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.

Defects of fermented milks

The sensory defects of fermented milks may be divided into the following groups:

•  Defects resulting in changes of appearance,
•  Defects of flavour and aroma
•  Defects of consistency, body and texture, and viscosity.

The defects of dahi and yoghurt are discussed here:

i. Defects resulting in changes of appearance

The most important defects affecting the appearance of yoghurt are presented here under:

Natural Yoghurt

Defect	Cause of Defect
Settled, whey separation	Separation of whey due to over acidification/ mechanical shaking of gel/ low solids content/ admixture of air in stored yoghurt.
Fermented	Contamination by yeasts and coliform organisms
Unclean appearance	Mud particles, dirt, etc.
Formation of colonies or film on the surface	Growth of yeasts or moulds
Aged, not fresh appearance	Formation of film on the surface due to drying up crystal structures on the surface due to freezing/ packages smeared over/ packages damaged/ lacking care during transport
Cream line	None or insufficient homogenization
Condensed water inside of lid of package	Great fluctuations in temperatures and air pressure

Additional Defects for Fruit- and Flavoured Yoghurts

Atypical colour	Addition of fruit juice for colouring or artificial fruit colouring
Pale colour	Discolouration of fruit colour due to heating/ too pale colour of flavouring and fruit base
Un-homogenous appearance	Insufficient stirring/ fault during stirring

ii. Defects of Flavour and Aroma

The most important defects affecting the flavour and aroma of yoghurt are presented

here under:

Natural Yoghurt

Defect	Cause of Defect
Feed	Feed flavour of raw milk
Bitter	Too long storage of yoghurt/ high proteolytic activity of cultures/ growth of proteolytic contaminants
Burnt (cooked)	Too severe heat treatment of the milk
Unclean sour	Cultures contaminated by wild lactic acid bacteria, coliform organisms, etc.
Yeast, fruit	Contamination by yeasts
Stale, empty (no aroma)	One-sided growth of the streptococci/ too short time for fermentation or too low temperature of incubation/ low aroma production/ low total solids content of the milk/ too strong slime production.
Mealy, gluey	Excessive addition of milk powder/ too high evaporation
Cheesy	Contamination by proteolytic organisms (e.g. moulds)
Oxidized flavour	Effect of light, Metal catalysator, Oxidized flavour spontaneously occurring after 1-2 days of cold storage (without the effect of light and metal traces).
High acid (over-sour)	Too high acidification during incubation and after that
Low acid	Too low acidification during incubation/ inhibitory substances present in milk/ bacteriophage attacking the yoghurt culture.

Additional Defects for Fruit- and Flavoured Yoghurts

Artificial flavour	The use of too high concentrations of fruit base and flavours/ artificial flavours.
Too sweet	Excessive addition of sugar
Atypical, flat	Addition of foreign stuffs affecting flavour/ blunting the fruit acids by neutralization/ sever heat treatment of the fruit base and flavouring concentrate/ faults in the manufacture of fruit concentrate.

iii. Defects of consistency and viscosity

The most important defects affecting the consistency and viscosity of yoghurt are presented here under:

Defects in the consistency of Set Yoghurt

Defect	Cause of Defect
Whey separation, settled	High acidification of yoghurt before cooling/ containers too early moved/ insufficient cooling/ strong acid producing cultures/ low heating temperature of the milk.
Soupy, soft	Low protein content of the milk/ too small inoculum of culture/ short time of incubation/ mechanical shaking of the gel before the completed coagulation.
Spitted, cleavage	Splitting of a spherical segment of the gel due to mechanical shaking.

Defects in the Viscosity of Stirred Yoghurt

Sandy	Contraction of the gel particles: Severe heating of the milk/ homogenization of the milk at too high temperature-pressure combination/ too much evaporation of the milk/ excessive addition of milk powder and/ or uneven stirring/ shaking during incubation i.e. too early moving yoghurt from the production line/ stirring the gel above 38oC, followed by holding above 38oC.
Slimy	Slime-producing contaminants/ slime production by yoghurt culture/ addition of slime-producing strains of yoghurt culture/ too low temperature of incubation of the yoghurt culture.
Gummy, gluey, tough	The use of unsuitable stabilizers/ high quantities of added stabilizers/ faulty incorporation of stabilizers.
Phase separation on containers (below whey, above gel)	Admixture of air: strong cooling/ pumps; stirring
Liquid	Too strong stirring the gel/ low solids content of yoghurt/ insufficiently concentrated flavours and fruit base, insufficiently cooled/ short time of cold storage/ culture of low slime production.

iv. Common Defects in Dahi

Defect	Cause of Defect
Green/ yoghurt flavour	Acetaldehyde accumulation
Oxidized flavour	Copper contamination and/ or exposure to fluorescent light or sunlight.
Yeasty/ Cheesy	Contaminating yeast growth
Rancid	High lipolytic activity
Insufficient flavour	Poor quality starter, low citrate level in milk
Acidic/ sour.	Excessive level of starter culture
Weak body	Insufficient heat treatment of the mix, milk SNF too low.
Grainy texture	Acidity too high, skim milk powder not properly stabilized.
Chalky/ powdery texture	Excess amount of skim milk powder, poor quality of powder.
Wheying off	Excessive acidity development

Factors affecting the keeping quality of fermented milks (yoghurt)

A number of factors can affect the keeping quality of yoghurt. Some of them are external, others are internal.

i. External Factors

i) Influence of Temperature: Unfavorable changes in the quality of yoghurt, which may occur during its storage, depend considerably upon the after-acidification ability of culture, the extent and type of contamination,the quality of packaging and the storage temperature above 0oC.

ii) Influence of Air: The atmospheric oxygen and a low relative humidity of air may contribute to some abiotic deterioration of yoghurt (oxidation,drying up of the surface). However, under practical conditions the effect of atmospheric oxygen has a secondary role in the spoilage of yoghurt.Also, the effect of the atmospheric relative humidity on the quality of properly packaged yoghurt with a storage life of 3-4 weeks practically has no significance.

iii) Influence of Light: Chemical changes in the milk fat occurring in the presence of gaseous oxygen and catalyzed by light, lead to the so-called oxidized flavour. It seems that the unstirred yoghurt is more sensitive to this defect than the stirred yoghurt.Under practical conditions the appearance of the oxidized flavour in yoghurt can be prevented by using the packaging materials with a reduced light permeability and by storing the product in dark.

iv) Packaging Material :A migration of the soluble substances from the packaging materials into the product may occur depending on the type of plastic materials, the time of contact with the product and the kind of fruit acids present in the product. Therefore, all packaging materials should be properly tested before use.

v) Period of Storage: The period of yoghurt storage before its consumption depends on the following factors: a) the interval of storage in a dairy plant (up to 4-6 days); b) the storage time in distribution channels (upto 7 days);c) the home storage by the consumer (upto 4-6 days). This means in total 2-3 weeks of storage time after the manufacture of yoghurt. Therefore, a reasonable effort should be made to produce yoghurt with a storage life of at least 3 weeks. This can normally be achieved in the careful manufacture.

ii. Internal Factors

i) After acidification of yoghurt: As mentioned, the metabolic activity of yoghurt organisms during the manufacture of yoghurt is considerably reduced by cooling after incubation. However, the final product undergoes after-acidification during its storage even at 0-5oC, because it is not possible to completely stop the enzymatic activity of lactic cultures through cooling.The rate of after-acidification of yoghurt depends upon the afteracidification ability of cultures, the rate of fermentation in bulk or in retail containers, cooling, the storage temperature and the initial pH value. The acid production of the streptococci at a pH of 3.9 to 4.3 and of the lactobacilli at a pH of 3.5 – 3.8. Therefore, the lactobacilli mainly contribute to acidification at pH value below 4.0. But at a pH above 4.0 both lactobacilli and streptococci contribute to after-acidification of yoghurt.

ii) The Consistency and Viscosity of Yoghurt: The consistency of set yoghurt and the viscosity of stirred yoghurt depend upon a number of factors. During cooling and storage of yoghurt for 48 hours after the manufacture, an improvement in its consistency, i.e. viscosity occurs. This phenomenon may occur due to the hydration of proteins, the structure solidification of the gel during cooling and eventual thyxotropie of the stirred yoghurt. However, quickly cooled yoghurt does not have optimum consistency, because the protein hydration and the structure solidification take a certain interval of time. Fruit yoghurt containing pectin-rich fruits shows a strong increase in its consistency during the first 10 days of storage due to the swelling of pectins.

iii) Flavour in Yoghurt: The flavour in yoghurt may be affected during storage.Practical observations show that during cooling of yoghurt and at the beginning of its storage, an improvement of flavour occurs probably due to the formation of specific aroma compounds, the structure solidification and the impression of freshness influenced by cooling. However, a significant after-acidification of yoghurt, which may occur during storage,can mask the flavour sensation.

Certain strains of the yoghurt culture active in the production of acetaldehyde, may reduce its flavour during storage. Other carriers of flavour such as fruit base, fruit concentrate do not impose problems in their stability during storage for 4-6 weeks. Also, the flavour in yoghurt may be affected by the migration of soluble matters from the packaging materials. The bitter taste of yoghurt can appear during storage as a result of the high proteolytic activity of cultures.

iv) Colour in Yoghurt:During the long term storage of fruit yoghurt, the colour of fruit base may become pale (e.g. strawberry and raspberry yoghurt).

The Cold Store

Cooling of yoghurt is carried out in the cold store which should fulfill the following requirements:

•  To allow the normal transport and stapling
•  To avoid any losses of cooling.
•  Lighting should be made by using special lamps with a little proportion of UVrays.

 Transport of Yoghurt and its Retail Distribution

Transport of yoghurt or other fermented milks imposes similar requirements in maintaining a refrigeration as those of storage. It is difficult to fulfill these requirements during warm days in the summer when the atmospheric temperature may rise to 40oC (e.g. in countries with a warm climate). It is recommended to maintain a refrigeration in the transport vehicles in two ways:

•  Large scale transport should be carried out in insulated vans with own refrigeration units.
•  Small scale transport can be done in cars with different possibilities for refrigeration (dry ice, liquid nitrogen, moist air, etc.).

 During retail distribution, the quality of yoghurt is influenced mainly by the mechanical shaking, the storage temperature and time of storage.

Storage and keeping quality of fermented milks

During the storage of fermented milks, various changes may occur such as microbial,enzymatic and abiotic spoilage. The spoilage of yoghurt is discussed hereunder:

i. Microbiological Spoilage

Microbial spoilage refers to an alteration of the outer appearance of yoghurt caused by micro-organisms. The culture of yoghurt does not cause this type of spoilage. On the other hand, contaminants capable to grow at lower temperatures and at a low pH, form colonies or film on the surface of yoghurt deteriorating its appearance. The microbial spoilage of yoghurt is presented in table . 

The microbial spoilage of yoghurt

ii. Enzymatic Spoilage

Enzymatic spoilage (Table ) is attributed to the activity of enzymes present in yoghurt. These enzymes derive from both the yoghurt culture and contaminants.Other enzymes such as milk enzymes or those originating from the microflora of raw milk, which eventual resist heat treatment of the milk have no practical significance.

Table  shows that contaminants are the main causative agents for the enzymatic spoilage of yoghurt. They adversely affect the taste and consistency of the final product.

The Enzymatic spoilage of yoghurt

iii. Abiotic (Chemical) Spoilage

The spoilage occurs without the activity of microorganisms and enzymes. Table  shows the abiotic spoilage of yoghurt (changes in protein, fat, lactose, vitamins,mineral salts and in additions such as fruit base or other flavorings) influenced by the gaseous oxygen and light.

The Abiotic spoilage of yoghurt