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Growth of yeasts in milk and associated changes to milk composition
1Department of Food Science and Technology, University of New South Wales, Sydney, Australia.
Abstract:
The growth of several yeast species in milk containing added sodium chloride (0-15%, w/v) at 25 degrees C and 10 degrees C was examined in conjunction with yeast metabolism of milk constituents. Depending on conditions, all yeasts grew to maximum populations of 10(7)-10(8) cfu/ml. Kluyveromyces marxianus gave strong utilisation of lactose and weak metabolism of citrate, protein and fat with the production of ethanol, glycerol, lactic acid and propionic acid. As measured by the production of free amino acids and free fatty acids, Candida lipolytica and Candida catenulata gave strong proteolytic and lipolytic reactions, the specificities of which appeared to be influenced by temperature and the presence of NaCl. These species also metabolised organic acids. Although giving strong growth responses, Debaryomyces hansenii and Saccharomyces cerevisiae did not metabolise lactose and gave only very weak lipolytic and proteolytic reactions. Citrate was metabolised by D. hansenii but not by S. cerevisiae. Both species produced small amounts of ethanol, glycerol and lactic acid.
Insights
This study details yeast growth and metabolism in salted milk, revealing species-specific utilization of lactose, fats, and proteins under varying temperatures and salt concentrations.
Area of Science:
- Food Microbiology
- Yeast Physiology
- Dairy Science
Background:
- Yeast plays a crucial role in dairy product fermentation and spoilage.
- Understanding yeast interactions with milk components under different environmental conditions is vital for food safety and quality.
Purpose of the Study:
- To investigate the growth dynamics of various yeast species in milk with added sodium chloride (NaCl).
- To analyze the metabolic capabilities of these yeasts concerning milk constituents like lactose, citrate, protein, and fat.
- To determine the influence of temperature (10°C and 25°C) and NaCl concentration on yeast growth and metabolism.
Main Methods:
- Culturing of selected yeast species (Kluyveromyces marxianus, Candida lipolytica, Candida catenulata, Debaryomyces hansenii, Saccharomyces cerevisiae) in milk with 0-15% NaCl at 10°C and 25°C.
- Monitoring yeast populations (colony-forming units per milliliter - cfu/ml).
- Assessing the metabolism of milk constituents through the measurement of end products (e.g., ethanol, glycerol, lactic acid, propionic acid) and the release of free amino acids and free fatty acids.
Main Results:
- All tested yeasts achieved significant growth (10^7-10^8 cfu/ml) under various conditions.
- Kluyveromyces marxianus efficiently metabolized lactose but showed weak utilization of citrate, protein, and fat.
- Candida lipolytica and Candida catenulata exhibited strong proteolytic and lipolytic activity, influenced by temperature and NaCl.
- Debaryomyces hansenii metabolized citrate, while Saccharomyces cerevisiae did not; neither utilized lactose effectively.
Conclusions:
- Yeast species exhibit distinct metabolic profiles in response to milk composition, temperature, and NaCl levels.
- Kluyveromyces marxianus is a significant lactose-utilizer, while Candida species are potent protein and fat degraders in salted milk.
- Debaryomyces hansenii and Saccharomyces cerevisiae show limited lactose metabolism and weak enzymatic activity on proteins and fats in this matrix.