Acidulants in Hot-Pack Products

Acidulants in
Hot-Pack Products

July 1995 -- Applications

By: Lynn A. Kuntz
Associate Editor*
*Editor since August 1996

  The severe heat encountered in retorting may provide a high level of safety, but it does not help food product quality. Buying, running and maintaining retorts can increase manufacturing overhead. And there are size constraints; you can't very well fit a 55-gallon drum into a retort.

  More and more food manufacturers are turning to hot-pack processing as an alternative to canning. One of the most important aspects of this process is maintaining product pH below 4.5. A number of food ingredients on the market perform this function.

History of hot-packing

  Hot-pack processing is not a new idea. Before modern food processing evolved, people discovered that adding hot, cooked foods to clean containers increased the length of time the products could be kept without spoiling. At some point, those interested in the underlying rationale discovered that acidic products were particularly safe. Eventually science determined that a pH of 4.5, while not a guarantee against spoilage, not only significantly delayed spoilage, but prevented growth of the deadly Clostridium botulism organism.

  High-acid foods, those with a pH of less than 3.7, do not support the growth of spore-forming bacteria, although yeast and mold can still cause spoilage. This means that if the pH is low enough, cooking, filling, then cooling yields a safe product without retorting.

  While many foods are naturally high in acid, especially tomatoes and other fruits, others need help to depress the pH to an acceptable level. When this is the case, product designers turn to food acidulants.

  Several substances, mostly organic acids, can acidify food products. We reviewed these in, "Acid Basics: The Use and Function of Food Acidulants" appearing in the May 1993 issue of Food Product Design. Some acids are more useful than others for the acidification of hot-pack products. Three major considerations come into play when determining the best ingredient or combination of ingredients: antimicrobial effect, flavor and strength.

Preservation by pH

  In a hot-pack process, the processor heats an acid food to temperatures near boiling for a sufficient time to destroy all pathogens and inactivate spoilage organisms. Containers are filled with the hot product, sealed, then cooled. Other factors influence the effectiveness of a particular heat process, including percent solutes and other antimicrobial elements, but pH provides the basis for the process.

  Although many food-grade acidulants exist, they all do not excel at pH control. Some perform other functions. For example, ascorbic acid acts as an antioxidant and prevents browning. The most effective acids for decreasing pH are the strong acids, such as citric, malic, tartaric and fumaric, although lactic and acetic often work well. The hydrogen ions in these acids will dissociate to a greater degree than those in the weaker acids.

  Weak acids, while not efficient at lowering pH, exhibit antimicrobial properties in acidic products. The weak acids perform best when the pH is low enough to keep them in a dissociated state. Acetic, lactic and benzoic acids work well in this regard. Lactic acid tends to inhibit the growth of bacteria, especially lactic acid bacteria. Acetic exhibits a strong bacteriostatic effect and suppresses yeast growth.

  "Benzoic acid is not very soluble, so it is usually handled in the form of sodium benzoate," says Tom West, technical services manager, Jungbunzlauer, Newton Centre, MA. "When added to acid systems, the benzoate dissociates back to the acid form where it functions as an antimicrobial agent."

  One of the problems with taking advantage of the antimicrobial properties of some of the weak acids is that their use may be limited by the FDA. For example, when adding functional preservatives such as sodium or potassium benzoate, the total preservative level must not exceed 1%. Also, when compounds are classified as a preservative, many consumers prefer not to see them on a label. Some can impart off-flavors when added at too high a level. Vinegar may be an effective preservative and it may be label friendly, but it would not make a very tasty apple pie filling.

  "Since we normally add some type of preservative to our hot-pack products to ensure shelf life, certain customers require aseptic processing to eliminate those ingredients," says Joseph Kuo, R&D manager, Ramsey Laboratories, Cleveland.

  Because the goal is inhibition of microbial growth, sanitation is one of the most important concepts in the production of hot-packed foods. In theory, the difference between this process and aseptic processing amounts to a difference in the degree of sanitation and how it is achieved. This means clean ingredients, clean equipment, clean packaging, and a clean environment. If the bacterial load is too high, no amount of acid will help the situation.

Picking up the pieces

  This "pH + heat = product safety" relationship seems simplistic at first, but the product designer must look out for pitfalls. For example, pH in a homogeneous product such as a sauce should not present any surprises. You formulate to a pH, mix the product, heat it and it is ready to go. But working with particulates can change that. If the product contains an acidic particulate, such as a fruit piece, the principle still applies. An acidic product receives a heat treatment that inactivates the microbes. If the piece is small or thin, in theory it will be fine.

  But what happens when you try to put a large chunk of potato through a hot-pack process? Even worse, what happens when a large particulate consists of a fabricated food, such as a meatball, where all those previously exposed ingredient surfaces now lie in the center of the piece?

  "In order to get the preservative effect in hot-packing, the entire product has to undergo the process temperature, typically around 200°F, at the lowered pH," explains West. "With a low-acid piece, you need to acidify it somehow before it is heated. You may heat the particulate with a pH of 7.0 and a week later it will equilibrate down to a pH of 4.5. If that happens, you will not have necessarily killed anything... except maybe the consumer."

  One answer could be steeping the particulates in an acidic bath until they reach the correct pH. When fabricating a particulate destined for a hot-pack application, the formulation should include enough acid so the product qualifies as a high-acid product. This can be done directly or by using encapsulated acids. However, unless there is a functional reason for leaving the pH high until the product is cooked -- perhaps it will affect the gel -- encapsulated acids are not an economical choice.

When the formulation sours

  Working with acidulants can create a number of problems. First, food-grade acids have different solubilities. Citric, malic and tartaric possess high solubility in water. Adipic and especially fumaric do not. These would not work well in applications where available water is limited. They also would be more difficult to handle from a production standpoint.

  Manufacturers often add acidifiers in a water solution to increase ease of handling and enhance dispersion. Acids with lower solubilities by their nature need less concentrated solutions, which may increase handling difficulty. They also have slower dissolution rates and can come out of solution as the temperature cools, making these solutions more difficult to deliver consistently.

  The acids with higher solubilities tend to pick up water more readily during storage. This means that under humid conditions what was once anhydrous acid is now part water. Therefore, every time a solution is prepared, the titratable acidity -- as well as the pH -- should be tested and confirmed.

  Adding acids to hot-pack foods can affect the other ingredients, as well as microorganisms. Many ingredients, including colors, protein and stabilizer systems, are sensitive to pH. Often, the effects will not be readily apparent until the product goes into storage. Here, colors can fade and stabilizer systems break down.

  When working with high-acid systems, it is important that the ingredients used remain unaffected. For example, caseins will form an insoluble precipitate when the pH of the system drops to 4.7. Unless a starch is cross-linked, it will hydrolyze under acidic conditions.

  "When a product contains soluble proteins, you must pay close attention to their isoelectric point," warns West. "The dairy proteins will behave differently above and below their isoelectric point. You can protect them with certain buffers, phosphates and citrates."

  Ingredients containing high levels of natural salts of weak acids (components such as minerals and proteins) may create a buffering effect, making it difficult to reduce the pH. Dairy products have significant amounts of soluble proteins and minerals such as calcium and magnesium that form salts and act in this manner. On the other hand, meat products typically have a negligible effect.

  A buffer is a compound, often a salt, paired with a weak acid that reacts with the free hydrogen ions in a dissociated acid. This causes the entire system to resist changes in the pH when acids or bases are added. But while the buffer affects the pH, it does not mask the flavor of the acid.

Favor the flavor

  Most acids will reduce the pH of a food when they are used in sufficient amounts -- as long as those amounts are legally allowed. Therefore, flavor ranks high on the list of deciding factors. Two factors influence the flavor of the food: the acidity level, or tartness; and the type of acid used.

  From a flavor standpoint, a pH of 2.4 to 2.8 marks the point where a food product is usually too tart to be palatable. Those levels would only be acceptable if the flavor of the food is normally associated with that level of acidity. A vinaigrette may taste acceptable at that level of acidity, a cream sauce would not.

  "Our main objective is to keep the products below pH 4.5 to control pathogenic organisms," remarks Kuo. "But some products actually taste better at a lower pH. With raspberry-flavored products, for example, you want the pH under 3.5 for the best flavor. With lemon-lime, you can go as low as 2.4."

  Often, sweeter products can be formulated at a lower pH while maintaining palatability. Using the correct sugar:acid ratio softens the impact of acidity in the mouth. If the balance is right, the product does not come across as sour.

  Besides their acidity, acidulants may impart their own characteristic flavor. Acetic acid, or vinegar, is the most familiar and obvious example. Phosphoric acid has little flavor impact other than sourness. The sensory effects of the acids also vary. Citric acid produces a strong, sharp flavor that quickly dissipates. Fumaric also gives a strong acid bite, but the sensation lingers. Malic acid lingers, but the flavor is milder.

  The profile of the acid must match the profile targeted in the finished product. Often the best flavor is achieved by using the type of acid naturally associated with a particular food. For example, the fruit acids malic, citric and fumaric typically give the best flavor profile in fruit flavored foods.

  "With a dairy-flavored product, you probably would want to formulate with lactic acid," recommends West. "Sour cream is fairly acidic in terms of pH, but it has a good flavor profile. You would see lactic in applications like a hot-pack cream sauce."

  These differences in acid flavor delivery affect the way other ingredients taste. Malic acid complements aspartame-sweetened products. Using citric acid for the same product brings out bitter, metallic notes. Acidity also can mask certain flavor notes or make them less pronounced.

  Because of their high acidity, hot pack products may be more difficult to design than products destined for retort. However, if done correctly the advantages gained in the finished product outweigh the expenditure in development.

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