Microbial Interactions

The interactions of mixed cultures are of great interest in studying the kinetics of population development in liquid

Table 1. The Principal Fermentation Reactions in Foods

Lactic acid fermentation

Homofermentative: C6H1206 (glucose) to 2CH3CHOHCOOH (lactic acid)

Heterofermentative: C6H1206 (glucose) to CH3CHOHCOOH (lactic acid) + C02 + C2H5OH (ethyl alcohol)

Propionic acid fermentation

3C6H1206 (glucose) to 6CH3CHOHCOOH (lactic acid) 3CH3CHOHCOOH (lactic acid) to 2CH3CH2C02H (propionic acid) + CH3COOH (acetic acid) + C02 + H20

Citric acid fermentation

CH2COOHHOCCOOHCH2COOH (citric acid) to 2CH2COCOOH (pyruvic acid) to CH2COCHOHCH3 (acetylmethylcarbinol) + 2C02

Acetylmethylcabinol can be oxidized to CH3COCOCH3 (diacetyl) or reduced to CH3CHOHCHOHCH3 (2,3, butylene glycol)

Alcoholic fermentation

C6H1206 (glucose) to 2C2H5OH (ethyl alcohol) + 2C02

Butyric acid fermentation

C6H1206 (glucose) to CH3COOH (acetic acid) + CH3CH3CH2COOH (butyric acid) + CH3CH2OH (ethyl alcohol) + CH3(CH2)2CHOH (butyl alcohol) + CH3COCH3 (acetone) + C02 + H2

Gassy fermentation

2C6H1206 (glucose) + H20 to 2CH2CHOHCOOH (lactic acid) + CH3COOH (acetic acid) + C2H5OH (ethyl alcohol) + 2C02 + 2H2

Acetic acid formation (oxidative)

C2H5OH (ethyl alcohol) + H20 02 to CH3COOH (acetic acid)

and solid fermented foods. The following is a discussion of the study on the interactions between an important fermentative yeast (S. cerevisiae) and an important industrial bacterium (Acetobacter suboxydans) and an important environmental contaminant (Escherichia coli).

Figure 1 shows the interaction between S. cerevisiae and A. suboxydans. For the yeast, both direct count (under the microscope) and electronic count (Coulter Electronic Counter, which can differentiate particles sizes) increased with time. The direct count registered about 10 times more cells than the electronic count. For the bacterium, the direct count increased with time, but the electronic count increased to about 7.8 log cell/mL and then started to decrease. This decrease in number is due to the adhesion of many of the bacterial cells to yeast cells. The electronic counter cannot differentiate these two populations when they are in clumps. However, microscopic observations can ascertain the different counts. The concentration of glucose completely disappeared in the first 10 h of yeast and bacterial interactions. The pH of the medium first decreased and then stabilized at pH 3. This is due to oxidation of alcohol (produced by S. cerevisiae) by A. suboxydans.

Figure 1. Interaction of mixed cultures of S. cerevisiae and A. suboxydans in terms of cell numbers monitored by direct count and electronic count as well as product formation (alcohol and acid production). Source: Ref. 3, used with permission.

Figure 1. Interaction of mixed cultures of S. cerevisiae and A. suboxydans in terms of cell numbers monitored by direct count and electronic count as well as product formation (alcohol and acid production). Source: Ref. 3, used with permission.

The interactions of yeast and E. coli (Fig. 2) showed interesting contrasts compared with the yeast-Acetobacter interactions. Growth of yeast as monitored by viable cell count, direct cell count, and electronic count all increased with time. After reaching a stationary phase, viable yeast count decreased, but direct count and electronic counts registered no reduction in numbers. A likely explanation is that the latter two biomass measurements counted both live and dead cells, but the viable cell count method only registered living cells. After the stationary phase, some of the yeast cell went through the death cycle. The growth of E. coli showed an increase in the viable cell count and direct cell count. The electronic count reached 8 log cells/mL and then declined, exhibiting a trend similar to that observed in the yeast and Acetobacter interactions. Alcohol contents increased to about 2%, and the pH first dropped to around 4 and then rose to 6. This pattern differed from that obtained from the interaction of yeast and Acetobacter. E. coli cannot oxidize alcohol, and this did not create large amounts of acid to counteract the basic metabolites in the reaction vessel. This resulted in a medium that reverted to a more alkaline state.

This study indicated that different methods of estimating biomass may provide different results in mixed culture fermentation. The electronic counting method is useful when a single culture is monitored. In mixed culture interaction that involves clumping, this method is not suitable for differentiating mixed populations.

The following sections are synopses of major fermented food. Detailed treatment of these subjects can be found in books listed in the reference section.

Figure 2. Interaction of mixed culture of S. cerevisiae and E. coli in terms of cell numbers monitored by direct count, electronic count, and viable cell count as well as product formation (alcohol and acid production). Source: Ref. 3, used with permission.

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