Culture quality is of utmost importance in the production of high-quality cultured dairy products. Culture manufacturers often work closely with processing facility operators to effectively meet specific needs. Culture manufacturers, through significant research efforts, develop unique culture combinations for a given product. For instance, all yogurt contains the thermophilic cultures Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, but the specific strains of each are carefully combined in order to obtain particular flavor, body, and texture characteristics in yogurt. Specifically, some strains, such as the so-called ropy strains of L. delbrueckii subsp. bulgaricus or S. thermophilus that produce exopolysaccharides, may be selected for their ability to add viscosity to yogurt (19,20).
Several types of culture forms are available, including (a) liquid (for propagation of mother culture; rarely used today), (b) deep-frozen concentrated cultures (for propagation of bulk starter), (c) freeze-dried concentrated cultures in powder form (for propagation of bulk starter or DRI-VAC, for preparation of mother culture), and (d) deep frozen or freeze-dried, super-concentrated cultures in readily soluble form for direct inoculation of the product (direct vat set or DVS). The use of frozen or freeze-dried cultures eliminates the need for small dairy plants to make cultures or operate a culture room (21). Larger plants are typically supplied with frozen or freeze-dried cultures for the manufacture of bulk starters in aseptic bulk culture rooms (21).
DVS cultures are so easy to use that they are commonly utilized today, but it is useful to describe the procedure for preparation of bulk starter. To begin, an understanding of the terminology is necessary. Commercial culture is the original culture obtained from the culture manufacturer. Mother culture is the culture prepared from the commercial culture, at the processing facility. An intermediate culture may be made for large volumes of bulk starter. Bulk starter is the starter used in production. The production of mother culture, intermediate culture and bulk starter are essentially the same. The steps include (a) heat treatment of the skim milk (or reconstituted skim milk with 9-12% dry matter) medium, (b) cooling to inoculation temperature, (c) inoculation, incubation, (d) cooling of the finished culture, and (e) storage of the culture (1). Heat treatment to 90-95°C for 30-45 min destroys microorganisms and bacteriophages, denatures proteins, and expels dissolved ozy-gen, thus improving the properties of the medium for culture propagation (1). Inoculation temperature and inoculum level are defined by culture manufacturers, but typically inoculation temperature ranges are 20-30 °C for mesophilic bacteria and 42-45 °C for thermophilic bacteria (1). Cooling and storage conditions and shelf lives of cultures vary. Generally, deep-frozen and freeze-dried cultures can be stored for at least 12 months at — 18°C and —■45°C, respectively (1). It is of utmost importance for consistency to be exercised in culture handling if a consistent product is desired.
A separate room in the dairy plant for preparation and propagation of starters is one important element in production of quality yogurt because it limits opportunities for contamination by airborne yeast, mold, and bacteriophages (1). Bacteriophages are essentially viruses that infect specific cultures and cause failure of lactic acid production. Each strain of culture has a different level of sensitivity to bacteriophage. Because it is impossible to entirely eliminate bacteriophage from a dairy plant operation, control measures must be employed. Aseptic techniques for propagation of starter cultures, sterilization of air and equipment, culture rotation of phage-unrelated, or use of phage-resistant strains is necessary to control phage (22).
Specific culture combinations, incubation temperature, and inoculation rates utilized in yogurt and sour cream production will be discussed in Secs. III and IV, independently.
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