Yogurt

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Key Factspending-external-review

  • Yogurt is made from milk cultured with live bacteria.
  • Yogurt is consumed in a variety of ways, including Greek yogurt, drinkable yogurt, and frozen yogurt.
  • The use of pasteurized milk is a key barrier to foodborne pathogen transmission in yogurt products. Raw milk can contain pathogens such as Escherichia coli, Salmonella spp., and Campylobacter jejuni. The acidity of yogurt is another barrier to foodborne illness. There is evidence of E. coli 0157:H7 exhibiting acid-tolerant properties, but this pathogen is readily destroyed via pasteurization.
  • Yogurt products have previously been associated with fungal disease.

Introduction

Yogurt has been a staple food product for numerous cultures throughout the world dating back many thousands of years. In the Middle East, primitive herdsman began carrying milk in containers made from intestinal gut lining, which they discovered could help extend the life of milk products because contact with the intestinal fluids of the containers caused the milk to curdle and sour, preserving it for an extended period. Other than drying, this was historically the only safe method of preserving milk.

Modern yogurt production involves culturing milk with live bacteria. The bacteria produce lactic acid which coagulates the milk proteins, making yogurt thick and slightly sour in flavor. The bacterial cultures required for producing yogurt are Streptococcus thermophilus and Lactobacillus bulgaricus. About 80% of all yogurt manufactured in the U.S. contains an additional culture called Lactobacillus acidophilus, and many commercial yogurt products also contain Bifidobacterium bifidum or Lactobacillus casei because of their potential health benefits.

Over the millennia, a multitude of health benefits have been attributed to eating yogurt. As for back as 6000 BCE, Indian Ayurvedic medicine made reference to the positive health benefits linked to yogurt consumption. Yogurt has been used in the treatment of everything from a variety of gastrointestinal maladies to sunburn relief. In the early 20th century, it was even sold in pharmacies as a medicine. Today, yogurt is promoted as a healthy “probiotic” food. The benefits of incorporating probiotic foods such as yogurt into the diet have been widely documented, and there is emerging research to suggest yogurt may have positive implications for improved gastrointestinal health and overall immune function.

Yogurt can be consumed directly, but it is also commonly used to make dips and dressings and is used as a lower-calorie substitute in cooking. Drinkable yogurt and frozen yogurt are also widely consumed yogurt products.

Types of Yogurt

The U.S. Food and Drug Administration (FDA) regulates and inspects commercial yogurt products. The FDA also sets the following guidelines for labeling yogurt:

  • Regular yogurt must be no less than 3.25% fat and contain no less than 8.25% milk solids.
  • Low fat yogurt must be between 0.5 to 2% fat and contain no less than 8.25% milk solids.
  • Non-fat yogurt must be less than 0.5% fat and contain no less than 8.25% milk solids.

Other terms used to describe yogurt are based on differences in processing:

Yogurt
Half heavy cream, half whole milk yogurt.
  • Set (equivalent to sundae or fruit-on-the-bottom): This is the firmest type of yogurt. During processing, the yogurt is set in a container and is left undisturbed. Fruit can be added to the bottom of the container so that, when turned upside down, it resembles a sundae.
  • Swiss (equivalent to stirred, custard, or blended): Most commercial yogurts are Swiss style. After the yogurt is set, it is stirred and dispensed into secondary containers, making it less firm than set yogurt. Fruit can also be stirred in to produce flavor varieties.
  • Liquid or drinkable: Liquid yogurt has milk, fruit, and/or fruit syrups added for increased fluidity and flavor. The pH of yogurt is raised when milk is added, so the shelf life of drinkable yogurt is generally only 4–10 days.
  • Yogurt cheese or strained yogurt: Yogurt is strained to remove liquid whey, resulting in a thick, creamy, concentrated product.
  • Frozen yogurt: To make frozen yogurt, regular yogurt is mixed with a pasteurized ice cream mix of milk, cream, and sugar. Other ingredients, such as stabilizers, fruit, and flavors are blended together and then the mixture is frozen. In some cases, frozen yogurt contains live bacteria; the bacteria become dormant when cooled, but regain activity post-ingestion due to the warm body temperature.
  • Greek yogurt: Greek yogurt contains about twice the protein and half as much carbohydrates and sodium as regular yogurt. During processing, the yogurt is strained three times, as opposed to two times with regular yogurt, which yields the higher protein content and creamier texture. There are also low-fat and fat-free options for Greek yogurt.
  • Lite or light: Yogurt that has 50% less fat or one-third fewer calories than regular yogurt is considered light.

Foodborne Outbreaks

In 2013, an FDA investigation into complaints of gastrointestinal illness found that commercial yogurt products sold by the brand Chobani had been contaminated with the fungi Mucor circinelloides. Symptoms, including vomiting, nausea and diarrhea, were reported by more than 200 consumers who had ingested the associated yogurt products. The company voluntarily pulled the products connected to the reported illnesses from the market. The risk linked to fungal pathogens is not well understood, but M. circinelloides may cause spoilage in yogurt, and it poses a particular risk to the immunocompromised.

Yogurt (along with ice cream) was one of the vehicles of infection named in a 2007 outbreak of Hepatitis A which was determined to have been spread by a food-handler in a Minnesota restaurant. Fifteen people were reported ill and six were hospitalized; no deaths were reported. A Norovirus outbreak involving frozen yogurt and ice cream occurred in 2004 during a school fundraiser in Arizona. Norovirus was determined to be the source of the illnesses after an employee who had handled the machine tested positive. 53 fundraiser attendees reported being ill; no one was hospitalized, and no deaths were reported.

Interestingly, yogurt is often recommended as a natural aid in returning gut after contracting a foodborne illness.

To contribute to the Yogurt Foodborne Outbreaks section, please follow this link: https://fsi.colostate.edu/suggest-a-topic/

Production

U.S. yogurt production in 2017 totaled 4.5 billion pounds at 170 processing plants. New York leads the nation in yogurt production, accounting for 15.7% of the total amount of yogurt produced in the United States. California is the second largest producer of yogurt.

The production of yogurt requires only two ingredients: milk and live cultures. However, producers may also include, dry milk powder, stabilizers, fruit, and sweeteners.

Milk is the main ingredient used when making yogurt. It can be cream, whole, low-fat, or skim. Whole milk is used to make full-fat or regular yogurt, low-fat milk is used to make low-fat yogurt, and skim milk is used to make non-fat yogurt. In general, the higher the fat content of the milk, the smoother and creamier the yogurt will taste.

Live cultures are the second key ingredient in yogurt. There are two required cultures, Streptococcus thermophilus and Lactobacillus bulgaricus, and several optional ones.

Cream may be added to adjust the fat content of milk. Non-fat, dry milk powder is used to adjust the content of yogurt solids above the 8.25% minimum to achieve better body and texture. Stabilizers also function to improve body and texture as well as to increase firmness. They help keep fruit uniformly mixed in the yogurt and prevent separation of whey. Examples of stabilizers include alginates, gelatin, gums, pectins, and starch.

Sugar, honey, and artificial sweeteners may be used to reduce the naturally sour flavor of yogurt. Fruit, fruit syrups, and pie filling are also optional and can either be mixed in with yogurt or added to the top or bottom of the yogurt.

Processing

Pasteurization: In order to prevent deactivation of the bacterial cultures needed in yogurt production, milk is pasteurized at 185°F for 30 minutes or 203°F for 10 minutes prior to the addition of the cultures. The high heat also denatures the whey proteins, which allows the yogurt to form a more stable gel. Lastly, pasteurization effectively kills disease-causing bacteria.

Adjusting Milk Composition and Blending Ingredients: At this point, stabilizers are added to the mixture, as are sweeteners if a less tart yogurt is desired. The nonfat dry milk powder is also added before heating to prevent coagulation of the milk proteins.

Homogenization: Not all yogurts are homogenized. If this step is taken, the ingredients are mixed well to ensure a more stable consistency.

Heating: The milk is then heated to 200°F for 10–20 minutes, depending on the desired thickness of the yogurt. Holding it longer will result in a thicker yogurt.

Cooling and Inoculation: The mixture is then cooled rapidly to 112–115°F. At this point, the warm mixture is inoculated with the live bacterial culture.

Incubation: The mixture is incubated for 4–7 hours at 105–115°F. The bacteria used in making yogurt are thermophilic and this is their optimal temperature range; they are killed above 130°F and do not grow well below 98°F. Yogurt will become firm when a pH of 4.6 is reached. Incubating the mixture any longer will result in an increased acidity and more sour taste.

Cooling: When the desired pH is reached, the yogurt is cooled to around 45°F to end the fermentation process.

Addition of Fruit and Flavors: For set style yogurt, fruit is added to the bottom of the cup and the inoculated yogurt is placed on top of the fruit prior to fermentation. For Swiss style yogurt, fruit is mixed with the yogurt after the fermentation and cooling steps. The yogurt is then packaged; at which point, it should be refrigerated at 40°F or lower.

Food Safety

The acidity of yogurt acts as a barrier to bacteria growth, as does the high temperature achieved during the yogurt-making process. However, milk must be pasteurized beforehand to sufficiently kill disease-causing pathogens such as E. coli 0157:H7, which may be acid tolerant.

It is essential that all equipment and workspaces used in the yogurt-making process remain clean and sanitized to prevent the addition of unwanted bacteria to the yogurt.

The FDA requires that yogurt be made with live cultures, but some yogurts are heat treated so that the final product contains no active cultures. The label should specify what microorganisms are present, and in what amount in terms of colony-forming units (CFUs), as well as the known health benefits associated with the particular strains used. The label should also disclose the expiration or use-by date, serving size suggestion, company name, and proper storage of the product. Regardless of the use-by date, yogurt with visible signs of microbial growth or off-odors should be discarded immediately.

The shelf life of yogurt is 10–21 days. For liquid yogurt, the shelf life is 4–10 days and for yogurt cheese the shelf life is 7–14 days when refrigerated at 40°F. Yogurt can also be frozen for several months, but this may alter its texture.

Consumption

In 2017, per capita consumption of yogurt in the U.S. was 13.7 pounds. Domestic consumption peaked in 2014/2015 at 14.9 pounds per capita. Although yogurt consumption in the United States has continued to rise over the past two decades, the per capita consumption is dwarfed by Sweden’s annual 62.8 pounds per person. Research by the NPD Group reports that Americans between the ages of 18–34 are driving the increase in yogurt consumption in comparison to the older generations. In recent years, the health benefits of yogurt (particularly the recognition of probiotics) has encouraged a higher consumption of yogurt and the incorporation of yogurt into many more food products. Yogurt has been introduced into a wider variety of products, including fast food establishment menus, toothpastes, beauty products, and even pet foods. In the 2006-2007 Foodnet Population Survey Atlas of Exposures, 43.3% of the survey cohort reported eating fresh or flavored store-bought yogurt within the past seven days.

Information on keeping yogurt fresh and safe to eat can be found at FoodKeeper App.

Nutrition

The health benefits associated with yogurt consumption are plentiful, and there is ongoing research suggesting yogurt may contribute more to overall health than is currently known. All types of this fermented dairy product are a nutrient-rich source of calcium, potassium, and protein, and yogurts that have been fortified with vitamin D and/or probiotics have additional health benefits.

Probiotics are live microorganisms that develop a symbiotic relationship with the host when administered in the proper amount. Lactic acid bacteria that survive in the gut are often used as probiotics. This includes Lactobacillus rhamnosus, L. casei, L. acidophilus, and Bifidobacterium lactis, among others. The bacteria acquire nutrients and energy from the food people eat and, in return, help maintain a healthy gut microbiota. Research suggests this may promote immune function, improve mental health, and protect against cognitive impairment. It may also lower the risk of some chronic diseases, such as cancer, high blood pressure, obesity, and diabetes. Different strains of bacteria confer different benefits, which may be specified on the product label.

Gastrointestinal Health: Consuming probiotics is associated with a reduced severity and duration of diarrhea in children with acute infectious diarrhea, in those taking antibiotics, and in those with lactose intolerances. There is also evidence that probiotic consumption may be helpful in managing the symptoms of irritable bowel syndrome (IBS) and reducing relapses of ulcerative colitis. Some probiotics inhibit the growth of Helicobacter pylori, which is associated with ulcers and cancer of the stomach. The role of probiotics in preventing traveler’s diarrhea is being investigated.

Immune Health: Researchers have demonstrated that consuming probiotics can decrease the incidence of upper respiratory infections in adults and reduce cold and flu-like symptoms in children (resulting in higher attendance in preschool and daycare). There is also evidence that consuming yogurt may lower the incidence of yeast infections.

Emerging Research: Changes in the gut microbiota may occur with obesity and type II diabetes. There is potential for probiotics to play a role in the prevention of obesity and diabetes, but more research is needed. There is also a new field of research devoted to studying the link between the consumption of certain probiotics and mental health. A recent study showed that probiotics may reduce anxiety and stress, but further research is needed to confirm these findings.

References

  1. Dairy Products Profile. (n.d.). Retrieved from https://col.st/MyXCZ
  2. Foodborne Illness Outbreak Database – Sponsored by MarlerClark. (n.d.). Retrieved from https://col.st/vHGbV
  3. Foodborne Illness Outbreak Database – Sponsored by MarlerClark. (n.d.). Retrieved from https://col.st/Bo8xx
  4. Lee, S. C., Billmyre, R. B., Li, A., Carson, S., Sykes, S. M., Huh, E. Y., … Heitman, J. (2014, August 29). Analysis of a Food-Borne Fungal Pathogen Outbreak: Virulence and Genome of a Mucor circinelloides Isolate from Yogurt. Retrieved from https://col.st/UaNob
  5. Mauro, Machado, & Rachel. (2015, July 11). History of yogurt and current patterns of consumption. Retrieved from https://col.st/Sa461
  6. U.S. yogurt per capita consumption, 2018. (n.d.). Retrieved from https://col.st/jzIp4

Picture acknowledgement: https://col.st/RcMtu

Author

Natalie Tsevdos

Natalie Tsevdos

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