Lactic Acid Fermentation

Lactic Acid Fermentation and the effects of live culture incubation in milk and yogurt

BIOL 1506E

Thursday Session

November 19th 2015

Introduction

In conditions with very little to no oxygen, cells will acquire much of their energy from anaerobic respiration. Under anaerobic conditions, eukaryotic cells undergo a form of cellular respiration called fermentation, which is quite similar to glycolysis. (Morton 1980). There are two types of fermentation, alcoholic fermentation and lactate or lactic acid fermentation (Russell et al 2015). The difference being, in lactic acid fermentation, the pyruvate obtained from glycolysis is converted into the three carbon molecule lactate, while alcoholic fermentation the pyruvate is reduced to ethyl alcohol, along with CO2 being released as a byproduct (Russell et al 2015). Fermentation can be shown by the following equations (Russell et al 2015):

Alcohol fermentation:

2 CH3-CO-COOH + 2 NADH + 2H+ -> 2 CH3-CH2-OH + 2 CO2 + 2 NAD+

(pyruvate)                                         (ethanol)

Lactic acid fermentation:

2 CH3-CO-COOH + 2 NADH + 2H+-> 2 CH3-CHOH-COOH + 2 NAD+

(pyruvate)                                         (lactic acid)

Lactic acid fermentation is found in some plant and animal tissues (Russell et al 2015), as well as several bacteria such as Streptococcus thermophilus and Lactobacillus bulgaricus. In terms of economical purposes, lactate fermentation directly influences the range of food available today, such as the bacterial cultures present in yogurt and the yeast in bread (Russell et al 2015).

Methods and Materials

Please refer to the Biology Lab Manual (Brosseau-Demore and Mallory 2015)

Results

[pic 1]

Table 1 exhibit the change in pH and texture/appearance in the milk and milk + yogurt samples after incubation (Brosseau-Demore and Mallory 2015).

[pic 2]

        The two bacteria above are Streptococcus thermophilus (left) and Lactobacillus bulgaricus (right). The Streptococcus takes on the coccus shape, while the Lactobacillus takes on a rod shape. The two bacteria present are considered symbiotic and thermophilic (Brosseau-Demore and Mallory 2015).