Graduate Thesis Or Dissertation


Preparation, evaluation, and use of lyophilized, concentrated dairy starter cultures in cheese and yogurt manufacture Public Deposited

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  • Cultures of Streptococcus lactis and Streptococcus cremoris were grown in sterile 11% nonfat milk (NFM) at 32 C for 17 hr with the pH maintained at 6. 3 with sodium carbonate. The cells were harvested and resuspended in a medium of 5% gelatin, 5% sodium citrate, 2% monosodium glutamate and 10% sucrose (GCGS) to 1/20th the original culture volume. Suspensions were then frozen in a dryice- acetone bath and lyophilized to a residual moisture of 2%. The lyophilized culture produced was approximately 1/600th the weight of a conventional culture containing the same amount of cells. Vials containing the dry concentrate were sealed under vacuum and stored at -22° and +22° for periodic examination for cell viability and acidproducing activity. Rehydration was accomplished by the addition of a sterile solution of 20% lactose containing 1% protein hydrolysate (rehydration medium RM) which permitted maximum activity of the culture. Culture activity was assayed by simulating Cottage cheese and Cheddar cheese making procedures and measuring the final pH obtained. Appropriate dilutions of the lyophilized concentrate were made so a comparison could be made on a per cell basis with an actively-growing conventional culture. The final pH obtained for S. lactis C10 was 5.3 for both the dry and the conventional culture; this activity resulted in a period of 5 1/4 hr from milk inoculation to milling in Cheddar cheese making. Several cultures maintained activity values similar to the conventional culture for periods of up to 3 months. Cheddar and Cottage cheese were made in 800 lb vats with lyophilized concentrates and conventional cultures with results similar to ghat projected by the activity tests. This procedure was also applied to cultures commercially available as frozen concentrates with good activity retained in the lyophilized concentrate. Modifications of the above methods required for the production of lyophilized concentrates of the yogurt starter cultures included supplementation of the growth medium with 0. 1% Tween 80, growth at 37 C for 14 hr, and the use of 6% malt extract as a cryoprotective agent. With these modifications and proper culture strain selection, approximately 10% survival was observed with Lactobacillus bulgaricus and 25% for Streptococcus thermophilus. Low (30 C for 16 hr) and high temperature (42 C for 5.5 hr) yogurt manufacturing conditions were simulated for evaluation of culture activity. The activity test consisted of inoculating 100 ml of steamed (30 min) 13% NFM with equal volumes (0.25 ml of culture dried from 2. 0 -ml volume and rehydrated in 10.0 ml RM) of L. bulgaricus and S. thermophilus and measuring the final pH attained after 5.5 or 16 hr at the appropriate temperature. Lyophilized concentrates provided pH values almost as low (4. 4 to 4. 7) as those achieved with conventional yogurt cultures (4.4 to 4. 6), and required only an additional 15 to 30 min of manufacturing time. Lyophilized concentrates of flavor-producing cultures which included S. diacetilactis, Leuconostoc and Propioniibacterium retained a high level of viability (90 to 100%) with GCGS as the suspending medium. The Propioniibacterium could be grown either in milk or non-milk media without reduction of viability while Leuconostoc and S. diacetilactis required growth in milk, which in case of S. diacetilactis had to be supplemented with yeast extract. All lyophilized cultures had reduced viability when vacuum was lost during storage indicating a deleterious effect of oxygen. Rehydration medium was important in the reduction of osmoticallyinduced cell injury during rehydration and to provide peptides for required nitrogen metabolism. The cryoprotective requirements were not the same for all species studied and varied within strains.
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