions for the "starter culture" must be as near perfect as possible. In order to achieve these conditions modern commercial Yoghurt manufacturers go to great lengths, when compared to the random inoculation of a pitcher of milk on a Turkish mountain side centuries ago.

To ensure that the "starter culture" has little if any competition from other organisms the milk will be heat treated to kill any organisms that may have been in the milk. Apart from killing unwanted bacteria the heat treatment will have a physio-chemical effect on the proteins and other additives within the mix. The heating may be necessary for some of the ingredients to achieve the required state to form gels and protein lattice that lead to the products final viscosity and texture.

As many ingredients used these days are dry powders, the heat treatment must be capable of coping with the large numbers of fungal and bacterial spores associated with dry powders. The inoculation and fermentation will take place in sealed hygienic vessels usually made from stainless steel. The temperature will be monitored and maintained at the optimum for the starter culture throughout the fermentation. The levels of lactic acid will be measured and monitored throughout the fermentation and the fermentation will be stopped by rapid cooling at the desired level of acidity.

Too long or too short a fermentation will produce a product that is inferior in either flavour texture. Too long a fermentation will give other organisms the chance to become established, with the associated risks of "off" flavours and smells.

The choice of raw materials will greatly affect the quality of the finished product.

Type of milk
Milk standardisation
Choice of starter culture
Culture preparation
Design of process pant
Heat treatment

The handling of the yoghurt during processing will also effect the texture and viscosity of the yoghurt.

The coagulum
This term refers to the complex but delicate protein lattice that forms as the pH drops and the protein reaches its isoelectric point. This lattice forms a sort of skeleton for the other milk constituents to bind to (fat, minerals, sugars).

Following the incubation the handling, of the yoghurt will greatly effect the texture and viscosity of the final product. Gentle agitation and pumping techniques need to be used to ensure that the viscosity produced by the coagulum is not destroyed.

Yoghurt is typically classified into the following groups:

Set Yoghurt
This type of yoghurt is incubated and cooled in the final package and is characterised by a firm jelly" like texture.

Stirred yoghurt
this type of yoghurt is incubated in a tank and the final coagulum is "broken" by stirring prior to cooling and packing. The texture of a stirred yoghurt will be less firm than a set yoghurt somewhat like a very thick cream. There is some slight reformation of the coagulm after the yoghurt has been packed, however this is slight and can not be relied upon.

Drinking Yoghurt.
This type of yoghurt is very similar to stirred yoghurt, having the coagulum "broken!" prior to cooling. In a drinking yoghurt the agitation used to "break" the coagulum is severe. Little if any reformation of the coagulum will reoccur after packing.

Frozen Yoghurt.
Frozen yoghurt is inoculated and incubated in the same manner as a stirred yoghut. However cooling is achieved by pumping through a Whipper / chiller / freezer in a fashion similar to ice cream. The texture of the finished product is mainly influenced by the whipper/ freezer and the size and distribution of the ice crystals produced.

Concentrated yoghurt
This type of yoghurt is inoculated and fermented in the same manner as a stirred yoghurt. Following the "breaking" of the coagulum the yoghurt is concentrated by boiling off some of the water, this is often done under vacuum to reduce the temperature required. Heating of low pH yoghurt can often lead to protein being totally denatured and producing rough and gritty textures. This is often called strained yoghurt due to the fat that the liquid that is released from the coagulum upon heating used to be "strained" off in a manner similar to making soft cheese.

Flavoured yoghurt
Yoghurt with various flavours and aromas have become very popular. The flavours are usually added at or just prior to filling into pots. Common additives are fruit or berries, usually as a puree or as whole fruit in a syrup. These additives often have as much as 50% sugar in them, however with the trend towards healthy eating gaining momentum, many manufacturers offer a low sugar and low fat version of their products. Low or no sugar yoghurts are often sweetened with saccharin or more commonly aspartame. The use of "fruit sugars" in the form of concentrated apple juice is sometimes found as a way of avoiding "added sugar" on the ingredients declaration, this tends to be a marketing ploy and has no real added benefit.


Typical composition of a commercial fruited yoghurt


0.1 - 3.5%


3 - 4.5%

Milk solids non fat

11 - 18%


0.2 - 0.4%


10 - 20%


Factor that alter the quality of yoghurt

Milk Quality

The milk used for yoghurt manufacture should be of the highest bacterial quality available. It should also have an absence of any material that will impede or prevent the growth of the starter organism (antibiotics, preservatives, disinfectants, bacteriophages).



Bacteriophages are a group of virus that attack the yoghurt starter organisms, a whole range of defects can be attributed to the action of these bacteriophage. Bacteriophage normally referred to just as "phage" are the most likely cause of long or never-ending incubations. Large manufacturers that have laboratory facilities to check incoming milk will often eliminate the possibilities of other starter inhibiting substances but "phage" is always a risk. "Phage" are usually found in the drains and floor gullies of a dairy producing any cultured product, poor hygiene and a lack of general housekeeping increase the risk. Cheese manufacturing and the subsequent whey handling are prime sources of "phage".

Starter culture

The starter culture is the term generally applied to the organisms used to ferment a cultured product, (cheese, yoghurt, Kefir, ). The organisms selected for this purpose need to produce the desired affect in the product, (although you could use a cheese starter in a yoghurt fermentation, the result would not be yoghurt). For normal commercial yoghurt the starter must be capable of fermenting lactose and producing lactic acid, little if any carbon dioxide is required and the flavour and aroma must be clean and fresh.

Traditionally when a suitable starter organism had been found a large quantity would be grown in a suitable nutrient medium (traditionally milk, but commercial blends of nutrients are now available), and small quantities would be used to inoculate each new batch of yoghurt. This technique with a main batch of starter culture is often referred to as using "bulk starter". The use of a bulk starter is becoming increasingly uncommon amongst commercial producers, mainly because of the risk of "phage" attack on the bulk starter, and the subsequent lost time while a new batch of starter organisms are prepared. A technique often referred to as DVI (Direct Vat Inoculation) is becoming the industry norm. DVI involves inoculating the yoghurt mix directly with a very large number of freeze dried starter organisms. The advantage of relative immunity to "phage" attack far outweigh the slightly longer incubation time required with this technique.

Fat %

The percentage of fat in the final yoghurt has a significant effect on the "mouthfeel", the normal range of fat content is from 0.5% to about 3.5%, however levels as low as 0% and as high as 10% are found in some speciality products.



Yoghurt is usually classified into the following groups


Minimum milk fat


Partially skimmed yoghurt

Maximum milk fat
Minimum milk fat

< 3%

Skimmed yoghurt

Maximum milk fat



In general the higher the fat level in the yoghurt the creamier and smother it will feel in the consumers mouth. A considerable amount of work has been carried out by the commercial manufacturers to reproduce this "creamy mouthfeel" without the use of fat. There are now a number of very low fat yoghurts on the market that have this "creamy mouthfeel" and still offer the health benefits of a low fat diet.

The Dry Matter content.

The higher the dry matter (solids non milk fat) the firmer the yoghurt will be. Commercial manufacturers control the dry matter in their yoghurt to ensure consistency of production. The normal methods used to standardise the dry matter content are :

Addition of skimmed milk powder,
Addition of milk concentrate,
Addition of the ultra filtration retentate from skimmed milk,
Addition or whey powder,
Addition of sodium Caseinate powder.
Sugars and sweeteners

Disaccharide sugars such as sucrose or monosaccharides such as glucose can be used alone or in conjunction to produce the sweetness level required. Levels of sugar greater than 10% should not be added to the yoghurt mix prior to the incubation, this is because the changes in osmotic pressure will adversely effect the starter culture. If higher levels of sugar addition are required then a means of adding the sugar after fermentation needs to be devised. The addition of sugar often improves the "body " and "mouthfeel" of a yoghurt.


Hydrophilic colloids will bind water and consequently increase the viscosity of a yoghurt, they also help prevent the separation of whey from the yoghurt, a problem known as synuresis. The most beneficial quantity os stabiliser to add to a yoghurt mix has to be determined experimentally by each manufacturer. Too much stabiliser and the yoghurt can take on a rubbery texture, far too much stabiliser and the yoghurt can become a hard solid mass. A traditionally produced natural yoghurt will require no stabilisers to produce a firm, fine gel, however commercially produced yoghurt that has to be pumped, stirred, fruited and filled will often break down to a runny liquid without the addition of stabilisers. Pasteurised yoghurt will definitely need to be stabilised as the nature of the heat treatment will adversely affect any naturally formed gel. The mechanical handling of a yoghurt after its incubation has a significant effect on its final texture and viscosity, consequently the design of the equipment needs to reflect this.

Common stabilisers are, gelatin, pectin, agar, starch. In quantities in the order of 0.1% to 0.5%.



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