Yogurt is enjoyed throughout the world, created by the natural industry of lactic acid bacteria (LAB). Transformation of milk into yogurt is a network of biochemical interactions featuring the unique characteristics of LAB strains.
Lactic Acid Bacteria
Lactic acid bacteria are a diverse group of microorganisms. They can ferment lactose, the sugar found in milk. When LAB are introduced to milk they metabolize lactose, producing lactic acid as a byproduct.
This increase in lactic acid lowers the pH, thickens the milk and gives yogurt its characteristic tangy flavor. The most commonly used LAB strains in yogurt production are Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus.
These work synergistically. While S. thermophilus acts quickly to acidify the milk, L. bulgaricus contributes to development of flavor and texture.
Yogurt-Making Process
1. Preparation of Milk
The first step is to select and prepare the milk. Whole, low-fat, or non-fat milk can be used, depending on desired final product. The milk is heated to around 85°C (185°F) to denature the proteins and create smooth texture by preventing curd formation during fermentation.
Heating also pasteurizes milk to eliminate bacteria, including the lactic acid bacteria already present, in about15 seconds. Bacterial death occurs at c. 71°C (160°F). After the temperature is raised, it's quickly cooled.
2. Cooling
The milk is cooled down to about 43-45°C (110-113°F), which is the optimal temperature for growth of new lactic acid bacteria. This temperature range allows for the best fermentation conditions. Quick cooling of milk prevents growth of unwanted microbes.
3. Inoculation with Starter Culture
Once the milk is cooled, a starter culture containing live LAB is added. This can be a commercial yogurt with active cultures or a specific freeze-dried starter.
Starter strains usually include a blend of LAB cultures, such as Lactobacillus bulgaricus and Streptococcus thermophilus. The inoculated milk is mixed thoroughly to ensure an even distribution of bacteria.
4. Fermentation
The inoculated milk is kept at a warm temperature 44°C (110°F) for 4 to 12 hours, depending on the desired thickness and bite. During this stage, the bacteria multiply and convert lactose, resulting in a thickened product with a tangy flavor.
The tangy bite is caused in the natural process as LAB lowers the pH, creating a more acidic environment. This causes milk proteins to coagulate and form a creamy thickened product.
Adjusting the fermentation time can yield different taste profiles.
5. Cooling and Storage
After fermentation, the yogurt is cooled again to stop the fermentation process and stabilize texture. It can then be flavored or sweetened as desired. Stored in the fridge, it continues to develop benefits and flavor and lasts several weeks.
Avoid storing it in the door of the fridge as this is most conducive to warming and mold growth. Eating from the same container it's stored in can cause cross-contamination and fuzzy filamentous visitors. Unlike cheese, if yogurt shows mold it's spoiled and must be tossed.
LAB Strain Differences
While S. thermophilus and L. bulgaricus are the standard LAB strains for yogurt, numerous others are used to create different varieties. These include
Lactobacillus acidophilus: Often added for its probiotic properties, this strain is known to promote gastrointestinal (GI) tract health.
Lactobacillus casei: Known for its ability to thrive in a wide range of pH levels and temperatures, it is often used in functional yogurts aimed at enhancing digestive health.
Lactobacillus bulgaricus: This essential strain brings a creamy texture and tangy flavor to yogurt. It’s often appreciated for balancing taste.
Streptococcus thermophilus: Works synergistically with Lactobacillus bulgaricus, improving the fermentation process and adding smoothness.
Bifidobacterium bifidum: Another probiotic strain to increase health benefits of yogurt, especially GI microbiota balance.
Facts About Yogurt
Historical Significance: Yogurt has been consumed for thousands of years, with evidence of its existence dating back to 5500 - 6000 BCE in Central Asia.
Health Benefits: Yogurt is rich in probiotics, which aid in digestion, enhance immune function, and contribute to overall GI tract health.
Versatile Ingredient: Yogurt can be used in cooking and baking, from sauces, marinades and dressings to smoothies and desserts.
Cultural Variations: Different cultures have their own versions of yogurt, such as the Indian lassi, the Middle Eastern labneh, and the Greek strained yogurt, each with unique preparation methods and flavors.
Low Fat or Non-Milk Varieties: Today, yogurt comes in many low-fat and dairy-free variants made from almond, soy, coconut or oat milk.
Other Heath Benefits: Yogurt is also rich in calcium and protein. One cup of plain yogurt contains about 30% of the daily value of calcium, necessary for bone health.
The yogurt-making process exemplifies the harmonious interplay between lactic acid bacteria and milk. This dynamic not only results in a nutritious and delicious product but also produces diverse flavors and textures.
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