Yogurt is made by heating, cooling and then fermenting milk using healthy streptococcus and lactobacillus bacteria. The final product can be consumed hot or cold. However, if heated past a certain point, the bacteria in yogurt will die.
The healthy bacteria in yogurt will die if exposed to temperatures above 130 F (54.4 C).
The Yogurt Production Process
Yogurt is a commonly consumed food that has been eaten for thousands of years. It has traditionally been made by culturing milk with live bacteria. These days, the production of most yogurt products is a bit more complex, since the yogurt you'll find sold in supermarkets is pasteurized. It's not always the yogurt itself that is pasteurized, though, but the milk that is used to make it.
This pasteurization process involves heating the milk at either 185 F (85 C) for 30 minutes or 203 F (95 C) for 10 minutes in order to kill any pathogenic bacteria. The pasteurized milk is then cooled before it is used to make yogurt.
The yogurt production process also involves heating and cooling. Most yogurt is made by heating milk to about 176 F (80 C), then cooling it to a temperature between 112 F and 115 F (44.4 C and 46 C). However, yogurt may be heated to as high as 200 F (93 C). The exact temperature and duration of time the yogurt is heated depends on how thick the final product is meant to be.
Heated and Hot Yogurt
If bacteria were added to yogurt while it was still hot, they would die. This is because the probiotic bacteria in yogurt are killed at temperatures above 130 F (54.4 C). They are consequently added only after the yogurt has cooled, then are allowed to ferment the product for four to seven hours.
Even though the Food and Drug Administration (FDA) requires yogurts to be made with active cultures, the products you find in supermarkets may not retain live and active cultures by the time they reach you. Some yogurts are also heat-treated after they've been cultured, which kills the bacterial cultures.
Heated Yogurt Products
Many shelf-stable food products contain yogurt. These products range from salad dressings to yogurt-covered cereal bars or candies. These products are often pasteurized again to increase shelf-life, which kills the healthy bacteria found in yogurt.
Similarly, any recipes that call for yogurt and involve heating will usually kill the healthy bacteria found in this food. However, if you're making a salad dressing or other unheated product, the bacteria in yogurt should remain active.
Influence of Bacteria on Yogurt
The healthy bacteria in yogurt have a variety of roles in this food product. Their primary role involves helping yogurt become more of a solid and less of a liquid. The four to seven-hour fermentation period results in a change in pH that increases the firmness of this product.
The bacteria in yogurt also influence this food's flavor. At this pH, yogurt tends to have a slightly tart but fairly neutral flavor. When allowed to ferment for longer periods, you can end up with an increasingly acidic, sour product. This is why yogurt is cooled to around 45 F (7 C) — this temperature stops the fermentation process.
Cold Versus Hot Yogurt
Although they will die at high temperatures, the bacteria in yogurt become dormant only when cooled below 98 F (36.7 C). They won't continue to ferment your yogurt or other food product — but remain viable. Bacteria that have been cooled or frozen will reactivate once ingested.
This means that, despite refrigeration or even freezing, live and active cultures can be found in many yogurt products. However, you should be aware that frozen yogurt is not regulated in the same way by the FDA as is regular yogurt. This means frozen yogurt is no more likely than regular yogurt to have live or active cultures in it.
Yogurt's Live and Active Cultures
The National Yogurt Association has a Live and Active Cultures Seal that can help you identify products that contain live, healthy bacteria in yogurt and yogurt-based products. Yogurt products that have been heat-treated or pasteurized a second time won't have this seal.
In order to receive the Live and Active Culture Seal, the National Yogurt Association's guidelines state that products must contain a minimum of 100 million live cultures per gram. The only exception to this are frozen products. Frozen products can have less cultures, a minimum of 10 million live cultures per gram.
Probiotic Bacteria in Yogurt
- Bifidobacterium bifidum
- Bifidobacterium lactis
- Bifidobacterium longum
- Enterococcus faecium
- Lactobacillus acidophilus
- Lactobacillus bulgaricus
- Lactobacillus casei
- Lactobacillus gasseri
- Lactobacillus plantarum
- Saccharomyces boulardii
Certain bacteria, like Bifidobacterium bifidum and Lactobacillus acidophilus, are the same type of bacteria that live in your gastrointestinal system. Ingesting probiotic foods with such bacteria will help healthy bacteria colonize your gut's microbiome. Other fermented foods, like miso paste, soy products and kimchi, have probiotics similar to those found in bio-yogurt.
The standard types of bacteria used to make yogurt, Lactobacillus bulgaricus and Streptococcus thermophilus., are good for your gut, but not considered to be probiotics. These bacteria don't reside in your gut's microbiome, but do help support the healthy bacteria that already live there.
- Dairy Council of Northern Ireland: How Is It Made? Yogurt
- International Journal of Scientific and Research Publications: The Evolution, Processing, Varieties and Health Benefits of Yogurt
- Colorado State University: Food Source Information: Yogurt
- Harvard Health Publishing: The Benefits of Probiotics Bacteria
- National Yogurt Association: Yogurt Varieties
- National Ocean Service: What Is an Extremophile?
- Behavioral Brain Research: Heat-Killed Lactobacilli Alter Both Microbiota Composition and Behaviour
- Journal of Microbiology and Biotechnology: Micronized and Heat-Treated Lactobacillus Plantarum LM1004 Stimulates Host Immune Responses via the TLR-2/MAPK/NF-κB Signalling Pathway in Vitro and in Vivo