Getting a ReactionGetting a Reaction

 

Getting a Reaction
August 1997 -- Cover Story

By: Suanne J. Klahorst
Contributing Editor

    Physico-chemical reactions involving flavor components -- whether between flavors, or between flavors and nonflavor components of food and the environment -- are loosely termed "flavor interactions." These interactions influence the quality, quantity, stability and the ultimate perception of flavor in food. Flavor is primarily a combination of taste and odor, and along with appearance and texture, comprises the criteria for sensory acceptance of foods.  The term "artificial flavors" refers to those flavors that are added to foods, or consisting of compounds not existing in nature. Naturally occurring flavors, or those formed by heating, aging or fermentation, are considered "natural flavors." Naturally occurring flavors that are synthesized for addition to foods take on the label "nature-identical" flavors.  Fruit flavors serve as frequent examples of how flavors are isolated and translated into a commercial food ingredient. Nature locks the flavor into fruits via shells and skins, relying more on aroma to entice animals into acting as seed carriers. Many fruits need some form of interaction in order to develop their optimum flavor. Enzymatic reactions that occur after the fruit is picked, or after cutting and chewing, are often desirable in terms of flavor development. While most fruits certainly yield their best flavor when eaten just after being picked, a fruit flavor must provide this taste experience in a specific application, as in a dairy product or a confection. Therefore, fruit flavors are formulated and compounded for specific applications. The goal of the product designer is to select flavors that perform optimally within the context of a chemically reactive food product. Successfully achieving this goal requires knowledge of flavor interactions.Basic interactions  Physical and chemical flavor interactions occur continuously during food growing, harvesting, processing, storage and consumption. Interactions can be attributed to various types of chemical bonding: covalent bonding, hydrogen bonding, hydrophobic bonding, and the formation of inclusion complexes. The most commonly measured physical aspects of flavor interactions are binding, partitioning and release. Binding refers to the absorption of volatile and nonvolatile components of flavor onto the constituents of the food matrix. Partitioning describes the distribution of flavors in the aqueous, lipid or gas phases associated with the foodstuff and the package. The point at which flavor is made available to human sensory receptors is termed "release." Optimizing the time for flavor release is product-dependent, since longer times are needed for foods that are well-chewed than for drinks that spend only a few seconds in the mouth.  Quantities of flavor compounds required for detection are very small, frequently in the part-per-billion range. The understanding of flavor interactions is complicated by the numbers of flavor compounds available to participate in them. Chocolate flavor contains more than 250 components; coffee flavor is estimated to have more than 700.  Lipids, including those termed "fats," play an important role in flavor stability and delivery. They are important to food flavor because they act as solvents for hydrophobic flavors, and serve as components of the interfaces that are involved in flavor release from food. Fat affects aroma, character, masking, release and reactions of flavors. Most aroma chemicals are partially soluble in fat, which in conjunction with aroma, provides mouthfeel and richness. Aroma compounds are responsible for the buttery, creamy rich flavor of dairy products high in milkfat. Fats are precursors for flavor reactions that occur during heating with protein in baked goods and roasted meats. They participate in aging reactions that give unique flavors to cheeses. The ability of fats to mask off-flavors is attributed to their capacity to solubilize off-flavors, decreasing their volatility.  Triglycerides in foods can attract flavor compounds, depending on the fatty-acid chain length and degree of unsaturation of the fat. Triglycerides with only unsaturated oleic acid bind more flavor than those with only saturated fatty acids.  Flavors partition themselves between the oil and water phases differentially, based on the chemical structure of the flavor and the chain length of the fatty acids present. In foods in which fat has been reduced, the flavor release is affected by this partitioning, since flavorants in aqueous systems possess a higher equilibrium vapor pressure than lipid systems. Volatiles release more quickly from aqueous systems, and dissipate, resulting in less of a flavor impression on the human sensory organs.  The role of lipids in flavor-partitioning determines the delay in release of fat-soluble flavors measured during time/intensity profiles. Water-soluble flavors are released first, followed by the release of their fat-soluble counterparts, resulting in successive flavor perceptions. Lack of sustained flavor perception poses one of the challenges in successfully formulating reduced-fat foods that exhibit a similar richness to their high-fat analogs.  Proteins possess little flavor of their own, but they bind several volatile flavor components particularly well in the presence of heat denaturation. Binding, due to hydrophobic interactions and hydrogen-bonding, is reversible, as in the case of ketones, hydrocarbons and alcohol-based flavors. Covalent binding, such as Schiff base formation (aldehydes and amino groups), often is irreversible. Some of the factors influencing protein binding to volatiles are: temperature, pH, concentration and water presence. Proteins may bind more or less of a flavor component, depending on length and extent of heat treatment. In dairy proteins, several flavor components, such as a vanillin, benzaldehyde and d-limonene, were reduced by as much as 50% in solutions containing whey proteins or sodium caseinate. Protein-flavor binding can reduce the impact of desirable flavors and carry undesirable flavors to sensory receptors. The most widely studied, documented protein-flavor interaction is the binding of off-flavors to soy proteins.Size and structure  Carbohydrates serve several important flavor-enhancement functions. Ranging in size from small to large, they function as sweeteners; browning-reaction participants; fat replacers; viscosity builders; and flavor encapsulators. Sugars serve as carriers for flavors by physical interaction in aqueous systems, and by chemical-binding in dry ingredients. Structures of larger carbohydrate molecules, such as starch and cyclodextrins, can form hydrophobic regions that serve as inclusion mechanisms for flavor compounds of a like, hydrophobic chemistry. The flavor molecules that fit into these hydrophobic regions are called "guest molecules." These interactions are highly reversible, since no other chemical reaction takes place between the starch and the guest, other than the hydrophobic attraction. This interaction forms the basis for the molecular encapsulation of flavors.  Polysaccharides, particularly hydrocolloids and gelling agents, bind flavor components to varying degrees. When the concentration of flavors is held constant -- and the level of polysaccharides increases -- perception of aroma and taste decreases, as a result of viscosity. The sweetness of sucrose, for example, is decreased when the viscosity of a solution of guar gum or carboxymethylcellulose is increased.  Carbohydrates also alter the volatility of aroma compounds. When compared to flavor compounds in a water solution, the addition of mono- and disaccharides increases volatility, and the addition of polysaccharides decreases volatility. The effect of carbohydrates on volatility is particularly important in food systems that use fat replacers, since volatiles are released at a faster rate when lipid content is low, due to the weaker interactions of carbohydrates with hydrophobic flavor compounds.  Food matrices often are composed of proteins, carbohydrates and lipids, so interactions with flavors often occur between two or more components. The Maillard reaction (also known as nonenzymatic browning), in which reducing sugars react with amino acids to produce aromatic volatiles and browning products, is responsible for the flavors formed during thermal treatment of foods, such as chocolate, coffee, roasted meats, bakery items and caramel. The number and type of flavors produced by these reactions depends on the quantity and type of amino acids available to participate in the reaction mixture. In combination with lipid oxidation reactions, the Maillard reaction generates flavor compounds when carbonyl compounds (from degradation of sugar or lipids) react with amines or thiols during heating. Flavor reactions within a complex food matrix seldom occur in isolation, and are affected by the reactants, the intermediates and the products of other reactions.Packaging problems  Among the oldest packaging materials, glass remains popular in spite of its fragility and weight disadvantage. Along with foil and metal, glass is considered inert and unreactive with flavor compounds. Consumer desire for conveniently packaged products has promoted a trend in packaging materials away from the inert toward more interactive synthetic polymers. Plastics used for packaging are either simple homopolymers (one material) or copolymers (combinations of materials). Flavors and packaging interact as a result of three factors: migration of packaging or food components; permeation of the package by gas, water and organic vapors; and exposure to light.  The transfer of vinyl chloride monomers from packages made of polyvinyl chloride (PVC), and migration of residual styrene present in polystyrene materials, are both examples of odor-active interactions with plastic that adversely affect food flavor and safety. When polymers for plastic films are manufactured, complete polymerization of the raw materials is rarely feasible, leaving low levels of residual monomers or oligomers. Processing aids, such as plasticizers, catalysts, color and strengtheners, also are added. Many of these low molecular weight materials are capable of migrating into foods.  Migration of food flavors into packaging also is a concern. The low molecular weights and boiling points of flavor components enhance the rate or extent of this flavor absorption. The loss of flavor to packaging is called "scalping," and results in significant flavor losses that must be compensated for in the formulation. For example, in toothpaste manufacturing, the advent of flexible tubes made of polyethylene or polyolefins required 5% to 10% more flavor, due to sorption in the contact layer of the tube. The addition of extra flavor was estimated to cost between $10 million and $18 million annually in the United States. Orange juice, which contains flavor components such as acetaldehydes, are scalped by polyethylene within a few days. Compensation for natural flavor loss in this type of product is not as easily accomplished.  Migration of contaminants from outside the package, such as strongly flavored mints, or perfumes from cosmetics and detergent, is controlled in the retail grocery industry by product segregation and separate bagging after purchase.  Permeation of packaging by oxygen and water vapor from the atmosphere figures in as a factor in flavor degeneration. The transfer mechanisms for flavors are similar to those for gas and moisture, but the gains and losses have more significance because of their lower initial concentrations. If oxygen permeates the package, oxidation may result. For example, oxidized phospholipids contribute to warmed-over flavor development in precooked meats.  Oxygen also participates in light-induced reactions. Light is responsible for the deterioration of food pigments, vitamins, proteins, amino acids, fats and oils. Oils are susceptible to more rancid flavor development, due to oxidation, as their degree of unsaturation increases. The resulting compounds directly and indirectly impact flavor by acting as precursors in flavor and off-flavor development. Oxidation of vitamins and oils, which protect and deliver flavor by serving as antioxidants or flavor carriers, contributes to flavor loss.  Limiting the flux of water vapor to keep moist foods moist, and dry foods dry, offers an obvious packaging advantage. Not surprisingly, water activity also represents an important variable that influences the rate of many chemical reactions of flavor compounds. A less obvious interaction is permeation into food packages by environmental contaminants -- such as printing solvents (glycol ethers), cigarette smoke, disinfectants, and diesel fuel odors -- during shipping and retail storage of packaged foods.  From inside of the food package, aroma compounds sorbed through the packaging result in flavor loss. Flavor compounds exhibit permeability through packaging materials selectively, based on their molecular weight. This can create an imbalance in the flavor profile after packaging. These problems are solved by the use of higher aroma barrier films.Methods for measuring  An integrated approach to studying flavor interactions encompasses combining human sense with instruments and methods. No universal method exists for isolating the entire flavor-compound spectrum. A more common approach is using methods suited for isolating and enriching specific fractions of flavor compounds.  The dynamic flavor profile method. Many sensory methodologies are suitable for investigating model systems. However, model systems used for sensory research aren't always practical for product development. New methodologies in sensory science provide product developers with contextual information about flavor perceptions of a food product, which are relevant to the dynamic environment of the human mouth. This information is utilized to design synergistic flavor combinations that either enhance or mask flavor notes in the final product.  The dynamic flavor profile method is well-suited to assessing flavor in a food product, because it combines established descriptive methods (spider web profiles) with time-intensity measurements that quantify flavor change over the tasting duration. The data generated are vector-based graphical representations of the flavor impressions of several trained panelists. This evaluation technique has proven useful in several problem-solving scenarios involving flavor selection: interactions with fat replacers; masking off notes produced during processing; rounding out flavor profiles; and determining flavor stability in shelf-life studies.  Sniffing out odors. Two methods for sensory analysis, Aroma Extract Dilution Analysis (AEDA) and CharmAnalysis, have been developed to gain more information on odor activities. These techniques can determine the relative odor activity of single odorants within a more complex mixture of odorants, even though their chemical structures are unknown.  The methods involve serial dilution of the volatile fractions of flavors, and subsequent evaluation, using gas chromatography olfactometry (GCO). The AEDA method determines the flavor dilution factor by diluting and detecting the flavor component, until the odorant is no longer perceivable.  CharmAnalysis is a proprietary GCO system developed in the Flavor Chemistry Laboratory at the New York State Agricultural Experiment Station in Geneva, part of the Cornell College of Agriculture. Gas chromatography (GC) is used to separate the flavor component, and a trained flavorist sniffs the effluent of the GC to describe the aroma peak. CharmAnalysis offers a high-resolution GCO and complementary software, called Charmware. The software takes advantage of a database of flavor compounds, known as Flavornet, compiled from GCO, AEDA and other methods that have detected odorants, as shown in published research. Maintained by Terry Acree at Cornell University, the database now is available through the World Wide Web.  The 'E-nose' knows. Another development during the past 10 years is the analytical tool known as "the electronic nose." "E-noses are arrays of organic volatile sensors, connected to a computer-based data analysis system," says Leslie C. Smith, Ph.D., vice president, technology, fragrance division, H&R Florasynth, Teterboro, NJ. "In anthropomorphic terms, the sensors emulate human olfactory receptors, and the computer emulates the human brain."  Initial claims for this method included its use for wine tasting, fragrance creation, and body-odor analysis. But it has found its niche in more pragmatic applications, such as a quality-control tool to spot batch-to-batch differences in raw materials and detection of off notes in packaging materials. E-noses are used for basic QC functions in several industries, including brewing, tobacco and cosmetics. As computational techniques develop, they will certainly be useful in more complex tasks of smell or taste.  But some believe the technology possesses inherent limitations. "It is difficult to imagine that they will ever replace the highly creative organ -- the human nose -- of the trained perfumer," Smith says.  Several manufacturers are offering instruments based on different types of sensors, including: Neotronics Scientific Inc., Flowery Branch, GA; Alpha M.O.S. America, Belle Mead, NJ.; and Aromascan, Hollis, NH. During the last two to three years, the big advance has been in sampling techniques and the use of neural networks to recognize the patterns, in conjunction with the appropriate sensory training sets.  Using your head(space). Aroma compounds are well-suited to the use of headspace methods. Headspace refers to the vapor space above a foodstuff, where volatile compounds are collected, providing a mechanism by which these compounds can be segregated from the foodstuff matrix. The aroma then can be isolated and identified, without interference from the matrix.  Capturing aromas from the headspace generally is achieved by one of three methods -- static headspace, dynamic headspace, or purge and trap -- and often are combined with solid-phase microextraction (SPME). Static headspace sampling uses a sealed vessel to collect volatiles, with little or no impact on the foodstuff. Dynamic headspace, and purge and trap, concentrate the volatiles. Purge and trap is best-suited for liquids, as it entails the bubbling of a purge gas through the sample.  SPME represents a relatively new, as well as simple and convenient, method. Once the sample is collected via one of these methods, it is injected directly into a gas chromatography column for separation. GC operating temperatures range between 40° and 300°C.  Headspace methods are often preferable to chemical extraction, because they do not require an organic solvent, so the chromatogram has no solvent peak to mask or dilute the peaks of the aromatics being separated.  Depending on the method used for flavor collection and concentration of aromas, the results can vary significantly in terms of final composition and flavor-component mixture. A flavor extracted from nature is first evaluated by these methods to determine how components will be blended into natural, or nature-identical, commercial flavors. Each type of flavor compound will be analyzed by a method appropriate for its chemistry and matrix to avoid artifact formation, and to obtain an authentic picture of the flavor composition. For example, low impact methods of flavor isolation are preferred for concentrating aromas from fruits and blossoms. However, these methods might not be suitable for reaction flavors that already have been subjected to heat treatment.  Flavor chemists at H&R Florasynth compared several methods of isolating and concentrating the fruit flavors of peach, passion fruit, strawberry and pear. They reported that the vacuum headspace method produced the most authentic fruit flavor concentrates of the highest sensory quality that were most typical of fresh fruits. The concentrates obtained with the dynamic headspace method were described as rich in the highly volatile components, but the higher-boiling components were under-represented. This produced flavor concentrates with an unbalanced fruit character.  The Likens-Nickerson method produced unwanted artifacts from the thermal impact of the steam distillation, although this was improved by conducting the distillation under vacuum. The Likens-Nickerson method utilizes an organic solvent for the extraction process, which is not suitable for polar, water-soluble compounds, and produces an inferior flavor concentrate of some fruits.Protection processes  Protecting flavors from interactions that diminish or degrade them involves minimizing processing influences (heat, pH); environmental factors (evaporation, oxygen); and chemical interactions with the food matrix. Protection mechanisms designed into commercial flavor delivery provide a mechanism for releasing the flavor at the appropriate time. Flavors can be encapsulated by several different processes to protect them from undesirable interactions.  The initial goal of encapsulating flavors was to design solid forms of liquid flavor ingredients. Solid flavors provide longer shelf life as a raw material and are compatible with dry ingredients. Extended shelf life is due to limited exposure to temperature, moisture, oxidation and light during storage and processing. Additional encapsulation benefits include: reduction of negative interactions with other compounds; and better handling characteristics. Advances in encapsulation technology allow encapsulation of hydrophobic or hydrophilic flavors, which are available in a range of particle sizes, shapes and solubilities.  The industry is developing micro and macro encapsulation technologies utilizing various coatings: polymers, waxes, fats, starch, hydrogenated vegetable oils, proteins, maltodextrins, mono- and di-glycerides, gum arabic, alginates, gelatin, and cellulose compounds. This enables product release by different mechanisms. Gelatin capsules are released by water; fat coatings melt at high temperatures; lipid capsules are degraded by lipid-degrading enzymes (lipases); gums are broken down during chewing; and so forth. The type of encapsulation chosen by the product developer determines whether flavor and aroma are released when the product is opened, heated, mixed or eaten.  The type of encapsulation process determines the cost, particle size, and shape of flavor capsules. Spray-drying represents the most economical method used to produce solid flavors from liquid ingredients. Spray-dried particles are generally smaller and dustier than agglomerates and granules. Spray-chilling is used for application of coatings that are applied warm and then formed by cooling, such as fats or waxes. Spray-chilled capsules may require additional agglomeration to improve the wetting properties for addition into liquid products. These capsules have found applications in bakery products, dry soup mixes and high-fat foods.  Numerous encapsulation choices exist, including:Extrusion. The production of particles using an extrusion process is useful for very sensitive and volatile compounds. The flavor is completely surrounded by the encapsulation material, and shelf life can be increased by years. Though costly, this method can be used to produce particles large enough to be visible in a food product, if desired.Coacervation. Proteins are typically used for coacervation, a process whereby the coating material forms microcapsules around the material to be encapsulated, and is separated from the aqueous phase by further processing. Cost is a limitation for this method, as is the availability of food-grade encapsulating materials.Molecular encapsulation. This is a method of encapsulation that occurs on a molecular level using cyclodextrins. Although a simple process, the disadvantages are a less uniform structure, and the need for temperature to release the flavor compound in the food product.Submerged-nozzle encapsulation. This process creates capsules possessing a distinct shell of gelatin, alginates, carrageenan, starch, fat or wax. Two or more polymers can be combined. The larger the capsule, the lower the price. Coatings can be fat- or solvent-based, and provide additional protection from heat in baked goods or microwavable foods.  Encapsulation processes can be combined to use the advantages of two or more methods in one granule. A product can be spray-dried, compacted and granulated in a fluidized-bed system. Granules can be layered, using a different flavor in each layer, and released sequentially. These custom granules may require higher use levels, but offer the opportunity to meet specific customer requirements.Howdy, partner  Flavor suppliers traditionally gauged their competitive advantage by how expertly they were able to translate a naturally derived food flavor into a commercial product for an "off the shelf" application in product development. In today's market, an excellent flavor is only the initial foundation for commercial success. As food companies become more competitive, and demand more return on their R&D and advertising investments, flavor suppliers are meeting these needs by providing flavor interaction know-how, product-development services, and exclusive custom flavors offering competitive advantages to their partners.  "What we are selling today is more than a high-quality flavor, it is the understanding and knowledge of how to make that flavor perform in the context of the food product," says Heinz Juergen Bertram, Ph.D., vice president of technical service at H&R Florasynth. "Flavor suppliers are participating in the R&D process, and are taking more responsibility for the performance of the flavor in the final product."  This level of buyer-supplier relationship requires a close working relationship with the client. Food companies choose a limited number of flavor suppliers with whom to form partnerships. In return, they obtain access to proprietary technologies, custom products and a variety of services. A recent flavor-industry trend involves recruiting culinary expertise to turn out food products designed with flavor profiles that meet customer specifications. Several companies now have chefs on staff who assist customers on collaborative product-development projects.  Partnerships benefit product development in food companies and flavor companies alike. Food companies gain expertise and assistance in their product-development functions, and flavor suppliers shorten their own product-development cycle time by taking the guesswork out of meeting customer needs. Flavor suppliers are developing core competencies in flavor-delivery systems and encapsulation strategies, and partnerships provide an avenue for testing new technologies in finished products.Up and coming  Bioengineered plants, such as soybeans, tomatoes, potatoes, corn and canola, now are being marketed to the food industry. The impact that genetic engineering will have on the flavor industry is still undetermined, but applications and benefits could prove significant. Calgene, Inc., Davis, CA, recently acquired by St. Louis-based Monsanto Company, introduced its Laurical® canola oil this year. Laurical is being evaluated for flavor enhancement in foods that traditionally incorporate tropical oils, according to John Diehl, sales and marketing manager, Calgene. Preliminary results indicate that up to 25% less flavor is required when the new canola oil is used in some applications. The mechanism of enhancement is still being investigated.  The approval process for biotechnology products in flavor applications has been integrated into the Flavor and Extract Manufacturer's Association expert panel of toxicologists, biochemists and other scientists. The panel assesses the safety of flavors and ingredients, including those produced using genetically modified organisms (GMOs), according to guidelines identified by the U.S. Food and Drug Administration in its approval of recombinant chymosin. A recombinant thaumatin produced by a GMO was recently determined to be GRAS, and provides an alternative source to the plant-derived thaumatin. As more biotech products are determined to be safe, their success is expected to encourage more research for biotech applications in flavor production.  Recently published estimates for the market demand for flavors and flavor enhancers climb as high as $2 billion annually by the next century. Recent activity in flavor R&D revolves around the use of flavors compatible with fat replacers, and the design of new flavor blends based on tropical fruits. Natural or nature-identical flavors will continue increasing in popularity, due to consumer demand. Consumer demand for aseptic packaging and microwavable containers will continue to require research on flexible packaging alternatives that minimize flavor interactions.  Suanne Klahorst is a free-lance food science writer and food scientist with 10 years of food-industry experience. She is currently with the California Institute of Food and Agricultural Research at the University of California, Davis.Back to top© 1997 by Weeks Publishing Company3400 Dundee Rd. Suite #100
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