Cheese Whizzes

Jeffery R. Broadbent refuses to name a favorite cheese. "That's like asking a person who loves books who their favorite author is," according to the director of the food science program at Utah State University, Logan. Broadbent was one of several participants in a symposium on the chemistry and flavor of dairy products at the American Chemical Society national meeting in August. The symposium was sponsored by the Division of Agricultural & Food Chemistry.

"I've had the privilege of being at a number of conferences with spectacular displays of cheeses from around the world, and it's fun to try many new cheese types," Broadbent told C&EN. "Unfortunately, you always have a few in there that grab you by the throat and make you wish you hadn't tried them. Sometimes it's even a variety that you're familiar with, but something has gone very, very wrong in the flavor development. So you get a cheddar that tastes ghastly. And some of the softer cheeses made from raw milk can have all kinds of different flavor backgrounds. Some are wonderful, but some are a little scary."

Whether made well or badly, cheese brims with flavor compounds. There are some 400 in cheddar, for instance, noted Michael H. Tunick, a research chemist in the Department of Agriculture's Dairy Processing & Products Research unit in Wyndmoor, Pa. These compounds originate from the metabolism of bacteria, mold, and yeast, as well as from chemical reactions involving other ingredients as cheese develops and ages. 

People have been making cheese for thousands of years, Broadbent said, "and empirically over that time they figured out how to make refinements in their manufacturing protocols to get what they wanted at the end." Yet this complex food has not surrendered all of its secrets. "There is still a lot of art to making a good fermented food today," he said.

Cheese production begins with milk, about 10 lb to make a pound of cheese. For cheese made in the U.S., the milk must be heated briefly to kill unwanted bacteria.

A starter culture of desirable bacteria such as streptococci or lactobacilli is then added. The particular strain and the amount used are key factors in determining the final flavor of the cheese. The microorganisms consume lactose in the milk, converting it into lactic acid, citric acid, and other metabolic products. This step begins formation of flavor compounds including acetaldehyde, acetic acid, and diacetyl. It also reduces the pH of the milk, which is usually 6.6–6.8, to around 5.3 in finished cheese. When the fermentation is done, coloring may be added.

Next, the enzyme rennet, which can be obtained from a calf's stomach or produced by microbes, is added to begin breaking down casein, a soluble protein in milk. The rennet also coagulates the milk solids into curds floating in a solution of milk sugar, minerals, and water-soluble proteins known as whey.

The mixture is then heated and stirred. The whey, which can lend a harsh taste, is drained off. Salt and seasoning are added, and the curds are pressed into blocks and left to cure. The cheese may be used as is or aged. The Oregon cheese producer Tillamook County Creamery Association, for instance, ages its cheddar for 60 days to achieve a medium flavor and for at least 15 months to obtain extra sharp.

Microbial and chemical reactions continue in the cheese as it ages. Some reactions produce peptides that break down into their component amino acids, including cysteine and methionine, Tunick explained. In turn, these chemicals release compounds such as methanethiol, dimethyl sulfide, and methional [3-(methylthio)propanal]. Although these compounds may not be especially enticing by themselves, they contribute appealing flavors to cheese. Concentrated methional, for instance, smells like boiled potatoes, but it lends a cheddary flavor to cheese. Other flavor compounds that develop during aging include ammonia, amines, and alcohols such as 3-methylbutanol.

Unwanted compounds including catabolites of amino acids such as tryptophan or phenylalanine are also formed. They are unavoidable because they arise from the breakdown of casein, Tunick said.

Lipids in the cheese yield ketones such as 2-heptanone, which is responsible for much of the flavor of blue cheese. Lipids also break down into lactones, which provide coconut, peach, and other fruity notes to cheese--as do esters. And volatile fatty acids including butanoic acid contribute "goaty" and "rancidy" flavors, "which are good for some cheeses and not others," Tunick said.

Even the cows' diet comes into play in determining cheese flavor. For example, artisan-type cheeses, which are produced by small operations with cows in a pasture eating flowers and a variety of grasses, have much more flavor than mass-produced cheese from cows standing in stalls eating regular silage, Tunick said. And the flavors are much different, too. Pleasant flavors from forage include carvone and citronellol.

AS RESEARCHERS strive to make cheesemaking more of a science than an art, one fruitful area of study delves into the biochemical behavior and genetic makeup of the bacteria that are used to influence cheese flavor.

"Many of the microorganisms that we use in cheese are being sequenced, so we have a huge new library of genetic information that is propelling research in understanding how these organisms affect flavor development," Broadbent said. Researchers combine the genome sequence data for a particular strain with recombinant DNA experiments to determine which enzymes and pathways are involved in flavor reactions.

Because of the public's aversion to genetically modified organisms, however, these engineered strains aren't used in cheese, Broadbent said. Instead, culture suppliers can use traditional methods to screen cultures for strains that, say, produce high levels of a desired enzyme identified through the genetically engineered culture.

At the symposium, Broadbent focused on Lactobacillus helveticus CNRZ32, a strain of lactic acid bacteria used to produce cheeses such as cheddar and Swiss. He carries out genomics work on this strain with his long-term collaborator James L. Steele, a food science professor at the University of Wisconsin, Madison.

CNRZ32 helps to minimize bitterness, a flavor defect that can develop in cheese during ripening. Bitterness arises when proteolysis breaks casein into hydrophobic bitter peptides. "If you can convert those peptides into free amino acids, you remove the bitter flavor," Broadbent said.

Lactic acid bacteria also convert amino acids in cheese into -keto acids via a transamination catalyzed by an amino transferase. The reaction requires a cosubstrate such as -ketoglutarate, whose "availability appears to be one of the real rate-limiting steps in amino acid breakdown," Broadbent said.

Using that knowledge, the dairy industry has been able to identify strains that can produce more -ketoglutarate. "Some processors are using these bacteria to accelerate amino acid breakdown in cheese, which ideally will lead to a shorter ripening time and more intense and mature flavor," Broadbent said. "The ripening phase of a cheese can range from months to years, so if we can accelerate that, it's a fairly large cost savings to the producer."

Like flavor, texture is a significant determinant of cheese enjoyment. In fact, "texture is about as important as flavor to a lot of people," Tunick said. "Cheese has to feel right."

The dairy products research unit where Tunick works can perform several texture-related cheese tests. For instance, the impressively named "universal testing machine" squeezes a cylinder of cheese a couple of times to imitate chewing. The force required to do so represents the sample's hardness, and the extent to which the sample returns to its original shape represents its springiness. This experiment also reveals the cheese's cohesiveness, or how well it holds together.

Tunick's unit has been called on to help with cheese conundrums. In the early 1990s, for instance, USDA asked Tunick's unit to help make school lunches more nutritious. One option was to use low-fat mozzarella cheese on pizzas, but commercially available products had unacceptable texture and flavor. Tunick and six colleagues took on the challenge. Through molecular modeling of caseins and peptide fragments, instrumental analysis of cheese texture, electrophoretic assessment of casein breakdown, electron microscopy and image analysis of cheese structure, and dairy pilot-plant work, they developed a tasty low-fat mozzarella for this market. The cheese is now used extensively in school lunches.

Many low-fat food tales don't have such a happy ending. Often, when fat is removed from foods, "nobody eats them," according to Gary A. Reineccius, a professor in the food science and nutrition department at the University of Minnesota, St. Paul. "There are some real problems with flavor, quality, and consumer acceptance."

In part, the trouble can be traced to the fact that in cheeses and in many other foods, "fat is really important for texture--the richness, the 'mouth feel,' " Reineccius said. Lowering the fat content generally changes the texture of a food, which in turn likely alters flavor, even when the food contains the same concentration of each flavor component.

Reineccius is trying to find out whether such a change in formulation has a broader impact than "the feeling in the mouth."

In particular, he is studying the influence of texture on aroma release, using cheeses that range from soft to hard. He plans to inject small blocks of cheese with flavors that aren't normally detectable in the food--either licorice or the fruity flavor of ethyl butanoate. "We will have people chew the cheese and spit it out. We'll measure how much it's broken down." If the cheese breaks up readily, Reineccius explained, flavor is more easily released into the mouth and perceived.

The tasters will judge flavor intensity. Using a mass spectrometer, he will measure aroma compounds exhaled in breath from each participant's nose. The results will shed light on the relationship between a given stimulus and the perceived flavor intensity.

Whatever the findings, there's still the matter of personal preference. One person's delectable taste experience is another person's culinary disaster. "That's one of the great things about cheese," Broadbent said. "There's a taste for every palate out there."