Date of Graduation

5-2015

Document Type

Thesis

Degree Name

Master of Science in Food Science (MS)

Degree Level

Graduate

Department

Food Science

Advisor

Ruben Morawicki

Committee Member

Julie Carrier

Second Committee Member

Griffiths Atungulu

Abstract

Several studies have verified that lactic acid bacteria are capable of directly producing lactic acid from starch. One of these bacteria, L. amylovorus, has been studied using corn, potato, and cassava starches. However, the use of sweet potato starch, a widely available starch, has not been considered. Furthermore, there exists the possibility that the efficiency of lactic acid production can benefit from the systematic exposure of bacteria to increasing concentrations of a particular starch. In this work, fermentation studies were conducted to understand the adaptation of L. amylovorus to rising concentrations of sweet potato starch. L. amylovorus was cultured in successively increasing ratios of starch to glucose as the carbon source in DeMan, Rogosa, and Sharpe (MRS) medium. The fermentations were conducted without pH control and with pH controlled at 6.5. At selected times over a 62-hour duration, optical density at 600 nm, pH, substrate consumption, and lactic acid concentration were measured to assess the growth and activity of L. amylovorus. The effects of the adaptation process were tested by comparing the growth and activity of bacteria that underwent the adaptation process with non-adapted bacteria that was grown in MRS medium in which glucose substituted with 20 g of sweet potato starch. Growth and activity assessments indicated that the bacteria were able to hydrolyze and ferment the sweet potato starch into lactic acid. In most cases, pH control resulted in better substrate utilization and larger amounts of lactic acid. The adapted bacteria produced 11.80 g/L (g LA/L fermentation broth) of lactic acid, nearly twice of the un-adapted bacteria (6.35 g/L) with no pH control. Under controlled pH conditions, 14.80 g/L and 4.20 g/L lactic acid were produced by adapted and un-adapted bacteria respectively. This concentration represented about 80% conversion of the starch into lactic acid.

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