Yoghurt

Yoghurt: Manufacturing - Making - Production

Yogurt (also spelled yogourt or yoghurt)
Yogurt is a dairy product, which is made by blending fermented milk with various ingredients that provide flavor and color.

Raw Materials

Yogurt is made with a variety of ingredients including milk, sugars, stabilizers, fruits and flavors, and a bacterial culture

The process of making yogurt includes modifying the composition of and pasteurizing the milk; fermenting at warm temperatures; cooling; adding fruit, sugar, and other ingredients

Yogurt mix formulation

When the milk arrives at the plant, its composition is modified before it is used to make yogurt. This standardization process typically involves reducing the fat content and increasing the total solids. The fat content is reduced by using centrifugation to separate fat from milk. From the separator, the milk is placed in a storage tank and tested for fat and solids content. For yogurt manufacture, the solids content of the milk is increased to 16% with 1-5% being fat and 11-14% being solids-not-fat (SNF). This is accomplished either by evaporating off some of the water, or adding concentrated milk or milk powder. Increasing the solids content improves the nutritional value of the yogurt, makes it easier to produce a firmer yogurt and improves the stability of The milk substance is fermented until it becomes yogurt. Fruits and flavorings are added to the yogurt before packaging. the yogurt by reducing the tendency for it to separate on storage. Yogurt mix should have a minimum SNF of 12% to increase the viscosity and also to increase the resistance to "wheying off" (Increased proteing content = increased viscosity in the final product as it is the proteins that are integral to the formation of the coagulum) (Casein in particular) This increased SNF can be achieved by Evaporation of skimmed milk to give a skimmed milk concentrate or simply by the addition of skimmed milk powders or milk protein powders or concentrates. Recent advances in UF and RO give more options for ingredients

The choice of raw materials will greatly affect the quality of the finished product.
Type of milk - cow, buffalo, sheep etc
Milk standardisation - standardize the fat casein ratio
Additives - Fruit, stabilizers, emulsifiers, preservatives etc
Choice of starter culture - bulk or innoculated freeze dried, mixed strain etc Bulk usually 2% of mix volume
Culture preparation -
Design of process pant - batch, continuous etc
Heat treatment - dependant upon many factors 10°c to 95°c for 15 seconds to 25 minutes
Incubation - temperature matched to starter strains, usually 40° c to 45° c until 1% Lactic acid
Homogenisation - 200+ bar
Cooling to between 5°c and 7°c
Addition of fruit and packing

Pasteurization and homogenization

After the solids composition is adjusted, stabilizers are added and the milk is pasteurized. This step has many benefits. First, it will destroy all the microorganisms in the milk that may interfere with the controlled fermentation process. Second, it will denature the whey proteins in the milk which will give the final yogurt product better body and texture. Third, it will not greatly alter the flavor of the milk. Finally, it helps release the compounds in milk that will stimulate the growth of the starter culture. Pasteurization can be a continuous-or batch-process. Both of these processes involve heating the milk to a relatively high temperature and holding it there for a set amount of time. The milk is homogenized and the fat globules in the milk are broken up into smaller, more consistently dispersed particles. This produces a much smoother and creamier end product. In commercial yogurt making, homogenization has the benefits of giving a uniform product, which will not separate. Homogenization is accomplished using a homogenizer or viscolizer. In this machine, the milk is forced through small openings at a high pressure and fat globules are broken up due to shearing forces. 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.

To ensure that the "starter culture" has least competition from other organisms the milk is heat treated to kill undesireable organisms in the milk.

The heat treatment also has a physio-chemical effect on the proteins and other additives in 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.

The inoculation and fermentation usually takes place in sealed hygienic stainless steel vessel. The temperature will be monitored and maintained at the optimum for the starter culture throughout the fermentation. The levels of lactic acid is measured and monitored throughout the fermentation and the fermentation is 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.

Fermentation

The milk is cooled to between 109.4-114.8° F (43-46° C) and the fermentation culture is added in a concentration of about 2%. It is held at this temperature for about three to four hours while the incubation process takes place. During this time, the bacteria metabolizes certain compounds in the milk producing the characteristic yogurt flavor. An important byproduct of this process is lactic acid.

The lactic acid level is used to determine when the yogurt is ready. The acid level is measured by taking a sample of the product and titrating it with sodium hydroxide. A value of at least 0.9% acidity and a pH of about 4.4 are the current minimum standards for yogurt manufacture in

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. There is usually a slight reformation of the coagulm after the yoghurt has been packed

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 yogurt requires a different recipe to yogurt and usually consists of a thin yogurt blended with a high solids ice cream base mix - for technical assistance with this contact jw@dairyconsultant.co.uk

Freezing is achieved by pumping through a freezer in a fashion similar to ice cream. The texture of the finished product is mainly influenced by the freezer.

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

Factors that alter the quality of yoghurt

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

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:
  • Evaporation,
  • 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.

Stabilisers

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%

For more information or to discuss your requirements please contact us.
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John Watson
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jw@dairyconsultant.co.uk

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