PH is a comprehensive reflection of microbial metabolism and affects the progress of metabolism, so it is a very important parameter.
The pH is constantly changing during the fermentation process. By observing the pH change pattern, you can understand whether the fermentation is normal or not.
◆The relationship between pH value and microbial growth during fermentation
❶Interaction between pH value and microbial growth
•Affects the nature of the membrane surface charge and the permeability of the membrane, thereby affecting the ability to absorb substances.
•Change enzyme activity, the rate of enzymatic reactions and metabolic pathways: For example, yeast produces ethanol at pH 4.5-5, and glycerol and acid at pH 6.5 or above.
•Environmental pH also affects the degree of ionization of nutrients in the culture medium, thereby affecting nutrient absorption, or the toxicity of toxic substances.
❷Different microorganisms have different pH requirements
Three basic points of pH for biological growth:
•Various microorganisms have their minimum, optimum and maximum pH values for growth. When the pH value is below the minimum or exceeds the maximum growth pH, microbial growth is inhibited or death occurs.
•Different microorganisms have different optimal pH values for growth. According to the optimal pH value for microbial growth, microorganisms are divided into:
Alkaliphilic microorganisms: nitrifying bacteria, ureolytic bacteria, most actinomycetes
Alkali-tolerant microorganisms: many Streptomyces species
Neutral microorganisms: most bacteria and some fungi
Acidophilic microorganisms: Thiobacillus spp.
Acid-resistant microorganisms: Lactobacillus, Acetobacter
❸PH within microbial cells
•The pH value within microbial cells is quite stable, generally close to neutral.
•This characteristic of maintaining a stable neutral pH value in cells can maintain the structural stability of various bioactive molecules in cells and the optimal pH value required by intracellular enzymes. The optimal pH value of intracellular enzymes of microorganisms is generally neutral, and the optimal pH value of extracellular enzymes is close to the environmental pH value.
❹The impact of microbial life activities on environmental Ph
Microorganisms will also change the pH value of the external environment during the growth process. The reasons are:
As organic matter decomposes:
Decomposes sugars, fats, etc. to produce acidic substances, which lowers the pH value of the culture solution;
Decompose protein, urea, etc. to produce alkaline substances, which increases the pH value of the culture solution
◆Measures to adjust pH value during culture
When overacidic: add alkali or an appropriate amount of nitrogen source to increase ventilation.
When it is too alkaline: add acid or an appropriate amount of carbon source to reduce ventilation