Chapter 1
Molecular Approaches to the Study of Food Quality A. M. Spanier Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 1100 Robert E. Lee Boulevard, New Orleans, LA 70124 Research in the realm of food quality involves many scientific disciplines from cellular to organ-system physiology, from neural impulse transduction to psychomotor and psychological response, from isolated molecules to their combined biochemical interactions, from mathematics to complex statistical correlations, from negatively impacting microbiological issues to positively impacting microbial safety issues,fromsimple methods of creating food to complex ones, andfromhuman means of assessing food flavor quality to instrumental methods performing the same objective. For continued growth of our global economy, food scientists need to know the answers to questions such as: What is food flavor and food quality? How are they measured? What mechanisms, such as processing, endogenous factors, outside (external) factors, contribute to the production and loss of food quality? What are some food safety, nutritional and health concerns? What are some of the management and production strategies to enhance food quality? This chapter was written to present the most recent information in response to these questions. For millennia, man has raised animals for food and has utilized the soil for planting crops for food and fiber. Initially via trial-and-error and later through more deliberate methods of breeding or design, man selected highyielding varieties of animals and plants, fashioned numerous implements for agricultural and husbandry use, and even developed rudimentary methods of pest control to satisfy his needs. With the advent of the Industrial and Scientific revolutions, scientific methods were applied on a large scale to agriculture in order to improve yields of plant and animal food products. The resulting increase in agricultural productivity was essential to the further development and growth of the industrializing community of man; the massive increase in production and distribution of agricultural commodities led to significant increases in the standard of living in most developing nations and in the United This chapter not subject to U . S . copyright Published 1993 American Chemical Society
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States in particular. This development is even more important today with the maturing global economy, increased global population, and decreased acreage of arable land. The science of agriculture is dynamic and ever evolving. It requires not only a significant investment in training of research personnel and research but also in the development of skilled individuals who can conduct and translate research into products that contribute to the well-being of mankind. Today's researcher must be knowledgeable of both scientific and non-scientific factors that impinge upon their research. The latter includes consumer desires, commodity interests, socioeconomic factors, political commitments, agribusinesses, government and regulatory agency requirements, and general public concerns. Today's scientist must solve technical agricultural problems to ensure the continued adequate production of high-quality food and other agricultural products to help meet the nutritional needs of consumers, to sustain a viable food and agricultural economy, and to maintain a quality environmental and natural resource base. Productive and innovative scientists tend to have a detailed working knowledge of their own field and a fairly broad knowledge of science and technology in general. From this knowledge base there often grows over time, an understanding and appreciation of market needs and requirements essential for significant creative innovation. Scientists often start with an idea that is rather vague and general, and that might languish without special effort. The successful inventor/scientist is the one who molds these general ideas into more concrete form and does the work required to transform the initial concept into a finished final result. A scientist may have knowledge, experience, and commitment, but without some outside assistance, knowledge, or insight, all this information may go unexploited. Agricultural problems are often multidisciplinary, requiring expertise covering a range of disciplines such as the biological, physiological, psychology, physical sciences, engineering, mathematics, and economics. These problems may seem insurmountable when viewed from the perspective of one discipline but may seen amenable to original or even elegant solutions when viewed from another discipline. Communication is thus essential for this cross-disciplinary interaction so that opportunities for innovation can be recognized and exploited. This symposium was created to give those scientists involved in food quality research some basic appreciation of the disciplines involved in the study of quality. This would, hopefully, not only impart important information to the food quality researcher, but would also present a forum or potential avenue for further communication and exchange of ideas between scientists. Since the list of published books, peer reviewed articles, and symposia in various disciplines in food science are so massive that citation of all of them would fill a volume in itself, all literature citations are omitted from this chapter in an effort to not offend any author or group by the listing of selected examples.
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FLAVOR PERCEPTION AND COMPOUNDS AFFECTING FLAVOR The acceptance of a food in the market place is largely dependent upon the flavor quality of that food. Thus, preparation of food products for the consumer requires the food scientist to have knowledge of sensory evaluation, psychology (of sensory panels), biochemistry (at the receptor level), and physiology (nerve impulse recognition and transduction). The perception of a food's flavor quality depends upon a multifaceted series of sensory responses. These include not only taste and olfactory responses, but also the oral sensations of coolness, astringency and irritation, referred to as chemesthesis and mediated through the trigeminal system {Chapter 2). Several of these sensations are the result of changes in pH or ionic strength, reactions with salivary constituents or direct interaction with free nerve endings. However, certain oral responses such as sweet, bitter and coolness, and most olfactory responses involve stimulus recognition through a weak, reversible binding to receptor proteins at the surface of a sensory cell. This initial recognition is followed by transduction events within the cell involving either cyclic A M P or the IP cascade as "second messenger", with coupling through G-proteins. In addition, the availability of flavor stimuli at receptors is dependent on their release in the mouth and on nonspecific parameters such as solubility and volatility. Several chapters in this volume discuss issues critical to our understanding of the recognition/reception process. Labows and Cagan discuss these complex interactions at the cellular and subcellular level {Chapter 2) presenting a molecular model for taste perception. Nakamura and Okai {Chapter 3) also offer a molecular model for sweet taste perception presenting data suggesting that a single receptor might be responsible for the recognition of both sweet and bitter. Tamura and Okai {Chapter 12) add additional evidence in support of this hypothesis. Further discussion is presented by Spanier and Miller {Chapter 6) who expand upon the Nakamura and Okai hypothesis and suggest that a single receptor (specific for proteins, peptides and amino acids) is stimulated at specific subsites to elicit the individual sensation of sweet, sour, bitter, or savory {umami). Distinction is made between olfaction and taste both of which impact the final sensory perception, but to different degrees {Chapters 2, 3, 6, 12). Human perception of flavor occursfromthe combined sensory responses elicited by the proteins, lipids, carbohydrates, and Maillard reaction products in the food. Proteins {Chapters 6, 10, 11, 12) and their constituents and sugars {Chapter 12) are the primary effects of taste, whereas the lipids {Chapters 5, 9) and Maillard products {Chapter 4) effect primarily the sense of smell (olfaction). Therefore, when studying a particular food or when designing a new food, it is important to understand the structure-activity relationship of all the variables in the food. To this end, several powerful multivariate statistical techniques have been developed such as factor analysis {Chapter 6) and partial least squares regression analysis {Chapter 7), to relate a set of independent or "causative" variables to a set of dependent or "effect" variables. Statistical results obtained via these methods are valuable, since they will permit the food 3
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scientist to design experiments and then use the molecular descriptors as independent variables to predict the molecular outcome of the experiment. These methods may eventually assist in the understanding of natural flavor and the development of synthetic ones (Chapter 8). It is often too expensive to have or maintain an inhouse descriptive sensory panel. Therefore, other ways of measuring flavor need to be developed. Off-flavor in many foods have been measured by using gas chromatography to assess the level of lipid volatiles associated with off-flavor development (Chapters 5, 6, 9) such as hexanal or by direct chemical determination of thiobarbituric acid reactive substances (Chapters 5, 6) as a marker of the degree of lipid peroxidation. A new method being tested for use in the assessment of food quality is impedance technology. This method is showing promise for use in the seafood industry (Chapter 20). QUALITY ANALYSIS AND RESEARCH APPLICATIONS TOWARDS PRODUCTION OF QUALITY FOODS One of the main issues confronting today's food scientist is the development of new products for the market. Today's consumer oriented products must address the consumer's desire and demand for nutritionally sound, highly flavorful, and more natural products. Food scientists must also address the ergonomics of the situation and maintain maximal utilization of food crops within that society while at the same time maintaining capital outlays. These are difficult tasks for today's food and agricultural scientist to meet. Proteins and their amino acid precursors have, for the most part, been overlooked as a significant source of flavor. The fairly recent discovery of amino acid derivatives such as "aspartame" with significant sweetness potential has made a significant impact on consumer product development. New methods for synthesizing and preparing flavor peptides are appearing in the literature (Chapter 11). Furthermore, new uses of several classes of protein derived materials have been described (Chapter 10). Modes of utilization of the surplus of high yield, high growth foods such as soy (Chapters 14, 15, 16), milk protein (Chapter 17), and meat by-products (Chapter 6) have been developed in recent years. Not only have the constituents of food been directly utilized for the formulation of new food products but also enzymes and other natural products of food have been shown to be useful in the preparation of both food and nonfood items. For example, native soy has been shown to contain at least three lipoxygenase isoenzymes which improve the characteristics of dough and thereby enhance the production of white bread (Chapter 15). The lipid components of food are known to be critical in the development of much of a food's flavor. Modifications to lipid modifying enzymes such as lipases have led to new products useful in the rapid preparation of other food components (Chapter 13, 14). Better utilization of lipid constituents in food products can be gained from a better understanding of the thermodynamic and physicochemical characteristics of emulsions. Significant advancement in emulsion chemistry and food engineering have recently appeared in the literature and are an important portion of this volume (Chapter 19).
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While "natural" is the current catch-phrase of today's consumer, research must still be performed for the development of synthetic compounds that can lower the cost of production of food that can be utilized to develop other less costly food items. Amino acids or proteins with O-aminoaçyl sugars as part of their residue have been examined for their taste impact (Chapter 12). Several of these components have been shown to be potential replacements for salt (NaCl); this would have a significant impact for individuals with high blood pressure or with a propensity to other coronary or renal problems. Some glycosides, represented by some sucrose esters, are approved by the United States Food and Drug Administration for food use. These glycosides have potential use in the preparation of food materials and can lead to more cost effective means of production (Chapter 18). FOOD MICROBIOLOGY AND SAFETY ISSUES In addition to nutritional value and flavor, health concerns must also be a part of any discussion of food quality. Among the most interesting of food products with a health related function are the CLAs or conjugated dienoic derivatives of linoleic acid. The CLAs were originally found in meat (beef) extracts and have been shown to be a potent inhibitor of carcinogen-induced neoplasia in the epidermis and forestomach in mice and of the mammary in rats (Chapter 21). The flavor enhancer monosodium glutamate (MSG) is currently used in virtually every type of savory prepared-food. Unfortunately, MSG has several deleterious side effects on a large proportion of the population. Fortunately, a naturally occurring peptide isolated from a muscle food (beef) can serve not only as a potential replacement for MSG but also as a nutritional adjuvant. The peptide, called BMP or beefy meaty peptide, acts as a flavor enhancer and is found to occur naturally in beef (Chapter 6). Research on BMP suggest that it is not only non-allergenic but, by virtue of its protein composition, is a nutritionally sound replacement for MSG. A product of microbial origin that has serious negative implications on consumer health are the aflatoxins. Aflatoxins are carcinogens produced by Aspergillusflavusand Aspergillus parasiticus when these fungi infect crops before and after harvest, thereby contaminating food and feed and threatening both human and animal health (Chapter 22). Traditional control methods (such as the use of certain agricultural practices, pesticides and resistant varieties) that effectively reduce populations of many plant pests in the field, have not been effective in controlling aflatoxin producing fungi. Future research must, therefore, consist of acquiring knowledge of the molecular regulation of aflatoxin formation within the fungus, environmental factors and biocompetitive microbes controlling growth of A. flavus and aflatoxin synthesis in crops, and enhancement of host plant resistance against aflatoxin through understanding the biochemistry of host plant resistance responses. Such research is currently underway in several laboratories and should lead to the development of novel
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biocontrol strategies and/or to the development of elite crop lines that are immune to aflatoxin producing fungi. After aflatoxin contamination, perhaps the next most important factor that has a negative effect on human health and food quality is the presence of food borne bacteria. Several routes for reduction of the risk are currently under extensive investigation. One such means of risk reduction is the utilization of ionizing radiation treatments on meat food products. Ionizing radiation has been demonstrated to be an effective method to reduce or eliminate several species of food borne human pathogens such as Salmonella, Campylobacter, Listeria, Trichinella, and Yersinia (Chapter 23). If proper processing conditions are used, it is possible to produce high quality, shelf-stable, commercially sterile muscle foods. A second promising means for control of food borne bacteria is use of antimicrobial proteins. Several organisms produce antimicrobial proteins including bacteria (bacteriocins), frogs (magainins), insect (cecropins), and mammals (defensins). The common denominator among these agents is their proteinaceous composition and antimicrobial activity. These agents, therefore, add to the quality of food by virtue of their ability to be a nutritional source of amino acids while concurrently contributing to food quality by its ability to enhance the safety of the food (Chapter 24). Production of antimicrobial proteins for food use may be fairly simple when one considers the bacteriocins found in the lactic acid bacteria, since these bacteria are found present naturally in many foods. Certain microbes that are not generally deleterious to the public's health are found to be a problem due both to their presence in the aquaculture ponds of commercially grown catfish and in drinking water supplies. These microbes include some species of bacteria and several of the blue-green algae. These organisms produce metabolites such as geosmin and 2-methyl-isoborneol, that enter the municipal drinking water supplies and deposit in the fat layers of commercially grown catfish, yielding a product with a muddy or musty odor. Therefore, presence of these metabolites decreases the marketability of those catfish containing these undesirable flavors and decreases consumption of and increases complaints by consumers using the contaminated municipal water supply. Significant progress has been achieved in research efforts designed to develop means to control the growth of these organisms (Chapter 25). CONCLUDING REMARKS In closing, a very real and important problem exists today because of the phenomenal growth in the data base in science and engineering generated during the last quarter century alone. Today's food scientist needs to have not only a strong background in his/her specialty area but also a very strong appreciation for associated areas. Making the most of agricultural materials and foodstuffs in the coming decades, particularly with the overwhelming demographic projections that will significantly burden the world's food system, will require effective knowledge of all of the areas covered in this symposium
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and knowledge of several other areas such as biotechnology and animal husbandry that were not covered in this volume. A working knowledge of the improvements in the scientific and engineering database of these technologies along with the ability to utilize this information will have far-reaching effects on societies the world over and will lead to enhancement in the quality of life along with improvements in agriculture, aquaculture, manufacturing, food processing, and the economic health of nations. R E C E I V E D February 8,1993
