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.