Biogeneration of Aromas - ACS Symposium Series (ACS Publications)

29 Biogeneration of Aromas

Downloaded by EMORY UNIV on January 21, 2016 | http://pubs.acs.org Publication Date: August 25, 1986 | doi: 10.1021/bk-1986-0317.ch029

In S u m m a t i o n

Ira Katz International Flavors & Fragrances, Union Beach, NJ 07735 Many important food aromas arise via biochemical pathways. These pathways include microbial fermentation, exogenous and endogenous enzymic action and plant metabolism. In the past, flavor research was concentrated on identifying the important aroma chemicals responsible for the specific food aroma. Using this knowledge, present day research is focusing on elucidating the biochemical pathways responsible for important aroma chemical production. In addition, advances in biotechnology is providing opportunities to produce important aroma chemicals. It is anticipated that increasing our knowledge of bioaroma generation will ultimately lead to foods with intensified flavor, production of specific food aromas and production of previously unavailable aroma chemicals. This book on the biogeneration of aromas comes at an opportune time. During the 1960's and 70's research in this area at the industrial and academic level was neglected in favor of classical analytical research aimed at unravelling the complexities of the chemical mixtures responsible for the aroma of food. In retrospect it appears that this was the proper way to approach this problem, since we had to identify the important aroma contributors before considering their biological origin. Factors Stimulating Bioaroma Research Ten years ago bioaroma generation was not a commonly discussed research topic and most scientists did not consider it to be a viable means of aroma production. Today, there is obviously a resurgence in this area of research and a number of factors appear to be responsible for the renewed interest in bioaroma research. The "new biology", being led by advances in genetic research and genetic engineering permits forward thinking food scientists to explore bioaroma production in practical terms and experimentally pursue the concept. Bioaroma generation may provide the ability to produce impor0097-6156/ 86/ 0317-0380506.00/ 0 © 1986 American Chemical Society In Biogeneration of Aromas; Parliment, Thomas H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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tant secondary metabolites that are not available by other conventional production procedures. It is d i f f i c u l t to separate this concept from economic considerations since many available aroma chemicals are too expensive to use. Conceptually, bioaroma generation holds the promise of significantly lowering the cost of many important aroma chemicals and thus allowing their broad, general use. There appears to be a realization that we must understand the biological world we live i n . The accumulation of basic knowledge dealing with bioaroma generation is part of this and, in addition to using i t in i t s own right, can lead to information that is important in other areas of food research. Another stimulation for research on the biogeneration of aromas comes from the market place where consumers appear to be rejecting the a r t i f i c i a l flavor label and requesting the natural label. Regardless of the motivation, and whether i t w i l l be long l a s t ing, the scientific aspects of this subject are important and worthy of study. The aroma or odor of food is an important part of the hedonic value of the food and thus becomes part of our food selection process. Since our nutritional state and well being are part i a l l y related to food selection, understanding the biological generation of aromas is not conducted only to satisfy the current need of the marketplace, but takes on an important scientific and nutritional role. Origins Of Aroma Approximately 5,000 volatile chemicals have been identified in our food supply and there i s l i t t l e doubt that others exist and w i l l u l timately be identified. These complex chemical mixtures, responsible for food aroma, can number as high as 1,000 for an individual food, and originate via a variety of processes that include enzymic action, autoxidation, microbial action, food processing, cooking and chemical interactions. It is no wonder there are over 5,000 chemicals contributing to food aroma. A convenient way to consider these chemicals is to divide them into three broad groups. (1) One is the heat derived or, Maillard Browning aroma chemicals that are formed when food i s cooked or heat processed. These are responsible for the aroma of cooked foods such as meat, coffee, poultry and similar products. They also contribute to the aroma of heat processed fruits and vegetables. (2) A second group of chemicals are those that form during heat processing but are dependant upon chemical precursor formation during a deliberate fermentation step. Examples of these are cocoa and bread. The non-volatile chemical precursors formed during microb i a l fermentation react via the Maillard reaction to form the aroma chemicals responsible for the typical aroma of the product. (3) The third group, and the subject of this book, are the biologically derived aroma chemicals, often referred to as secondary metabolites. This diverse and important class of aroma chemicals generally arise via: (a) microbial fermentation; (b) enzymic action from endogenous enzymes; (c) enzymes added during processing; (d) end products of plant metabolism.

In Biogeneration of Aromas; Parliment, Thomas H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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Examples of products whose aroma are primarily biologically derived are vegetables, f r u i t s , berries, essential o i l s , fermented dairy products and alcoholic beverages.


Consumer Perception of Flavors Several of the chapters in this book present arguments that a p r i mary reason for understanding and ultimately manufacturing biologi c a l l y derived aromas i s the consumers demand for natural flavors. There is l i t t l e doubt that in addition to the scientific aspects, a major impetus behind the interest in the biological origin of aromas is the present day consumer demand for natural flavors. Also, the present day health orientated, better educated consumer understands that there is a relationship between health, diet and nutritional well being. As a result, this has manifested i t s e l f as a demand for natural flavors and foods or, perhaps more importantly, a rejection of the word a r t i f i c i a l . It is not the purpose of this book to argue the merits of consumer attitudes or to consider the complex marketing problems this has presented to the food industry. However, we recognize that the demand for "natural flavor" is partially responsible for the present day intensified research on the biological origin of aromas. Therefore, i t is important that we understand the legal definition of natural and a r t i f i c i a l flavors as i t applies to the United States. Definitions In the Federal Register, the term natural flavor or natural flavoring means the essential o i l , oleoresin, essence or extractive, protein hydrolysate, d i s t i l l a t e or any product of roasting, heating or enzymolysis, which contains the flavoring constituents derived from a spice, fruit or fruit juice, edible yeast, herb, bark, bud, root, leaf or similar plant material, meat seafood, poultry, eggs, dairy products, or fermentation products thereof, whose significant function in food is flavoring rather than nutritional. The term a r t i f i c i a l flavor or a r t i f i c i a l flavoring is the opposite and is defined as meaning any substance, the function of which is to impart flavor, which is not derived from a spice, fruit or fruit juice, vegetable or vegetable juice, edible yeast, herb, bark, bud, root, leaf or similar plant material, meat, f i s h , poultry, eggs, dairy products, or fermentation products thereof. It then goes on to l i s t , the permitted substances that can be used in a r t i f i c i a l flavors. It is apparent that the above definition of natural is broad and the fine points are subject to legal interpretation. What is important, from the standpoint of those interested in the biological derivation of aroma, is that products derived by enzymolysis and fermentation are considered to be natural substances. Historical Perspective The use of natural flavors by the flavor industry is not new. Hist o r i c a l l y , the industry began by the creative use of natural substances derived from botanical sources. During the 1960 s, we experienced an intense effort in analytical research that s i g n i f i !


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In Summation

cantly increased our knowledge of the chemical constituents of food aroma. As the flavor industry grew and cost effective synthetic chemicals, structurally identical to the natural became available, the synthetics were combined with the naturals to create cost effective and improved flavors. As our knowledge base increased, and more synthetic aroma chemicals became available, there was an i n crease in the creation and use of totally a r t i f i c i a l flavors and a concommitant decrease in the use of naturals. Many food scientists predicted that most natural flavors and botanicals would ultimately be replaced by the less expensive and more cost effective a r t i f i c i a l s . Undoubtedly, this would have occurred to some extent, were i t not for the consumer demand for naturals that began in the late TO*s and accelerated through the 80 s. Ultimately, the replacement may s t i l l occur since the underlying economic and technical factors are s t i l l in place. Downloaded by EMORY UNIV on January 21, 2016 | http://pubs.acs.org Publication Date: August 25, 1986 | doi: 10.1021/bk-1986-0317.ch029

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Biochemical Pathways Our knowledge of the aroma chemical composition of foods is extensive compared to what we know about the biosynthetic pathways which result in the formation of many of the important aroma chemicals in these products. The excellent work presented by Tressl points out the complexity of the subject and our lack of fundamental knowledge. His elegant approach of incubating tissue slices with isotopically labelled precursors and subsequently separating the aroma compounds, including enantiomers, has greatly contributed to our understanding of aroma chemical biosynthetic pathways. The biosynthetic formation of the mono- and sesquiterpenes is reviewed by Croteau. In addition, he proposes pathways for the formation of oxygenated terpenes that are the dominating aroma chemicals in many essential oils and citrus products. The carotenoids are a class of compounds important not only for their vitamin A activity but also as aroma precursors. Weeks describes the degradation of these compounds to produce numerous volatile products including the C13 ionones, important in fruit aromas. There are many ionones that occur in trace quantities in fruits that greatly contribute to the overall quality and perception of the aroma. Because of the unique aroma effect of these ionones i t is interesting to speculate on the aroma of the end product i f we could increase the concentration of the important ionones i n fruits and berries by breeding or processing. These fruits could have significantly enhanced aromas or, could be perceived as new, unique foods. Precursor Studies Two papers delve into the topic of non-volatile bound forms of aroma compounds. Schreier and Winterhalter show that terpenoids in papaya are present as water soluble glycosides. Acree's group investigated the non-volatile precursor of 3-damascenone, a chemical with one of the lowest known aroma thresholds. The latter were able to purify the precursor by a factor greater than 22,000. It i s interesting to speculate on how many other important aroma compounds are bound through oxygen linkages and how we might be able to take advantage of such aroma chemical precursors.


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Present Day Applications A number of chapters deal with the emerging f i e l d of biotechnology and i t s possible application to flavor and aroma chemical production. If economic barriers can be overcome, there is l i t t l e doubt that this rapidly emerging technology w i l l become a significant factor in the production of certain aroma chemicals and essential o i l s . Ultimately, this may lessen our dépendance on important agriculturally derived botanicals that suffer from periodic shortages and price fluctuations. The excellent paper by Armstrong shows that ethyl acetate, an important fruit aroma compound, can be produced by the yeast Candida u t i l i s . He was able to increase production of ethyl acetate by manipulation of the carbohydrate source and regulation of iron levels in the media. Alkyl methoxy pyrazines are an important class of aroma compounds exhibiting intense green bean/green pea aroma notes. Reineccius's group at the University of Minnesota show that mutant strains of Pseudomonas perolens produce the isopropyl isomer to a final level of 15 mg/L of culture. Since the threshold is 2 x 1 0 " ° ppm, the reported yield is substantial. Scharpf et a l extensively review the production of aroma compounds by fermentation processes. Of particular interest to those interested in this technology is the discussion on the factors to be considered in scale-up and the potential problems. They conclude that aseptic fermentation is appropriate when the product has a value of $ 1 0 0 to $ 8 0 0 per kg. They describe and reference a issued patented procedure for the production of γ-decalactone by aseptic fermentation using castor o i l as the carbon source. One important class of foods possessing natural flavors that arise via enzymatic and microbial action are "fermented" dairy prod ucts. This important food group, consisting of cheeses, yoghurt, buttermilk, sour cream and similar products is very interesting and illustrates the importance of taste and odor in food selection. There is no doubt that this group of fermented dairy products s i g nificantly contributes to our caloric intake and our over-all nutri tion. As consumers we use these products in a multitude of ways. We consume them directly, combine them with other foods directly or through cooking, convert them into sauces, etc. There seems to be no end to the creative way we u t i l i z e and consume fermented dairy products. The lesson is clear, we use these products because we en joy their odor and taste. In the selection process, their n u t r i tional value i s secondary when compared to their flavor. Since food consumption is a necessary part of l i f e , and in most societies food selection is primarily based on hedonics, then a p r i o r i , the biolog i c a l origin of aromas is an important subject. Ho's group at Rutger's describe the use of enzymes from Candida rugosa to convert butterfat to a series of neutral and acidic com pounds possessing a Romano cheese flavor. Similar technology is used by the food industry to produce enzyme modified cheese from young cheddar cheese. The f i n a l product possesses a more intense natural aroma and taste. Similar techniques could undoubtedly be used for the production of other natural cheese flavors. Gatfield reviews various fermentation reactions that occur in the presence of endogenous or exogenous microbial enzymes. Among


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the aroma compounds that can he generated are acetoin, acetaldehyde, ethyl acetate, pyrazines and the so-called C6 green notes. Essential oils have been used since the beginning of recorded history as sources of aroma chemicals. Lawrence discusses how factors such as photoperiod, stress, etc. can effect o i l production. He describes various essential oils that can be used as sources of specific natural flavor chemicals. Among them are leaf alcohol, benzaldehyde and tolualdehyde. This represents a technology which can be employed at the present time, since only classical isolation and separation techniques are required.


Future Applications Can we learn to control the enzymatic pathways to produce desired chemicals in good yield and prevent undesirable reactions resulting in off-flavor production? Hatanaka shows in detail the reaction scheme in plants whereby linolenic acid is converted via lipoxygenase and lyase enzymes to cis-3-hexenal. This is subsequently converted by other enzyme systems to leaf aldehyde and leaf alcohol. These three aroma chemicals are important "green" aroma notes in strawberries and other berries. If these enzymes were commercially available, then the production of natural cis-3-hexenal from inexpensive vegetable o i l is theoretically feasible. The work reported by Josephson and Lindsay illustrates the broad nature of aroma biogenesis. In the past, most food scientists did not consider that the desirable aroma of fresh fish was enzymatically generated after harvest. In seafood, post harvest aroma chemical formation is generally associated with negative odors. This work clearly shows that after harvest, polyunsaturated fatty acids are enzymatically converted to physiologically active compounds which are subsequently converted to volatile alcohol and carbonyl compounds that are characteristic of the odor of freshly harvested fish. Conceptually, i t i s interesting to speculate on the bioconversion of inexpensive secondary metabolites to others of greater value. Along these l i n e s , Schreier and co-workers use Botrytis cinera to convert linalool to a series of other terpenoids as well as to the furanoid and pyranoid linalool oxides. Reactions of this type are good examples of converting inexpensive, available aroma chemicals to higher valued products. Finally, Whitaker discusses the promise of plant tissue culture. This technology was originally received by scientists with great enthusiasm. However, we now realize that much more must be learned about biochemical and genetic regulation of plant secondary metabolites before large scale production becomes r e a l i s t i c . There is l i t t l e doubt that these goals w i l l evenutally be achieved. As we accumulate knowledge about the biogeneration of fruit aroma, i t is theoretically possible that someday we w i l l be able to "turn on" the appropriate enzyme systems to produce a complete natural fruit f l a vor. This is a d i f f i c u l t objective, and some may consider i t impossible, but the results are worth the effort. That i s why symposia like this are invaluable for stimulating aroma research. It w i l l be interesting to compare the conclusions of this symposia with a similar one five to ten years from now.


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The present use of biotechnology consists of the production of aroma notes and s p e c i f i c chemicals by fermentation; the application of improved enzymes and enzyme technology to food processing; and the app l i c a t i o n of new genetic technology f o r the production of improved food crops, spices and essential o i l s . The future application of biotechnology to bioaroma production i s l i m i t e d only by our own i n t e l l e c t u a l capacity to apply the technology as i t emerges. Increased food crop y i e l d s with i n t e n s i f i e d f l a v o r s , t o t a l aroma production of s p e c i f i c foods, production of important aroma chemicals previously unavailable and s i g n i f i c a n t l y lowered costs are some of the promises of the future.


RECEIVED May 23, 1986


Biogeneration of Aromas - ACS Symposium Series (ACS Publications)

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