DR. ZEYNEP USTONOL
Professor and director of graduate programs, Michigan State University

Fermentation is the oldest method of food processing and preservation, Dr. Zeynep Ustunol, professor and director of graduate programs, Michigan State University (pictured), told attendees of the 102nd Annual NAFFS Convention. Foods such as bread, wine and cheeses have been made using this method throughout human history. There are records going as far back as 7,000-8,000 BC of clay containers that separate curds and whey, she said.

Fermentation, Ustunol said, is produced by action of microorganisms and enzymes, under controlled conditions, primarily bacteria, yeast and mold. By controlling the time and temperature, fermentation imparts desirable biochemical changes to the food, instead of having it spoil.

Lactic acid (used in cheese production) and yeast (used in beer) offer two desirable biochemical changes to the food, Ustunol said, adding that there’s a renewed interest in fermented foods due to data accumulating on their health benefits and benefits of ingesting probiotics and their metabilites. 

"The concept of probiotics offering health benefits, such as improved gastrointestinal health, to support the immune system, is not new," Ustonol continued. "Elie Metchnikoff, considered the father of probiotics, believed they could provide anti-aging benefits. He wrote a book in 1907 called the ‘Prolongation of Life—Optimistic Studies’ where he provided his example of Bulgarian peasants living for over 100 years because they consumed significant quantities of fermented foods such as yogurt. Metchnikoff’s idea was that the toxic materials and deleterious organisms in food could be replaced by good bacteria like lactic acid bacteria by fermenting dairy foods. He believed the toxins produced by the gut microflora was accelerating aging and causing other health risks,” Ustunol said.

Today, she added there’s renewed interest in fermented foods. Data is accumulating on improved gastro-intestinal health from eating these foods, ingesting these organisms and getting the benefits of these metabolites.

Fermentation, Ustunol said, consists of series of chemical reactions catalyzed by enzymes in the living cell. Fermented food reactions caused by the action of microbes and conditions of fermentation that impact microbial metabolism will impact flavor. Fermentation, she added, has long been used as a natural flavor-enhancing tool for food processors and today is also used in production of flavors.
Ustunol explained yogurt is a cultured dairy product containing Lactobacillus delbrueckeii subsp bulgaricus and Streptococcus thermophilus(1:1) ratio. Other bacterial cultures (i.e. bifidobacteria, Lactobacillus acidophilus) may be used.

Sweeteners, flavoring and other ingredients are often added. She said the Code of Federal Regulations has three categories for standard of identity for yogurt (yogurt, low-fat yogurt and non-fat yogurt):

• Yogurt: >3.25% fat, >8.25% SNF, 0.9% TA
• Low fat yogurt: >0.5 <2.0% fat, 8.25% SNF, 0.9% TA
• Nonfat yogurt: <0.5 % fat, 8.25% SNF, 0.9% TA
• There’s no standard of identity for frozen yogurt, drinkable yogurt, Greek yogurt or plant-based yogurts

Conditions of fermentation that impact microbial metabolism will impact flavor, too, Ustunol said. “Flavor produced in fermentation is very complex in food production. First, there’s the product dependent, composition and ingredients used. Next is the culture strain and mix. There are several types of fermentation and conditions that can apply. These can change the need for incubation temperatures. Processing conditions are also important; for example, whether the curd is cooked or not and temperature, in the case of cheese. Storage conditions will also have their effect, and finally, the added flavors.”

Ustunol used cheese as an example to illustrate the above points. “The starting ingredient”, she said, “is milk, which can come from many sources. Then the culture is added for fermentation.  The added flavors are different if it will be a cheddar variety, where protein breakdown is important, or if it’s an Italian cheese, where fat becomes more significant. It’s when the final product requires added ingredients like peppers or fruit that flavor interactions become more important,” she said.

There are complex issues for flavor during fermentation, Ustunol said. Fermented dairy foods, such as yogurt, come in many flavors. Yogurt, she said is perhaps the best example of this and she used yogurt to make her point. “There are so many varieties of yogurt products on the market today,” she said. “Yogurt is not a new product but there is new and current interest in yogurt and it experiences about 6% growth annually. There are drinkable yogurts, spoonable yogurts, Greek yogurts, dessert yogurts, those for children, etc.,” she said.

Ustunol said nearly all yogurt is manufactured in one of two ways; blended/stirred or set, or what she referred to as “sundae style”, where the fruit will be on the bottom or on the top. The difference, she said, is relevant for flavors and whether the product can withstand fermentation. Both formats, she said, are offered the same pretreatment of the milk. Additional ingredients, sweeteners, stabilizers and milk solids are added, if applicable; the mix is then pasteurized and homogenized. Sometimes it’s put through an evaporator to reduce moisture and increase the solids.

“If the product is a stirred or blended yogurt, it gets the pretreatment above, then goes to fermentation tanks,” Ustunol said. “The culture is added and fermentation occurs. A liquid becomes a gel, the milk becomes yogurt. After fermentation, the blended yogurt goes into tanks.  Then flavorings are added, it’s put into package and then storage. The shelf life for this yogurt is long, 20-40 days. The yogurt is able to retain its flavor until the consumer eats it,” she said.

The set or sundae-style yogurt, she said, is pumped directly into its container and the fermentation happens there. Since the fruit puree or flavor is at the bottom or top of the container, this protects the flavor, she added.

The major flavor compounds in yogurt are acids (lactic acid), acetaldehyde and more than 90 flavor compounds, including nonvolatile and volatile, carbonyl compounds, alcohols, esters, sulfur and other miscellaneous compounds.

While there are many key aroma compounds, Acetaldehyde is considered the primary flavor compound and is key to the popular unique flavor in yogurt, Ustunol said.
Flavor compounds are produced by lactic acid bacteria which is the culture that starts the fermentation process. Ustunol said there’s a symbiotic relationship between two organisms; the first is Streptococcus thermophilus, which reduces the pH and produces amino acids for the second, which is lactobacillus delbruekii subsp bulgaricus, the key organism that produces the needed acetaldehyde. Both organisms are needed for the process to be completed, she said.

Ustunol said there are many factors that impact the flavor of yogurt. First is the starter culture – the milk source and composition. Second is the processing – heat treatments and other conditions in homogenization and pasteurization. Storage conditions are duration are other factors, she said.  Ustunol said fruit flavors (strawberry, in particular) are the most popular types of flavored yogurt. Flavors added to yogurt come in in various forms, she added, noting the key components of yogurt are protein, fat and carbohydrates. “The key things in yogurt are fat and protein,” she said. “The fat serves as a carrier for flavor compounds, with the fat definitely changing the way flavor is perceived. Protein also has an impact on the flavor. A high-solids item such as Greek yogurt with added proteins will carry flavor differently. The acidity is also a consideration. Because of a pH of 4.6, it’s compatibility with fruit flavors is very high. Other flavors like coffee, chocolate or caramel are more challenging,” she said.

Ustunol said sweeteners can come in many forms for yogurt. They tend to suppress the fruit flavors, so the selection of which to use can be important. Stabilizers, she said, are used to thicken the yogurt to keep the fruit in suspension. Starches, she added, tend to mask the flavor.

“So, these products that are still metabolizing after they’ve been moved to refrigerated storage can also have their flavor affected,” she said. “The concentration of flavor components decreases over time, so that’s an important consideration.”

Ustunol told attendees to look for more entries into the yogurt market. “For example,” she said, savory yogurts are coming to market – flavors such as carrot, sweet potato, parsnip and butternut squash.”