Date of Award

1999

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

School of Nutrition and Food Sciences

First Advisor

Donal F. Day

Abstract

Lactobacillus casei subspecies rhamnosus grew on non-supplemented blackstrap molasses. The bacteria grew best at the optimal temperature of 40 +/- 2°C, and at pH 5.5. Addition of yeast extract to molasses shortened the lag phase and increased cell mass. In batch fermentations without pH control, the highest ratio of lactic acid produced to cell mass was 12 from non-supplemented molasses. For pH controlled batch fermentations, the ratio's of lactic acid produced/cell mass were 81, 79, 51 and 31 for 0, 0.2, 0.4 and 0.8% yeast extract supplementation, respectively. Non-supplemented molasses, 8% Brix, produced 28 +/- 2 g/l of lactic acid in batch fermentations, pH 5.5. Molasses clarification reduced lactic acid production by 53%. Inversion of the sucrose in molasses with 0.2% invertase resulted in lactic acid production of a molar conversion of 1.99, with no residual sugars. Invertase also acted as a better nutrient supplement than did yeast extract. L. casei subsp. rhamnosus was inhibited by L-(+) lactic acid at a Ki of 120 mM when grown on MRS media, and 360 mM on molasses. The bacteria were inhibited by sodium azide at a Ki of 2.1 mM. L. casei subsp. rhamnosus immobilized onto solid supports. At a dilution rate of 0.2/hr, the most lactic acid (14 +/- 1 g/l) was produced from non-supplemented molasses. Doubling the dilution rate halved the yield of lactic acid. Residual fermentable sugar in molasses was primarily sucrose. At dilution rates of 0.2 and 0.4/hr, nutrient supplementation of molasses did not enhance the production. A 3-stage mini-pilot plant, 5.4 liters working volume, was constructed. Most of the lactic acid (78%) was produced in the first segment of the bioreactor. Cost of carbohydrate and nutrient supplement of lactic acid production from batch fermentation was $0.35/kg, and from the immobilized bioreactors was $0.24/kg. If molasses is resold, the production cost of lactic acid from immobilized bioreactors drops to $0.075/kg.

ISBN

9780599474758

Pages

149

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