Friend and Foe

Volatilization is the loss of flavor from volatile flavor components that "flash off" when heated in an open processing system. This phenomenon happens because many flavor substances have lower boiling points compared to other ingredients in the product. Other mechanisms also contribute to the flavor flash-off effect in open systems.

  "It's not just volatility, but whether flavor components will co-distill," says Simmons. "Some flavor components are more steam-volatile. If you're cooking with a lot of steam, these components might co-distill."

  Whatever the mechanism, various flavor components are affected differently. Consequently, volatilization/co-distillation not only can reduce overall flavor impact, it can throw the flavor system off balance. Just how out of balance the flavor will be depends on the processing conditions and the potential of flavor components to flash off.

  "The environment of each process is different -- significantly so, in some cases -- so the environmental factor is critical," says Mike Popplewell, Ph.D., manager of process design and development, McCormick Flavors, Hunt Valley, MD. "It affects volatilization, particularly with certain compounds."

Degradation of flavor components differs from volatilization in that it can occur in either an open or closed processing system.

  "You get dimerization, polymerization and degradation of flavor components," says Simmons. "The pH of the system is key in those areas. You have to be aware of the pH of the system as it can contribute to the effect."

  The pH, of course, varies depending on the flavor system and the product. In a tomato sauce, the designer may be limited to a low pH. This affects the flavor profile and the degradative reactions that may take place. Moisture also is a factor in how degradative reactions proceed.

  "If you have a system with the right components like amines, reducing sugars and amino acids at very low moisture, you're more likely to form pyrazines than you are with a higher moisture system," says Simmons. "The mechanism of the chemical reaction is obviously different than when you're working at low water activity."

Chemical interactions. In addition to degrading flavor substances, heat can accelerate chemical reactions among flavor components and other ingredients.

  "Degradation is typically thought of as a flavor molecule breaking down," says Popplewell. "That's probably less common than some sort of combination with another ingredient that causes a change in the flavor."

  The food systems that product designers work with tend to have more protein and starch than a home-cooked food. In the presence of heat, both of these ingredients can interact with flavor components and affect the product's organoleptic quality.

  "Certain flavors will go into the starch helix. It can absorb or adsorb certain flavor types which will be lost irreversibly," says Simmons. "Others can be brought back in the consumer's mouth, but most people don't chew long enough to release captured flavors from a starch or protein matrix."

  Many flavor components can disappear into these matrices, depending on the chemical and the nature of the protein and/or starch matrix. Products with reduced fat levels are particularly susceptible to these interactions due to the significant presence of starch- or protein-based fat mimetics.

  "Using flavors in a low-fat product -- where there may be a lot of starch -- after a few days you may find the flavor character has changed or has been lost, or has undergone selective flavor loss unbalancing the flavor," says Simmons.

  Even if flavor components don't react directly with other ingredients, these other ingredients may serve to de-stabilize an otherwise heat-stable flavor system.

  "In many instances, flavors go through significant heat processing before they're in the form used by the flavorist or product developer, and they're relatively stable at that point," says Popplewell. "Moisture and other active components in a formula can really drive flavor changes. By itself, the flavor might be heat-stable, but if you combine it with other ingredients, that's when stability issues arise."

Accentuating the positive

  The chemical reactions that heat promotes are not always bad ones that must be kept in check. Some chemical reactions are necessary for foods to obtain their expected flavor profile. The reaction that results in many of the more common "cooked" or "browned" flavor notes is the Maillard (non-enzymatic) browning reaction. Chicken pot pies help illustrate the potential differences between industrial and home preparation. In a home kitchen, the chicken meat usually is cooked in the sauce before making the pie.

  "In a processed pot pie, IQF chicken might be used. It may not impart the roasted, fatty notes and flavors of cooking juices associated with home-cooked chicken," says Jane Van Vliet, industrial marketing manager, FIDCO, Solon, OH. "In this case, flavors that replicate those derived from home cooking may be formulated into the recipe."

  To pinpoint where flavor development reactions may be lacking, a product designer must have an understanding of other reagents in the system. Reducing sugars in the system, free amino acids from meat extracts or sulphur sources will all contribute.

  "The flavorist must give consideration to the reactions taking place and what end chemicals might form," says DeRovira. "If a Maillard reaction is occurring, it may need bolstering through a natural or artificial source."

  Start by developing the product target in the lab with whatever traditional cooking is appropriate. Next, evaluate this "gold standard" organoleptically using descriptive techniques and compare the results side-by-side with the results from preliminary attempts to make the product on production equipment. This will help identify the flavor notes that are missing. Appropriate flavor ingredients then can be selected or compounded to fill in gaps. Of course, these added flavors would be subject to the degradative situations mentioned previously.

  "That's really the rub with creating flavors to survive heat processing," says Popplewell. "Some you want to degrade or react while other flavors are expected to remain as they are."

Protecting a flavor

  Many of the undesirable effects that heat has on flavors can be minimized by reducing the heat exposure of the flavor -- adding it later in the process, etc. Although this approach is simple and effective in many applications, some products demand a more novel solution.

  The first step in trying to protect flavor ingredients from heat is to analyze the stresses to which the flavor will be subjected. Will the product be retorted or hot-filled? Does the product have to go through a freeze/thaw cycle? All of this information is necessary in order to select the correct approach for preserving/ building flavor.

Rebalancing the flavor is simply having the flavor re-compounded so that it generates the desired profile after processing. Flavors can be rebalanced in different ways. First, the flavor can be designed for more heat stability by choosing more heat-stable components and/or those with less volatility. Higher molecular weight homologs of flavor chemicals tend to have higher volatilization temperatures, yet often they can duplicate the flavor effects.

  "You may have a choice between a sulphur compound that is more heat stable and will create a flavor profile similar to the more reactive ingredient," says Simmons. "Often it's pointless to use something that's very volatile. It's a waste of money."

  The second approach to rebalancing flavor compensates for the fact that some notes may be more volatile than others by adding higher levels of the more volatile flavor components. Spiking these may make the flavor seem harsh and out of balance initially, but it will be correct in the completed product.

  These first two approaches are effective for counteracting volatility and flavor binding. But what about off-flavors due to chemical interactions? If the offending ingredients can't be replaced with something less reactive, the flavorist must figure out how to mask them.

  "Off-flavors are a different, sometimes complex issue," says DeRovira. "We have to identify the undesirable flavorful substances that are formed, then figure out how to mask them."

  Flavor profiling methods are one way to identify where off-notes appear in the time sequence of a flavor profile. This helps the flavorist determine the appropriate way to mask the off-note.

  "If you have an off-note coming in from the middle to the end of the profile, then you choose positive flavor volatiles that also would come in at that point. But then you have to rebalance the flavor," says DeRovira. "It's choosing a particular flavor attribute, choosing where it comes in the flavor profile, and using a volatile or blend of volatiles at the same time in the profile."

  In other words, an off-flavor that occurs at the end of a flavor profile will never be masked by, say, acetic acid, which enters the profile up front. Adding garlic or spice or creaminess -- something that comes in the background -- will be more effective.

Alternative carrier systems for the flavor components offer another method that helps reduce volatilization. Changing from alcohol to a lipid-based carrier is one way to reduce volatilization. Even changing from alcohol to propylene glycol may change the volatility of the flavor enough to correct a minor flavor imbalance.

  "A water soluble flavor compound in a sugar or polyol carrier might reduce their volatility, at least initially," says Popplewell. "They are going to mix into the mass of the food product, so if this will work in the long run is somewhat doubtful. It all depends on how the flavor is used and what the application is."

Encapsulation preserves flavor components by delaying their release into the general food system matrix. This not only can help protect them by delaying volatilization, but also by separating them from potentially interactive/degradative ingredients.

  Many encapsulation methods exist; they were reviewed in the April 1993 issue of Food Product Design. For flavors, however, the encapsulation method is not as critical as the encapsulating material because this will affect how the flavor is released.

  Water soluble encapsulating materials will release flavors when exposed to moisture. In aqueous systems, a water soluble encapsulate will slow the release of the flavor, but not prevent it. In dry systems, the flavor will not be released at all until the product is either reconstituted with water or mixed with saliva in the consumer's mouth.

  "Oftentimes, the system itself doesn't allow you to use encapsulation," says DeRovira. "Sometimes you get hot-spotting; sometimes you don't get good enough flavor release. In a spray-dried system with a fat-based flavor, you may not have enough water to break down the encapsulation system by the time it gets to the consumer's mouth."

  Fat soluble encapsulates release their cargo when the temperature of the system exceeds that of the encapsulating fat's melting point. This could take place during processing or home preparation.

  "If you're encapsulating in fat and the product is a fat-based system that is cooked for a long time, the fat will dissolve and release the flavor," says Simmons. "You have to consider all those aspects and even the molecular size of what you're encapsulating. Some people think they're encapsulating, but only doing it partially."

  Insoluble, heat-stable encapsulating materials such as corn zein or shellac will hold their flavor cargo until forced to release it by exposure to shear. Depending on the material and the encapsulation method, this could be the shear experienced in an industrial mixer or the bite force of the human mouth. Unfortunately, getting an encapsulate to survive the former type of shear yet release with the latter is next to impossible.

  "In some applications, the particle size of the encapsulate is so small that you're not going to get release," says Popplewell. "Many foods will be processed with shear. An encapsulate would have to survive processing shear, yet be sheared in the mouth."

  Adds Simmons, "You have to consider how long something is in the mouth. If you have to chew it for 25 seconds before the flavor is released, most consumers won't get the flavor."

  Selecting the correct encapsulation solution depends on what the flavor needs to be protected from and through what stages of processing the encapsulate must survive prior to releasing its cargo. Imagine a product that is made in a batch kettle and will be heat-treated or pasteurized later in the process. Although this is an aqueous system exposed to heat, either a fat- or carbohydrate-based encapsulation system can slow the release of the flavor ingredients into the mass of the food until they're at a stage in the process where they would be less prone to volatilize.

  "There are a number of specific processing instances where just delaying that release a certain amount of time can be beneficial," says Popplewell. "Once you have that mass of flavor compounds released into the general food system, you're going to lose control. It's going to be processed like the rest of the system. The key is to try and keep those flavors in their own micro-environment for as long as possible."

  Canned soup provides a useful illustration. After initial preparation in an open kettle, a fat-based encapsulate flavor system could be added with mild agitation just prior to canning. When the cans are retorted, the heat from this stage of the process would melt the encapsulate and release the flavor. By this stage of the process, though, the can would be sealed and the volatiles confined.

  Nevertheless, volatility may not be the only issue in this situation. It's still possible that the heat of retorting could change the flavor components into something unexpected and, possibly, unpleasant. Several protective measures may have to be designed into the flavor system to account for all the processing stages. Keep in mind, also, that these processing stages include the consumer's in-home preparation. There, the product could very well be opened, dumped into a pan and heated for an uncontrolled amount of time so that any remaining flavor completely degrades and/or volatilizes.

  "The ultimate question is, 'Has it all been worth it, or is the flavor just going to be flashed off in the consumer's home?'," says Popplewell. "You begin to lose control of the product at that stage, but you set it up so it could be the best possible eating experience up to that point."

Don't beat the heat, use it

  With so many concerns for the heat stability of flavor ingredients, it's easy to forget that there are many desirable flavors created when foods are heat processed. Largely based on variations of the general Maillard reaction pathways, carbonyl compounds and amines react during the cooking process to form a variety of flavorful substances. Because processing procedures often differ from home cooking methods, these reactions often can proceed differently from what is expected and desired.

  "When replicating home products on a manufacturing scale, there will be times when flavors won't develop in the desired way or are lost because of equipment used, the scale of manufacturing and the fact that harsher processing conditions are required for food preservation," says Van Vliet. "Combinations of flavors and flavor enhancers may be used to compensate for flavor loss or reduced flavor development."

  For other products, such as dry mixes, the necessary reactions won't have an opportunity to occur at all. In both of these situations, the missing flavors can be restored through the application of reaction flavors. (The term "reaction flavor" is considered to be an outdated misnomer by some industry sources who prefer "process flavor." Because both are in common usage in the industry, this article will use them both interchangeably.)

  Reaction flavors, for the most part, are a vehicle by which desired "cooked" flavors can be added to a food formula. Three primary approaches to reaction flavor application exist. One uses chemical precursors that will complete the desired reaction while the food is processed. Others are semi-reacted precursors that have been isolated at a relatively stable point in the reaction; these complete the reaction during processing. The third approach is to add fully reacted precursors. Each approach has advantages and disadvantages.

  Although using flavor precursors may seem like an ideal way to get "freshly cooked" notes into a product, this approach actually puts an unnecessary burden on product designers and limits their chances for success in developing their own process flavor profile.

  Admittedly, designers will be able to adjust process parameters and change the course of the reaction to devise a "signature" flavor profile for the product. Nevertheless, taking this approach will require strict process control in order for the flavor profile of the finished product to be consistent batch after batch.

  "They're never going to be able to predict that their flavor will be the same each time," says Wipf. "If they under-process, don't process to the same temperature consistently, or process for too long, you may end up with flavor components that differ from those of the desired reaction."

  Using a partially reacted flavor is a more reliable way to add a process flavor that will complete its reaction in the product.

  "If precursors, such as basic reducing sugars and amino acids, are added to a food to react during processing, you must be sure that these components will come together because it is an intermolecular reaction," says Simmons. "The partially reacted flavor can be designed so that it only has to react with itself (intramolecularly) to produce the desired flavor reaction."

  On the downside, the similarity to precursors also carries over into the need for strict process control. With such careful control required to assure the reaction proceeds properly and gives the desired flavor characteristics, it's unlikely that precursors or intermediates could ever successfully be added to a product for reaction during consumer preparation.

  "They might put it in the microwave or they might put it the oven and the heat will develop in different ways, depending on the exact conditions," says Wipf. "If they put it in the microwave it will be totally different than if you bake the product or broil it."

  Says Popplewell, "You also have issues of labeling. If we were to put a finished reaction flavor into a system, it could be labeled 'natural flavor.' This may not be the case for certain precursors, which then may appear on the label as something the consumer may not like."

  The final approach is to process reaction flavor raw materials all the way to the desired flavor components and add them just like ordinary flavor ingredients. This has the clear advantage as far as consistency is concerned.

  "To the best of my knowledge, a lot of the effort seems to be going into finished reaction flavors rather than creating precursor systems that are put into the product and reacted through the processing heat," says Popplewell. "There is the potential to flavor as you process with certain types of processes. That's essentially what happens during toasting, drying, etc. There might be some way to accentuate that."

  Advances in the techniques for creating process flavors now allow them to contribute several flavor notes, such as "roasted," "brothy" or "fatty."

  "It's a means of producing very efficient flavors, and it allows for tremendous simplification of ingredient statements," says Wipf.

  Although fully reacted flavors are relatively stable, keep in mind that heat processing may affect these by promoting further reaction which may create unexpected and undesirable flavors.

  "One thing you can't have is a processed flavor with a brothy character that is put into a product, further processed, and comes out with roasted notes," says Wipf. "You try to design the flavor so you don't get additional reactions and maintain the stability of the flavor."

  Creating a reaction flavor almost always requires working directly with a skilled flavorist. The flavorist can look at the control product formula and identify what reactions are taking place. In this way, the correct precursors or process flavor can be selected to duplicate what's missing in the profile - much like rebalancing a compounded flavor.

  "It's possible to get something that works by trial-and-error. But to get the optimum product and have control of the product, you really need to divulge the formulation," says Popplewell.

  Working with the flavorist, examine the system to determine what flavor components you wish to end up with and examine what environments result in these flavor notes -- the sort of heat that results in a "fried" flavor versus one that produces "cooked meat" flavor, for example. Compare this with the actual environment the product will have from both a processing and ingredient perspective.

  "The pH has a big effect on reaction flavor. Other ingredients such as proteins and amino acids also affect reactions," says Popplewell. "You pretty much need to know in-depth what's in a product beyond just the basic ingredients. You have to know what free amino acids are present, what reducing sugars, etc. That will give you the foundation for designing a precursor system."

  Even when using finished process flavors a certain degree of process control is necessary for best results. This includes control over pH and moisture level, as well as designing the process procedures so that contact between the reagents and certain necessary phases of the product (the protein in a meat product, for example) takes place.

  "Variations in the ingredients can add variations to the product. Production processing control and QC systems certainly will not be as simple as with an ordinary product," says Popplewell.

  Says Wipf, "The major difference between a process flavor and a compounded flavor is that the processed flavor has less of the more volatile flavor constituents than a compounded flavor that will undergo further heat processing."

  Still, keep in mind that process flavors may continue reacting.

  "With reaction flavors, typically the reaction is not fully complete," says Popplewell. "If you were to do additional heat treatment, you might get some other flavor development. You may have consumed the initial flavor precursors, but you may have formed other compounds that are precursors to other reactions."

  If this is the case, some of the concerns of over-cooking by the consumer apply to reaction flavors as well as to traditional compounded ones. Are the reactions that form a finished process flavor ever complete? Probably not.

  "What you hopefully can do is get the reaction flavor to a point where it is relatively stable so that the consumer heating it at home won't change it that much," says Popplewell. "If they were to heat it for a long time would it change? Maybe, and it would definitely be something that you would want to look into.

  "Reaction flavors definitely add a level of complexity," continues Popplewell. "You have to weigh the benefits over what it costs you. If you can make a highly unique product, it might be worth it. If you can achieve the same thing by doing a careful compounding job with traditional flavor ingredients, that might be preferable."

  Heat can be a product designer's dearest ally while being a clever enemy. Whether you are using traditional compounded flavors or the latest in process flavor technology, evaluating the product formula and process is a requirement. This is the only way heat -- whether it is acting as a friend, foe or a combination of both -- can truly be tamed.

Basic Precursors Used in the Development of Reaction Flavors

Amino Acids: cysteine, glutamic acid, valine, glycine, hydrolyzed vegetable protein (HVP), hydrolyzed animal protein, etc.

Reducing Sugars: glucose, xylose, ribose, ribose-5-phosphate

Vitamins: thiamine

Sulfur Compounds: Furanones, sulfides, thioles (cysteine, thiamine)

Nucleotides: inosine 5'-monophosphate, guanosine 5'monophosphate

Acids: lactic acid, aliphatic carboxylic acids, acetic acid, etc.**Source: "Bioprocess Production of Flavor, Fragrance and Color Ingredients," 1994, John Wiley & Sons Inc., New York.

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