Culture Production, Distribution and Storage
Commercial culture preparation
Genetic techniques offer much opportunity to develop cultures with specific technological characteristics. However, at the commercial level, culture preparation is relatively simple.
- Lactic cultures are grown in buffered media to facilitate maximum growth without acid inhibition
- The cells are concentrated by centrifugation
- The cell concentrate is fast frozen or freeze dried (lyophilized). Frozen (-40C) or lyophilized cultures can be stored for several months without substantial loss of activity. Lyophilized cultures usually require a longer "lag time", i.e. time between inoculation and rapid cell growth.
Culture Practice in the Cheese Plant
Direct to the vat cultures need only be stored under prescribed conditions and opened and delivered to the vat under aseptic conditions. The following comments relate to the preparation of bulk culture at the cheese plant.
- Culture preparation should take place in a separate culture room which is kept at positive air pressure with hepa-filtered air (0.2 µm filter).
- All surfaces in the culture room must be of a material that can be sterilized.
- Use sterile pipettes and sanitize surfaces and equipment with 200 ppm chlorine.
- Alternative culture media are:
- Milk, but care must be taken to avoid rancid milk, mastitic milk, milk containing antibiotics, and milk with high bacteria counts.
- 10 -12% reconstituted skim milk powder is adequate provided that the powder is tested and certified antibiotic free.
- Whey and reconstituted whey powder may be used, but may not achieve the same cell counts as skim milk (due to less buffer capacity).
- A number of commercially prepared culture media are available. Most of these are based on milk protein powders.
- Culture media may be buffered with phosphates to increase cell counts but some cultures particularly Lactobacillus. bulgaricus appear to be inhibited by phosphates.
- Addition of phosphates also confers phage resistance because phosphates bind calcium, and phage require calcium to attach themselves to the bacterial cells.
- Calcium reduced skim milk powder and addition of anhydrous ammonia have also been used to inhibit phage in bulk cultures
- Culture media should be heated (at >88C for 1 h) to destroy bacteria and some inhibitory substances. Heating also reduces the redox potential (lowers oxygen concentration) which encourages the growth of LAB.
- Optimum pH endpoint before cooling is between 4.5 and 5.0. At pH less than 4.5 some cultures will pass from growth (log) phase to stationary phase and will be less active when added to the cheese vat.
- Cell count can be increased by:
- Internal pH control using buffered media
- External pH control by adding sodium hydroxide or ammonium hydroxide to maintain pH at 5.0 - 5.5.
- Generally cultures should be cooled to 4C after the desired minimum pH and cell counts are obtained. However, the optimum storage temperature depends on the particular culture. Consult with the culture supplier. For example, some thermophilic cultures should not be cooled below 20C. Storage time should be as short as possible, but I am aware of plants which successfully use a single bulk set culture for a week before making a new batch.