Doctoral Dissertations

Date of Award

5-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Food Science and Technology

Major Professor

James L. Collins

Committee Members

David A. Golden, Marjorie P. Penfield, Luther R. Wilhelm

Abstract

The objectives of the experiment were: (1) to prepare a soymilk mixture that could be fermented by a select type of starter culture to produce an acceptable level of lactic acid at 37 or 43°C within 7 hr, (2) to measure pH and acid levels of the soymilk mixtures during fermentation for 7 hr and during storage for 72 hr at 4°C and (3) to enumerate the counts of inoculated Echerichia coli O157:H7 and Listeria monocytogenesto determine their survival in the soymilk during fermentation and storage.

A soymilk mixture was prepared with water, soy flour (8.63%, w/v) and sugar (7.0%, w/v) and fermented with starter cultures YC-180 (Streptococcus thermophilus and Lactobacillus bulgaricus) or a combination of YC-180 and NU-A (Lactobacillus acidophilus) at 37 or 43°C for 7 hr. The total solids content of the water and soy flour mixture was 7.67%. These conditions comprised four treatments. The fermented soymilk mixture was placed in storage at 4°C and held for 72 hr. In Part I, during fermentation and storage, pH and total acidity (% lactic acid) were measured on each of the four treatments on each hour during fermentation and on 24-hr cycles during storage. In Part II, four treatments as described above were inoculated with starter cultures, fermented and stored; however, E. coli O157:H7 or L. monocytogeneswere added at the time the starter culture was added. Treatments in Part II were tested for pH and acidity and enumerated for CFU/g for the respective pathogenic bacteria and starter cultures.

In Part I, after 7 hr fermentation, pH (3.93) was lowest and acidity (1.02%) was highest for treatment 43-3 (43°C, YC-180 and NU-A starter cultures with 3 bacteria), and pH was highest (4.32) and acidity was lowest (0.63%) for treatment 37-2 (37°C, YC-180 culture with 2 bacteria). The pH and acid concentration did not change during storage. In Part 11, with E. coli O157:H7 or L monocytogenes, pH and acidity levels at the end of the fermentation period were similar to those in Part 1, with no changes occurring during storage. The correlation coefficient between pH and acidity over the fermentation period was always > -0.99.

Survival of E. coli O157:H7 was affected by all the experimental variables. At 4 hr of fermentation, a decline in the population occurred among the treatments. As fermentation continued, survival of E. coliO157:H7 in all treatments except 37-2 decreased rapidly. In treatment 43-3, counts were non-detectable (< log 2 CFU/g) at 5 hr. In treatment 43-2, counts were non-detectable at 7 hr, and in treatment 37-3, counts were non-detectable after 24 hr of storage at 4°C. After 72 hr of storage of treatment 37-2, the mean count had decreased from log 7 CFU/g to log 5 CFU/g. Correlation coefficients between the mean pH values and the mean CFU/g and between the mean acidity values and the mean CFU/g over the fermentation period were 0.6952 and -0.8177, respectively.

Survival of L. monocytogeneswas affected by all the experimental variables. By 4 hr of fermentation, changes began to appear in the population of the pathogen. After 7 hr of fermentation, the mean count for treatment 43-3 had decreased to log 2 CFU/g; survival in the other treatments was higher. In storage, the counts for treatment 43-3 decreased to non-detectable by 24 hr. Counts in the other treatments after 72 hr of storage were log 4.0 CFU/g or higher. Correlation coefficients between the mean pH values and the mean CFU/g and between the mean acidity values and the mean CFU/g during fermentation were 0.5850 and -0.7266, respectively.

In general, fermentation of soymilk mixtures at 43°C produced lower pH and higher concentration of lactic acid and was more effective in destroying both of the pathogenic bacteria than fermentation at 37°C. Addition of L. acidophilus to the inoculum caused greater destruction of the pathogenic bacteria than inoculum without L. acidophilus. E. coli O157:H7 was less able to survive under the conditions in the fermenting soymilk mixture than L. monocytogenes.

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