Lactic acid bacteria (LAB) create delicious flavors and tangy textures in dairy products like cheese, yogurt and fermented milk. These beneficial microbes maintain gastrointestinal (GI) and immune health. Food artisans use diverse strains of bacteria for unique taste and textures.
Known for their ability to ferment sugars, lactic acid bacteria are used to preserve food and improve flavor in gastronomy. Common genera include Lactobacillus in yogurt production, Streptococcus in cheese making and Lactococcus for buttermilk.
These bacteria are non-spore forming and catalase-negative. They thrive in anaerobic or microaerophilic environments. Catalase-negative bacteria might be anaerobes or facultative anaerobes; they exclusively ferment, and don't use oxygen for respiration.
Step 1: Gather Materials & Ingredients
These include:
Samples of dairy products such as yogurt, milk and cheese
Nutrient agar plates MRS agar is ideal for LAB
Sterile Dilution Medium: Peptone water or saline solution
Sterile inoculating loops or swabs, petri dishes, pipettes
Incubator set at a temperature range of 30-37°C (86-98.6°F), perfect growth temperature for Lactobacillales and other industrious bacteria.
pH Meter, optional, for measuring acidity
Microscope for further strain identification
MRS agar is short for De Man–Rogosa–Sharpe agar, a selective culture medium to cultivate prosperous proliferation of Lactobacilli for lab study. This agar has a clear brown color.
It contains sodium acetate, which suppresses growth of competing bacteria. Other Lactobacillales, like Leuconostoc and Pediococcus, might grow however, for better or worse.
Step 2: Preparing Nutrient Agar Plates
When samples are collected prepare nutrient agar plates. Using MRS agar is best because it is formulated specifically for LAB growth. Pour the agar into sterile Petri dishes and allow it to set completely. Once solidified, it's ready for inoculation.
Step 3: Inoculation
Using a sterile inoculating loop or swab, take a small amount from each dairy sample and carefully streak it onto the agar plates. It’s recommended to label each plate according to the sample type (yogurt, milk, cheese) to keep track of progress and microbe behavior.
Step 4: Incubation
After inoculating the plates, set them in the incubator temperature. Check temp to be sure it's between 30-37°C (86-98.6°F). This temperature range is ideal for LAB growth.
Now it's time for patience. Leave the plates to incubate for 48 hours. During this period, colonies begin to emerge.
Step 5: Identification of LAB Colonies
After incubation, closely inspect the agar plates. LAB colonies appear as small and circular. They're creamy white or opaque spots.
To further identify the LAB, take a small sample from each distinct colony and do a Gram stain. Typically, LAB will show up as Gram-positive cocci or rod-shaped bacteria under the microscope. They're too tiny to be seen with the unaided eye.
Step 6: Subculturing and Preservation
When the desired LAB strains are identified, subculture the keepers. This means transferring a loop or swab of the identified colony into new MRS agar plates for further growth.
For long-term preservation, freeze bacterial strains in glycerol stocks at -80°C or store them at -20°C. Strains stored at -80°C can maintain viability for up to 10 years.
Facts About Lactic Acid Bacteria
Health Benefits: LAB are often referred to as probiotics, contributing to gut health and supporting immune function. They can help in the prevention of certain gastrointestinal disorders.
Fermentation Process: During fermentation, LAB convert lactose (the sugar found in milk) into lactic acid. This not only preserves the product but also lowers the pH, making it an inhospitable environment for spoilage organisms.
Diversity: There are over 100 species of LAB, with each having distinct fermentation profiles. For instance, Lactobacillus bulgaricus and Streptococcus thermophilus are commonly used in yogurt production, while Lactococcus lactis is essential for cheese-making.
Cultural Significance: LAB are integral to many traditional dairy products worldwide. For instance, Kefir from the Caucasus region and Labneh, a strained yogurt popular in the Middle East, have unique flavors attributed to local LAB strains.
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