Commercial Starter Culture Preparations

Manufacturers of cultured dairy foods have several options for meeting their culture needs. The simplest (and usually most expensive) is to purchase frozen concentrated cultures that can be used to inoculate directly milk from which product will be manufactured. Using these ''direct-to-vat'' or ''direct-vat-set'' cultures avoids the possibility that starter culture will become contaminated with phage during preparation within the plant. Also, appropriate strain balance is assured. Alternatively, culture can be prepared at the plant. This culture, called bulk culture, can be prepared from commercially available frozen concentrated or freeze-dried cultures, or the inoculum can be prepared at the plant. Preparing inoculum at the plant involves starting with a ''mother'' culture maintained in small amounts (approximately 100 mL) of medium. The mother culture is used to inoculate successively larger amounts of medium (using a 1% inoculum) until sufficient inoculum volume is obtained to prepare the bulk culture. Preparing bulk culture inoculum at the plant carries an increased risk of phage contamination, so most plants purchase an inoculum either as a frozen concentrated or freeze-dried preparation. A new process for continuous production of mixed-strain lactic starter cultures employs immobilized cells in supplemented whey permeate medium. The advantages of this process are increased acid production and maintenance of strain balance (Lambboley et al. 1997; Sodini et al., 1998).

A. Frozen Concentrated Cultures

Frozen concentrated cultures contain 1010-1011 cfu/g, a sufficient concentration to allow 70 mL to inoculate 1000 L of medium for bulk culture preparation (Sandine, 1996). Preparation of frozen concentrated cultures involves (1) growing cultures under optimal conditions using pH control, (2) harvesting the cells via centrifugation or ultrafiltration, (3) standardizing the cell suspension to a specific activity, (4) adding a cryoprotectant, (5) packaging, and (6) rapid freezing using liquid nitrogen. The pH of the cell concentrate should be 6.6 for lactococci and 5.4-5.8 for lactobacilli (Stadhouders et al., 1971). There are many cryoprotective agents that can be used, including glycerol, monosodium glutamate, sucrose, and lactose (Mayra-Makinen and Bigret, 1993). Rapid freezing can also be accomplished using a dry ice-alcohol mixture (Sandine, 1996). The frozen concentrate should be stored at -196°C (liquid nitrogen) for best retention of activity, although storage at -40°C (dry ice) is also acceptable. Rapid thawing minimizes cell injury. This is accomplished by immersing the unopened can of cell concentrate in cool chlorinated water immediately before use.

B. Freeze-Dried Cultures

When transportation and storage of cultures at -40°C is not possible, freeze-dried cultures are a good alternative to frozen concentrates. Current technology can provide highly active freeze-dried cultures that, like some frozen concentrated cultures, can be added directly to milk in the cheese vat. The major disadvantage of using freeze-dried preparations in this manner is the longer lag phase they exhibit, adding an additional 30-60 min to the time required to make Cheddar cheese (Sandine, 1996). Freeze drying reduces the ability of a culture to utilize exogenous but not endogenous carbohydrates (Riis et al., 1995). Preparation of freeze-dried cultures is initially similar to that of frozen concentrates. After freezing, the culture concentrate is placed under high vacuum to dehydrate by sublimation. Usually 60-70% of the cells that survived freezing will survive the dehydration step of the freeze drying (To and Etzel., 1997). The dry cells are then packaged under aseptic conditions, preferably in the absence of oxygen. Exposure to oxygen rapidly damages the cells (Yang and Sandine, 1979).

C. Spray-Dried Cultures

Survival of cultures after spray drying is usually much lower than after freeze drying, because cells are simultaneously exposed to both thermal and dehydration stresses. The viability of spray-dried cultures depends on many factors including, growth conditions, age of culture, cell paste loading, processing, rehydration conditions, and cryoprotective used (Champagne et al., 1991).

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