What are starter concentrates?

Traditionally 'bulk starter' in liquid form was used to inoculate the milk used in the manufacture of cheese, yoghurt, buttermilk and other fermented products. Over the past 10-15 years, the use of starter cell concentrates designated as either Direct Vat Set (DVS) or Direct Vat Inoculation (DVI) cultures have increasing being used, particularly in small plants, to replace bulk starter in cheese manufacture. Note that the terms DVI and DVS are used interchangeably although particular culture suppliers will tend to use only one term.

In addition to these high activity cell concentrates, lower activity commercial cell concentrates have been used for many years to inoculate milk for bulk starter manufacture, and in the manufacture of 'long set products' that require extended incubation. This contribution while mainly concerned with DVS or DVI cultures will also comment on concentrates use in bulk starter manufacture.

Starter concentrates used in DVI cultures are concentrated cell preparations containing in the order of 10¹¹-10¹³ CFU/g. They are available as frozen pellets (fig. 1) or in freeze-dried granular form (fig. 2).

Under normal conditions starter growth in milk results in a cell concentration of about 10⁹ CFU/ml. Growth of starters in milk is limited by a number of factors including the accumulation of lactic acid. Concentrates can be produced by neutralisation (traditional fermentation technology) or removal of the lactic acid (using diffusion culture), recovering the cells by centrifugation, not required if diffusion culture is used, and by maintaining starter activity by freeze drying or freezing. Freeze-dried concentrates can be stored for some months at 4° C. Frozen concentrates are usually stored at -45° C or lower. Some suppliers recommend that their frozen DVI cultures are stored at -18° C.