Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Ibrahim, Dr. Salam A.


This study investigated the use of sweet potatoes (Ipomoea batatas) as a basic component to develop a medium for cultivation of lactic acid bacteria (LAB). Extract from baked sweet potatoes was used to form a sweet potato medium (SPM). SPM was supplemented with 4 g/L of each nitrogen source (beef extract, yeast extract, and proteose peptone #3). Lactobacilli MRS was used as a control medium. Ten LAB strains were used to determine the suitability of SPM serving the growth of LAB. Our results showed no significant (p < 0.05) differences in the optical density, maximum specific growth rates, and bacterial populations between MRS and SPM. SPM also maintained higher pH values throughout the incubation period compared to that in MRS. The cost of SPM was 47% less than the cost of MRS. Further step was taken to determine the suitability of SPM serving LAB enzymatic activity. LAB strains growing in SPM showed relatively higher β-glucosidases, acid phosphatase, and phytase activities and lower α– glucosidase compared to that in MRS. Strains of L. reuteri showed the highest enzymatic activities of α–glucosidase, acid phosphatase, and phytase whereas L. delbrueckii subsp. bulgaricus showed the highest β-glucosidases activity. Thus the enzymatic activity of L. reuteri growing in SPM was enhanced using six different metal ions. The response of L. reuteri strains to metal ions was found to be strain dependent. The addition of Mg2+ and Mn2+ followed by the addition of Ca2+ showed the highest enhancement effect on all tested enzymes. These findings indicated that SPM is a suitable medium serving the growth and bioactivities of LAB and thus could be used as an alternative low cost medium.