Enhancing the shelf life of fermented milk products

The storage life of yoghurt refers to maintaining its characteristics unaltered for sufficient period of time, i.e. until the product is consumed. Certain circumstances require the manufacture of yoghurt with considerably increased storage life, which in turn involve the use of corresponding methods.

i. Methods for Enhancing the Shelf Life of Yoghurt

Table  shows different methods for prolonging the storage life of yoghurt. They involve aseptic manufacture, biostabilization, preservation by heating, drying, freezing or addition of chemical agents. Microbiological spoilage is more or less restricted by using all methods. Enzymatic spoilage is controlled entirely or partially by pasteurizing the gel, drying, freezing or biostabilization.

A biotic spoilage is not essentially restricted by using any method. However, spoilage plays a slight role in the storage life of yoghurt within 4-6 weeks, when product is properly packaged and stored.

The following possibilities for using long life yoghurt are:

•  Countries with a warm climate.
•  When distribution of foods is difficult and time-consuming.
• Use in countries with suddenly occurring social-medical attention (hunger) protein-deficiency, etc.).
•  For addition in culinary preparations (sauces).
•  For yoghurt beverages which must have a rather long storage life.

 ii. Temperature and Time of Storage in Different Methods for Prolonging Shelf Life of Yoghurt

 At room temperature only pasteurized yoghurt can be stored for many weeks and dried yoghurt for many months.At refrigeration temperature the aseptic made yoghurt. with or with out gassing with CO2 of the head space in containers, has the storage life of 4 -6 weeks. Freezing yoghurt may prolong its storage for 3-12 months.

i) Aseptic Manufacture: As mentioned, the storage life of yoghurt is adversely influenced by contaminants such as yeasts and moulds and by the afteracidification ability of cultures. Therefore, aseptic manufacture should be combined by using culture of a mild after-acidification ability.

Manufacture with Special Aseptic Technique: The following requirements should be accomplished in the aseptic manufacture of yoghurt:

•  The yoghurt culture must be free of contaminants.
•  Yoghurt milk should be germ-free or at least vegetative germ-free
•  The closed production line must be used.
•  The production line must be designed for sterilization and aseptic operation.
•  Filling and capping to be carried out by using aseptic or semi-aseptic machines.
•  Containers for yoghurt should be germ free.

 ii) Aseptic Acidification: This method consists of inoculating the sterile milk under aseptic conditions with a special culture, followed by acidification,homogenization and aseptic packaging. Another possibility consists of inoculating the sterile milk (UHT sterilized), aseptic packaging (Tetra-pack or Brick-pack) and acidification in packages. The whole method of aseptic acidification can be applied in large-scale production of long life yoghurt.

iii) Hygienic Manufacture:This method involves the application of common hygienic measure in production phase (milk, cultures, inoculation, acidification,cooling, packaging) without the use of special aseptic equipment. Practical experiences show that it is possible in this way to obtain a reasonably good storage life of yoghurt. It is important to exclude as much as possible contamination by yeasts and moulds, as major causative agents of the spoilage of fermented milks.

iv) Biostabilization: Biostabilization is a method for prolonging the storage of yoghurt. It involves a regulation of the bacterial growth (streptococci-lactobacilli ratio), and the enzymatic activity (low after-acidification, reduction of proteolysis,inhibition of lipolysis). The above biotechnical measures should be supplemented with aseptic manufacture.

Biostabilization enables the production of long life yoghurt without heat treatment of the gel or the addition of chemicals. The main purpose is to prevent the acidification of the final product and to exclude air borne contamination by yeast and moulds.

v) Gassing:The filling with intent gas carbon dioxide or nitrogen of the head space in containers of yoghurt can improve the keeping quality of the finished product by inhibiting the growth of air borne contaminants such as yeasts and moulds.The packaging material must be sufficiently impermeable.However, this procedure shows a much better effect when it is combined with the hygienic or aseptic manufacture of yoghurt. According to the investigation CO2 treatment extends the storage life of yoghurt, made with the exclusion of contamination with moulds and yeasts by about 25 per cent.

vi) Chemical Preservation: This method refers to the addition of chemicals into foods in order to prevent the microbial spoilage (e.g. addition of sorbic acid or sorbates for inhibiting the growth of moulds). The concentration of sorbic acid used in making cultured milk products of prolonged storage life usually from 0.025 – 0.15 per cent. Yoghurt beverages of prolonged storage could be produced too by the addition of 50-100 per cent water to yoghurt after incubation together with sorbic acid, followed by homogenization, cooling, incorporation of CO2 and bottling.However, fermented milks including yoghurt are regarded as natural products which should not contain any foreign chemicals. Therefore, the above method for prolonging the storage life of yoghurt is not recommended for the practical application.

vii) Pasteurization of Yoghurt: Pasteurization of yoghurt after incubation aims to reduce microbial and enzymatic spoilage. By pasteurizing the yoghurt, organisms such as yeasts and moulds are destroyed completely together with the majority of the lactic acid bacteria. Since microorganisms in an acid medium are more susceptible than in a non acid medium, relatively low temperatures such as 60-70oC are bactericidal. The additional safety measure is either to fill the product into retail containers while hot or when pasteurized yoghurt has been cooled before its filling to use aseptic or semi-aseptic packaging machines. Thus, fresh product is changed to a preserved product with a storage life of many weeks.The main problem arises in the reduced consistency of yoghurt and losses of aroma during heating. The latter is slightly affected in flavored yoghurt.

viii) HF/UHF Multiples Frequency Method: This method refers to fast bioelectric treatment of the product filled into plastic cups by using high frequency and ultra high frequency frequent alternate electromagnetic field. Yeasts and moulds are killed and lactic acid bacteria “shocked” but remain viable. The storage life of yoghurt is increased to 6 weeks without refrigeration. The process is applicable to set and stirred yoghurt or similar fermented milk products.

ix) Prolonging the Storage Life of Yoghurt by the Artificial Acidification:

The partial or complete artificial acidification, carried out by the addition of edible acids to milk. This method can improve the storage life of product through shortening a time for the growth of contaminants during incubation, as well as through reducing after-acidification of the final product. Since the above method involves the manufacture of products which is not natural, it does not correspond to the desire of consumers.

x) Freezing Yoghurt: This method is only applicable for stirred yoghurt. In set yoghurt ice crystals damage the gel structure causing whey separation. Freezing stirred yoghurt is little used in dairy plants, although it was shown experimentally that storage could be considerably prolonged. Stirred yoghurt may be successfully frozen in a deep cold store at below – 18oC or at -26oC, provided that its total solids content is sufficiently high as is the case with fruit yoghurt (20-25%) and often with natural yoghurt (13-14%). Addition of stabilizers has a similar effect as high total solids. Rapidly frozen yoghurt may be stored at a low temperature, e.g. -26oC for 3 to 12 months. The defrosting should be carried out slowly in the cold store, e.g. at +5oC for 24-36 hours. The freezing does not affect the microflora but the surface of defrosted yoghurt has a less fresh appearance.

The following advantages of freezing yoghurt may be quoted:

a) The efficient prevention of spoilage; b) simple equipment; c) maintenance of the biological properties of yoghurt and its freshness; d) possibility to rationalize yoghurt distribution.

Disadvantages are: a) After defrosting the product loses its prolonged storage life (e.g. compared with pasteurized yoghurt); b) the closed deep-cold channels between dairies and consumers are required.

xi) Drying Yoghurt: Drying yoghurt is a very old method of the preservation of yoghurt. At present drying of yoghurt is carried out by using modern machines and processing. Dry yoghurt is particularly suitable for export to developing countries, for use in warm regions and in countries with social-medical indications such as hunger and protein deficiency. The use of yoghurt powder in the above regions usually involves the erection of small scale production plants for its reconstitution under hygienic conditions. The use of dried yoghurt for manufacturing other foods and preparation may be present to considerable extent, too.Drying of yoghurt can be carried out by two methods: a) Freeze drying; b) spray drying.

Freeze-Drying: Freeze-drying of fermented milks, including yoghurt, is carried out in a similar way as used with lactic cultures. This method is featured by removing water from the frozen product under a high vacuum. The freeze-dried yoghurt is packaged under vacuum into suitable containers. The final product is a fine powder which can be stored at room temperature until consumption. The reconstitution of dried product is carried out by adding the original quantity of water and stirring with the powder. The reconstituted freeze-dried yoghurt has a weaker consistency (the addition of stabilizers may be needed, e.g. alginates), the reduced flavour in natural yoghurt and lower numbers of the lactobacilli than the initial yoghurt from which it is made.Also, it is possible to make tablets using 46 per cent dried yoghurt, 50 per cent water soluble dried starch, 3 per cent tale and 1 per cent steering: the last three ingredients are sterilized before use. These tablets are usually used for the preservation of yoghurt cultures, as an alternative to the sealed ampoules. In tablets coated with paraffin wax, the flora retained its activity for 18 months, while non-waxed tablets lost their activity within 9 months.

Spray Drying:Spray drying of yoghurt applies the same principle in removing water as does drying of milk. As recognized, the drying time of milk is very short (about 30 sec) and high air temperatures are used, normally around 200oC. In the first phase of the spray drying process, an intense evaporation gives the advantage that the product receives a gentle thermal treatment, but in the final phase of drying, the product is exposed to the relatively high temperatures which may have an adverse effect on its quality (e.g. solubility).

In contrast to milk, spray-drying of yoghurt or other fermented milks is carried out using a specially mild thermal treatment. The powder must be removed from the drying chamber as rapidly as possible. Furthermore, it is recommended to the heatresistant strains of yoghurt bacteria, which can survive the spray-drying process. At present there are two methods of spray-drying yoghurt. One method involves concentrating the milk to 45-48 per cent total solids, homogenizing and cooling to 45oC, adding 1-15 per cent of cultures while stirring vigorously and spray-drying at a temperature of 55-60oC in the drying zone.

Another method involves concentrating the yoghurt previously prepared (a real yoghurt) and spray-drying at a temperature of 55-60oC in the drying zone. Yoghurt powder reconstituted has practically the same nutritive value as the initial yoghurt from which it is made. Due to the reduced lactose concentration, it is particularly suitable for lactase deficient people. The reconstituted spray-dried yoghurt has a considerably lower number of the live yoghurt bacteria than the initial yoghurt from which it is made. The surviving rate of yoghurt bacteria during the drying process could be expected to be about 20 per cent. However, when heat-resistant strains are used in making yoghurt, the surviving rate may be considerably higher. A weaker consistency of the reconstituted yoghurt can be improved by adding precooked starch or alginates.