All About Acid
From yogurt to buttermilk and Cheddar to ricotta cheeses, a key step in the manufacture of these very different dairy foods is lowering pH though the development or addition of acid.
Acid has many functions in dairy foods. "First, acids have a major role in protecting dairy foods against spoilage and growth of pathogenic organisms. This is especially true in yogurt, buttermilk, sour cream and other products at or below pH 4.6. They also play a role in products with higher pH, such as cheese," says Dave McCoy, principal scientist, Chr. Hansen Inc., Milwaukee, Wis.
Acid not only ensures food safety, it assists in preservation, thus extending the shelflife of fluid milk, a food that rapidly spoils if let untreated. "As the pH of the product drops, the ability of spoilage and pathogenic organisms to grow and produce toxins decreases," adds McCoy.
The substrate used to lower the pH of milk also contributes to flavor development and often physical changes, including protein coagulation and/or thickening. The lower the pH, the more rapid is coagulation time. This goes hand-in-hand with promoting syneresis, which occurs when the protein matrix in the curd contracts and squeezes out moisture. This is a most critical means of controlling moisture content in cheese.
"Many flavors are much more potent at low pH than they are at higher pH," he adds. Research done by Chr. Hansen in Denmark has shown that the flavor impact of low concentrations of acetaldehyde in yogurt increases by about 15% when decreasing the pH from 4.3 to 4.1.
Faye Lee, dairy applications, Tate & Lyle, Decatur, Ill., adds, "Citrates are usually used together with acidulants to provide a buffering system and thus modifying the tartness in the food system."
In cheesemaking, "Acid production also results in the loss of calcium from the casein protein particles, which improves melt and stretch properties," says John Lucey, professor, Department of Food Science at the University of Wisconsin-Madison.
Lowering the pH of milkAcid can be developed through the addition of what are called starter cultures. These lactic acid-producing organisms are selected and combined depending on their application. For example, fast-acid-producing, thermophilic organisms are typically used in yogurt manufacturing; whereas buttermilk production requires a slower acid production.
It is true that lactic acid bacteria and other microorganisms are present as contaminants in fresh milk. And, in the production of cheese, which typically employs a slow-acid development process, it is possible to make cheese without any additional cultures. However, relying on inherent cultures is unpredictable and can be quite a lengthy process, so the normal practice is to add starter cultures.
For some manufacturers, starter cultures are not fast enough for meeting production quotas. With certain dairy products, processors can create an acidic environment through the addition of what is called a food acidulant. There are a variety of food acidulants, each producing unique end results.
"Some acidulants have their own characteristic flavors, which may or may not be attractive to consumers. Acids can also produce flavors themselves, and they can alter the perception of other flavors," says Lucey. "For example, malic and citric acids are often added to fruit yogurt, enhancing the fruit flavors.
"Citric acid, which is also present in milk and may sometimes be added to cultured buttermilk, is a substrate that is used by certain types of dairy starter cultures that are used for aroma and flavor development," Lucey says.
Terri Rexroat, business line mgr., Degussa Food Ingredients-Cultures & Enzymes, Waukesha, Wis., adds, "Generally an acidulant is chosen over a culture when a processor has a tight schedule and therefore believes that he or she doesn't have time to wait for completion of a fermentation. In some cases, use of an acidulant can result in a valuable efficiency gain."
According to Lucey, "Acidulants provide a more rapid rate of acid development and less variability than the growth and acid production by starter culture. They also provide better control of pH than a biologically active starter and can be used at high temperatures, which would be a problem for bacteria."
Keep in mind that acidulants come with their own set of issues and concerns. "Acidulants need some way of being added or dosed into the product," says Lucey. "And, it is important to monitor pH or acidity, to determine if more is necessary."
"Because acid curdles milk protein immediately, it is strongly recommended adding acid slowly to the mix under proper agitation for developing a smoother texture, as in the case of sour cream," says Lee. "An in-line injection device adds acid solution by producing fine droplets in an even and consistent way, thus a better option of adding acids to the mix.
"Using modified starches in the mix not only provides a smoother texture and heavier body, but also reduces milk curdling and syneresis upon storage," adds Lee.
Indeed, acidulants help processors achieve a desired pH quickly, but these potent additives need to be used judiciously. If you add too much, there's no going back.
Furthermore, when using acidulants instead of cultures, "There are significant trade-offs, one of which is a loss of the complex, intermingled flavor notes that can only be generated via fermentation," Rexroat says. "In some applications, for example Cheddar cheese, the flavor is so complex that it has never been successfully duplicated by an acidulant."
And, unlike acidulants, "Acid-producing cultures also produce flavor and a lot of different textures for cultured products," adds Lucey. "A wide range of different rates of acid development (fast to slow) are available. For yogurt cultures, there have been recent developments in making sure that post-manufacture acidification in chilled distribution does not continue, as this can lead to a harsh acid flavor."
He adds, though, "That select acidulants can act as buffers." This helps a product maintain its desired pH through shelflife.
Depending on application, lactic-acid producing cultures and acidulants can be used together to improve efficiency. For example, "Coagulating enzymes are sensitive to pH variations, so acidulants are occasionally used to standardize the pH of milk entering the cheese manufacturing process," says McCoy. "Lactic acid or vinegar (acetic acid) is added before pasteurization to standardize the pH to about 6.4. This allows for a more uniform production process."
Lucey adds, "Generally, pH values lower than the natural pH of milk are used because this accelerates the rennet coagulation process and dissolves some calcium from the cheese, which can alter the texture."
Beth Jones, product mgr.-fresh dairy cultures, Danisco USA, New Century, Kan., cautions, "This type of process-pre-acidification of milk followed by culturing-can result in a product different than one produced by acidifying cultures alone.
"Enhancing the culture speed and consistency are key culture goals for producers and for Danisco's research program," says Jones. "Our patented isogenic technology for the control of phage in cheese cultures results in cultures that demonstrate both excellent speed and consistency for the acidification of milk."
Indeed, starter culture suppliers are proactively seeking ways to help processors reduce costs. "We know that faster cultures can reduce fermentation times and thereby improve process efficiency and reduce costs," says Rexroat. So, rather than having their customers turn to acidulants and assume a new learning curve, companies such as Degussa have introduced cultures for cheese, yogurt and sour cream that can help processors reduce fermentation times without sacrificing desirable finished-product characteristics.
A new sour cream culture from Dansico is a unique blend of freeze-dried cultures that gives vat sets in 12 hours or less, with the balanced flavor profile and texture of a traditional set.
And recently, Chr. Hansen introduced new cultures for use in cottage cheese. These are mesophilic/thermophilic cultures that are added directly into the vat and result in manufacturing times comparable to those of some bulk starters, according to the company.
In addition, the company recently was issued a patent for "liquid starter cultures having improved storage stability and use thereof" (U.S. Patent No. 6,787,348). The company's primary objective for the new patented invention is to supply commercial liquid microbial starter cultures that can be stored at the dairy, food or feed manufacturing plant for extended periods of time without significant loss of initial metabolic activity.
Acidulants in dairy foodsIndeed, acidulants play a variety of roles in the manufacture of dairy foods. Adipic acid, for example, can improve the melting characteristics and texture of processed cheese and cheese foods. Citric acid and sodium citrate are also used in these products for emulsification, buffering, flavor enhancement and texture development.
"Lactic acid is the acid of choice in making sour cream," says Janet Payne, senior market development specialist, Purac America Inc., Lincolnshire, Ill. "Lactic acid's smooth acid taste perfectly matches the sour cream flavor without the overpowering tartness of other acids.
"Lactic acid also has some unique anti-microbial properties for both pathogenic and spoilage bacteria," Payne adds. "This property can contribute to an extended shelflife and can provide a safer product without creating a strong dependency on additional preservatives."
Phosphoric, lactic, citric and hydrochloric acids all have application in direct-set cottage cheese. The term direct-set describes the fact that an acidulant is used in manufacturing as opposed to cultured cottage cheese, where a lactic acid-producing culture is used.
Glucono delta lactone (GDL), which is the neutral ester of gluconic acid, hydrolyzes in aqueous solutions to form gluconic acid. It, too, is sometimes used in the manufacture of direct-set cottage cheese, as well as for pre-acidifying milk for cheesemaking.
In direct-set cottage cheese, the process begins when cold milk is mixed with an acidulant to obtain a pH up to 5.0. After heating the acidified milk to 90
The whey drained from authentic mozzarella is packed full of protein and has traditionally been used to make ricotta cheese. The residual protein in the whey is not readily coagulated by starter cultures, but will coagulate under the influence of an acidulant like lactic or acetic acid, though use of the latter might result in a product with too strong of a vinegar bite. In general, after the whey is drained from the mozzarella curd, it is transferred to a vat where it is heated to 176