How heat affects flavour

It’s a paradox. Heat is both a primary enemy of flavour ingredients and essential to the development of many desired flavour components in foods.

By studying how heat affects flavours, R&D professionals can be better equipped to create formulas and processes that deliver consistent food products with the desired flavour profile.

Why is studying heat’s effect on flavour so important? After all, consumers subject foods to heat all the time when they cook at home. First, “industrial” processing differs from home preparation. Certain techniques are impractical on the industrial scale and alternative processing methods won’t always develop flavours in the same way. When preparing a chicken pot pie, for example, the chicken may be prepared by steam cooking instead of simmering.

This difference can cause the same flavour precursors to undergo different reactions and produce different flavours.

Compensating for differences in flavour development and needing to maintain the flavour profile over a product’s shelf life require the addition of flavour ingredients that ordinarily wouldn’t be in a home-prepared food. These ingredients may be sensitive to heat. Even flavours like vanilla are subject to change because of heat exposure.

In the home, a gravy would be prepared and immediately served. In foodservice, that same gravy may have to survive freeze/thaw cycles and heat during final preparation.

Studying heat/flavour interactions is also important to obtain the flavour consumers expect from alternative preparation methods, microwaving for instance. During microwave heating, moisture migrates to, and evaporates from, the surface of the product. This prevents browning reactions from occurring because the evaporation keeps the surface temperature too cool. At the same time, surface evaporation can cause steam distillation of certain flavour components. Another factor is that the speed at which microwave ovens heat simply doesn’t allow enough time for many flavour producing reactions to occur.

A particular challenge is creating a flavour that works in products that can be prepared in several different ways. Here, the flavour system must produce a consistently acceptable product following exposure to different heating methods.
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On the plus side

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 flavour profile. The reaction that results in many of the more common “cooked” or “browned” flavour notes is the Maillard (nonenzymatic) browning reaction. Chicken potpies 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 restaurant kitchen or production facility, IQF chicken might be used.

To pinpoint where flavour development reactions may be lacking, a product designer must have an understanding of other reagents in the system. Start by developing the product target with whatever traditional cooking is appropriate. Next, evaluate this “gold standard” for taste and aroma, 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 flavour notes that are missing. Appropriate flavour ingredients then can be selected or compounded to fill in gaps.
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What does heat do?

Volatilization is the loss of flavour from volatile flavour components that “flash off” when heated in an open processing system. This happens because many flavour substances have lower boiling points compared to a product’s other ingredients.

Various flavour components are affected differently. Consequently, volatilization/co-distillation not only can reduce overall flavour impact, it can throw the flavour system off balance. Just how out of balance depends on the processing conditions and the potential of flavour components to flash off.
Degradation of flavour components differs from volatilization in that it can occur in either an open or closed processing system. For example, the pH varies depending on the flavour system and the product. In a tomato sauce, you may be limited to a low pH. This affects the flavour profile and the degradative reactions that may take place. Moisture also is a factor in how degradative reactions proceed.

Heat can also accelerate chemical reactions among flavour components and other ingredients. 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 flavour components and affect the product’s taste or aroma.

Many flavour 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. Even if flavor components don’t react directly with other ingredients, these other ingredients may serve to destabilize an otherwise heat stable flavour system.

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Protecting flavours

Many of the undesirable effects that heat has on flavour can be minimized by reducing exposure to heat—for example, adding it later in the process. Though this approach is effective in many applications, some products demand a more novel solution. The first step in trying to protect flavour from heat is to analyse stresses to which the flavour will be subjected. Will the product be retorted or hot filled? Must it go through a freeze/thaw cycle?

Rebalancing the flavour is simply having it recompounded so that it generates the desired profile after processing. Flavours can be rebalanced in different ways. Flavour can be designed for more heat stability by choosing more heat stable components. Higher molecular weight homologs of flavour chemicals tend to have higher volatilization temperatures, yet often they can duplicate the flavour effects. Another approach compensates for the fact that some notes may be more volatile than others by adding higher levels of the more volatile flavour components. Spiking may make flavour seem harsh initially, but it will be correct in a completed product.

What about off flavours due to chemical interactions? If the offending ingredients can’t be replaced, you must figure out how to mask them.

Flavour profiling helps the flavorist determine the way to mask off notes. An off flavour that occurs at the end of a flavour profile will never be masked by, say, acetic acid, which enters the profile up front. Adding garlic or spice or creaminess will be more effective.

Alternative carrier systems for flavour components can also reduce volatilization. Changing from alcohol to a lipid-based carrier is one option. Even changing from alcohol to propylene glycol may change volatility enough to correct a minor imbalance. Some heat-generated chemical reactions are necessary for foods to obtain their expected flavour profile. Baked products would taste pretty flat without the “browned” flavour notes from the Maillard reaction.

Encapsulation preserves flavour 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. Water-soluble encapsulating materials release flavours when exposed to moisture. In aqueous systems, a water-soluble encapsulate will slow the flavour’s release, but not prevent it. In dry systems, flavour will not be released until the product is reconstituted with water or mixed with saliva.

Fat-soluble encapsulates release their cargo when temperature exceeds that of the encapsulating fat’s melting point. Insoluble, heat stable encapsulating materials such as corn zein or shellac will hold their flavour cargo until forced to release it by exposure to shear.

Selecting the correct encapsulation solution depends on what the flavour needs to be protected from and through what stages of processing the encapsulate must survive prior to releasing its cargo. Imagine a product made in a batch kettle and heat treated or pasteurized later. Although this is an aqueous system exposed to heat, either a fat- or carbohydrate-based encapsulation system can slow the flavour’s release.

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

Nevertheless, volatility may not be the only issue. It’s still possible that the heat of retorting could change the flavour components into something unexpected and, possibly, unpleasant.
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Don’t beat the heat, use it

There are many desirable flavours 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 flavourful substances. As processing procedures often differ from home cooking methods, these reactions often can proceed differently from what is expected and desired.

Reaction flavours are a vehicle by which desired “cooked” flavours can be added to a food formula. Three primary approaches to reaction flavour 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. Another approach is to add fully reacted precursors.

Although using flavour precursors may seem an ideal way to get “freshly cooked” notes into a product, this actually puts an unnecessary burden on product designers and limits their chances for success. This approach requires strict control in order for the flavour profile of a product to be consistent. Using a partially reacted flavour is a more reliable way to add a process flavour.

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 flavour characteristics, it’s unlikely that precursors or intermediates could successfully be added for reaction during preparation. The final approach is to process reaction flavour raw materials all the way to the desired flavour components and add them just like ordinary flavour ingredients. This has the clear advantage as far as consistency is concerned.

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