Lipids in Food Flavors - American Chemical Society

Chapter 6

Relationship of Temperature to the Production of Lipid Volatiles from Beef A. M. Spanier, A. J. St. Angelo, C. C. Grimm, and J. A. Miller

Downloaded by TUFTS UNIV on June 2, 2018 | https://pubs.acs.org Publication Date: July 7, 1994 | doi: 10.1021/bk-1994-0558.ch006

Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 1100 Robert E. Lee Boulevard, New Orleans, LA 70124

Analysis of flavor volatiles is typically performed by gas chromato graphy utilizing variously coated columns for separation. This method is fraught with many potential problems such as artifacts that arise due to the amount of water contained in the sample, the preparation and end-point cooking temperature of the sample, the temperature used to sparge the volatiles from the sample, and the capillary oven temperature and rate of elution or retention of the volatiles on the capillary column. This paper presents data demonstrating the effect of end-point cooking temperature and sparging/purging temperature on the development of volatile profiles in cooked/stored beef. Both types of heating produced different volatile profiles. Based on these data, the manuscript describes how various analytical methods can lead to potentially faulty impressions of the true perceivable meat flavor volatiles when the temperature parameters are not fully considered.

MEAT FLAVOR PERCEPTION Human perception of flavor is a fine balance between the sensory input of both desirable and undesirable flavors. It involves a complex series of biochemical and physiological reactions that occur at the cellular and subcellular levels (2). The final sensory response or perception to food is both initiated and transduced via stimulation of two major neural networks, the olfactory and gustatory systems (the smell and taste systems, respectively) by the food's lipids, carbohydrates, proteins, and their reaction products. While all three major flavor producing groups are important to the overall flavor perception, this report will focus on those flavor components that are principally olfactory and thus volatile in nature. This chapter not subject to U.S. copyright Published 1994 American Chemical Society Ho and Hartman; Lipids in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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SPANIER ET AL.

Temperature & Production of Lipid Volatiles from Beef

The flavor of food is a main driving factor involved in a consumer's decision to purchase a food item. Therefore, it is necessary that the food technologist have a thorough understanding of how food flavor deteriorates if s/he is to prepare products that consumers will purchase repeatedly. Such knowledge is particularly important in meat and meat products, since the deterioration of meat flavor is a serious and continual process (2-5) that involves both the loss of desirable flavor components (5, 6) and the formation of offflavor compounds (7-20). Many of these flavor producing components are associated with the process of lipid oxidation (22). The term "meat" encompasses a wide variety of foods of muscle origin; these include, and are not limited to, beef, poultry, pork, lamb, and their associated products. It stands to reason that this impressive list of muscle foods constitutes a commercially important food product. It is for this reason that muscle foods have long deserved, and still warrant, a great deal of research effort designed to reveal their flavor secrets. The species specific flavor of muscle foods (beef in particular) and the largest proportion of muscle food flavor come from volatile components primarily of lipid origin. While much is known as a result of prior research, much of the chemistry of meat flavor still remains a mystery to flavor chemists. Meat flavor development and deterioration have been shown to be affected by the actions and interactions of several antemortem and postmortem factors. Antemortem factors include, and are not limited to, the age, breed, and sex of the animal as well as the animal's nutritional status, and fat level and composition. Postmortem factors include the length of aging, the manner of cooking, and the manner of storage after cooking. The manner of cooking can include items such as wet vs. dry cooking, convection vs microwave, the rate of heating, the final end-point cooking temperature, and the animal's final fat level and composition. The manner of storage after cooking also affects the final flavor of the beef and includes refrigerated vs. frozen, aerobic vs anaerobic. Of all these factors, temperature is perhaps the most important in the production of flavor in beef. The effect of temperature on meat flavor is a function of both the cooking temperature and the analytical temperature used to determine the composition of the meat's flavor components. This chapter will present information that should shed some valuable light into areas where much of the previous research efforts may have been sidetracked. The remainder of this report will address the effect of these two temperature factors on meat flavor development and analysis. COOKING TEMPERATURE Top round (semimembranosus) muscle was used in this investigation. It was either USDA select or USDA choice having a fat content of 3.22% and 4.55%, respectively (22). Select cuts were used for the intact roast model while choice was used in studies utilizing ground beef. Effect of end-point cooking temperature (Packed Columns): One inch cubed (40 gram) mini-roasts were prepared from top round obtained from

Ho and Hartman; Lipids in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Angus-cross beef. Figure 1 shows representative examples of the mini-roasts cooked to end-point temperatures of 125 F , 140 F , 155 F , and 170° F (or 52 C , 60 C , 68 C , and 77 *C, respectively). The photograph indicates that there is a geometric progression of temperature into the interior of the roast and the coloration changes to a more "well done" appearance as the temperature increases. These photographs of roasted meat, by virtue of their different structural and color appearance, suggest that the internal portion of the roast may have undergone different chemical reactions and thus has a different chemical composition than the outer portion of the roast. This would suggest that the inner and outer regions of the roast might share differences in flavor. The direct effect of temperature on the enzymatic activity of a roast has been demonstrated previously (5). These earlier studies demonstrated the progressive inactivation of specific proteins and enzymes (notably hydrolases) with cooking of beef to higher end-point temperatures (EPT). In addition to measurement of enzyme activity in cooked beef, evaluation of the protein profile/composition of extracts of meat cooked to various EPT revealed that the protein composition of the muscle, measured by gel electrophoresis, changed as a function of increased EPT (3). The presence of significant remaining proteolytic activity along with the increased proportions of low molecular mass protein material appearing in cooked meat suggests that increases in cooking temperatures are responsible for the production of large numbers of flavor components as well as flavor precursors. The latter, particularly the sulfur containing peptides, can react with reducing sugars to form Maillard reaction products and other heterocyclic flavor components upon cooking or re-cooking. The process of cooking disrupts the tissue and membranes that serve as a source of many of the lipid precursors and lipid volatiles associated with desirable and undesirable flavors (3, 7, 10, 11, 13-15). In addition to the membrane alterations due to cooking temperatures and proteases activation (3), lipases could also be activated and thereby contribute to the production of additional flavor precursors or reactions. Cooking exposes or liberates various lipid components from their normal subcellular localization (via thermal denaturation and dissociation), making them more accessible to oxygen and other catalysts of lipid oxidation such as free iron. The process of lipid peroxidation should be measurable by assessment of the volatile profile and by assessment of the levels of thiobarbituric acid reactive substances (TEARS). These analyses were performed on samples taken from inner and outer portions of the minicube roast model. Examination of the data (Figure 2) reveals that storage leads to a progressive increase in TBARS and in lipid volatiles (as demonstrated by the increase in hexanal levels), no matter what the final EPT. The higher the EPT (155 F/68°C > 125°F/52°C) the greater the level of these lipid oxidation markers in the meat. The inner portion of the roast (Figure 1), having a lower temperature than the outer region of the roast (2) always shows lower levels of these markers than does the outer layer of the roast such that 155 F > 155°F > 125°F > 125 F . e


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SPANIER ET AL.

Temperature & Production ofLipid Volatiles from Beef


6.

Figure 1: Photos of mid-section of beef minicube roasts [semimembranosus muscle; top round; 40 grams/whole minicube; -3.8 cm/edge (55.3 cm )] cooked to the end-point temperature identified in the photo. Cooking was in a typical convection oven set to 350°F/177°C. 3


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TBARS (ppm)

Cfday

2 day

4 day

End-point Cooking Temperature Figure 2: Effect of storage (40°F/4°C) on the development of off-flavor volatiles (hexanal) and markers (thiobarbituric acid reactive substances; TBARS) in the core (IN) and outer (OUT) sections of minicube roasts cooked to end-point temperatures of 125°F/52°C and 155°F/68°C. Storage was for 0, 2, and 4 days (see 4 for details).


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The temperature dependency in off-flavor volatile production is shown to occur through the range of 125 F to 155 °F. On the other hand, at 170 °F, the TBARS and hexanal levels appear to decrease (Figure 3). This apparent decline in measurable volatile (hexanal) level in the miniroast is thought to be due to the release of these volatiles from the sample into the environment, much like the bouquet of odors one would perceive if one walked into the kitchen before dinner is served. Another observation is to note that these temperature-dependent changes appear to become readily evident or measurable only after a period of storage (compare front two rows of data in Figure 3 with back two rows of data) since no appreciable difference is seen in the samples of freshly cooked meat. This suggests that while the structure and chemistry of the meat are affected immediately during the initial cooking process (Figure 1), the development and/or appearance of off-flavor volatiles is seen only after the meats are stored. In other words, increased EPT causes both structural (Figure 1) (23) and compositional (2, 3, 7, 23, 14) changes in beef whereby the structure has been modified and precursors are available for oxygen and other free-radical initiating components (e.g., free iron; 4) to drive the peroxidation of lipids and thus enhance the undesirable flavor in meat after storage. Storage dependent changes are well documented and are discussed briefly below (4-7, 10, 13-15). Precooked beef products, often referred to as "convenience" and "institutional" foods, comprise 35% of all the beef sold and consumed in America today; this represents almost $10 billion in consumer expenditures on meat. Therefore, a thorough understanding of the flavor of beef and what factors affect the flavor would be critical to continued sales in this large market. Hornstein and Crowe (16) and others (17-19) suggested that, while the fat portion of muscle foods from different species contributes to the unique flavor that characterizes the meat from these species, the lean portion of meat contributes to the basic meaty flavor thought to be identical in beef, pork, and lamb. The major differences in flavor between pork and lamb result from differences in a number of short chain unsaturated fatty acids that are not present in beef. Even though more than 600 volatile compounds have been identified from cooked beef, not one single compound has been identified to date that can be attributed to the aroma of "cooked beef." Therefore, a thorough understanding of the effect of storage on beef flavor and on lipid volatile production would be helpful to maintain or expand that portion of the beef market. Storage of precooked meat leads to the production of flavor volatiles typically associated with off-flavors (25). These volatiles include, but are not limited to, pentanal, hexanal, heptanal, 2,3-octanedione, 2-pentylfuran and nonanal when examined in packed columns. Figure 4 presents data from a typical chromatographic profile for precooked/stored ground beef patties.


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Effect of purging temperature (Packed Column): The data presented above raises a very important question. If temperature will affect the volatiles produced by the beef, can the temperature used to sparge, purge and desorb the


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T B A R S (ppm)

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H e x a n a l (Area c o u n t s )

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End-point C o o k i n g Temperature

Marker H T B A R S (2d) 0 H e x a n a l (2d) • T B A R S (Od) S H e x a n a l (Od)

Figure 3: Effect of cooking temperature on the production of lipid oxidation products in freshly cooked (Od) and cooked/stored (2d) whole beef miniroasts (40 grams/cube).



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Figure 4: Packed column (Tenax G C - 8% polymetaphenyl ether; 10 ft. χ 1/8") gas chromatographic (GC) profile of volatiles from either freshly cooked and unstored ground beef (top round), or pre-cooked and then stored (40°F/4°C) for 1, 2, and 4 days. Volatiles identified and noted directly on the figure.

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volatiles from their matrix (the meat and the column) also affect the types and kinds of volatiles seen during analysis? To answer this question, we utilized dynamic purge-and-trap gas chromatography using a packed column. Packed columns were chosen because of their tolerance for high levels of water such as that found in muscle foods. Freshly cooked/reground beef patties, and those stored at 4°C for 0, 1, 2, and 4 days, were analyzed by dynamic purge-and-trap gas chromatography (Figure 5). Volatile detection was via a Tracor Model 100AT detector, described by Brown et al. (20) for egg volatiles. This detector combines flame photometric detection (FPD; used in sulfur mode) with flame ionization detection (FID). A Tekmar 25 mL needle impinger-assembly was used to substitute for the generally accepted Tekmar semi-automatic purge and trap concentrator (Model LSC-3; 21). One (1.0) gram of freshly cooked/reground beef patty was sparged for 30 minutes at either 50 ° C, 75 ° C or 100 C within the needle impinger assembly. Nitrogen was used as a carrier gas at flow rate of 20 mL/min. The beef volatiles, after passing through a transfer line and a six port, 1/16", Valco™ valve (Valco Instruments, Houston, T X 77255), were trapped/concentrated directly onto a packed Tenax GC - 8% poly-metaphenyl ether column (10 ft χ 1/8") held at ambient temperature (20 ° - 21 C). Samples were allowed to concentrate on the column for 30 minutes. Volatiles were eluted from the column by purging with nitrogen for a total of 60 minutes (nitrogen flow: 20 mL/min; column heated from 25°C to 250°C at 3°C per minute). The sample tube was removed from the injection port after completion of volatile stripping/sparging process and replaced with a tube containing a few milliliters of water to effect steam distillation. The six port valve permitted switching to a valve purge position that allowed the valve and transfer lines to be cleaned between runs by steam distillation from an impinger tube containing water alone. The carbonyl components found in precooked/stored meat sparged at three different temperatures (50°, 75°, 100 °C) were analyzed (Figure 6). Volatile profiles were similar to those seen in Figure 4 with the exception that the intensities and areas of various peaks differed significantly. Notable variability in the temperature-dependent quantitation of the volatiles was seen with low carbon number materials, i.e.,frommethanol to 2,3-butanedione, while the typical increases in volatile levels with storage ware seen for volatiles ranging in mass from 3-methyl butanal to decanal. These latter carbonyl compounds showed their lowest levels and most difficulty in quantitation at sparging temperatures of 50° C. Replicability, peak heights, and peak patterns appeared to reach their optimum level at sparging temperatures of 75 ° C; their patterns at 100 °C were similar to that at 75 °C (Figure 7). Previous studies have shown similar results (6). Unlike the carbonyl compounds, sulfur containing components show a very high degree of temperature dépendance (Figure 8). Precise assessment of the content and composition of sulfur compounds is essential in the study of meat flavor since the sulfur containing compounds [long known to be involved in the generation of Maillard reaction products, MRPs (22)] play a vital role in


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Figure 6: Packed column gas chromatographic profile with flame ionization detection (FID) of cooked ground beef stored for 0, 1, 2, and 4 days at 40°F/4°C (as described in Figure 4). The samples were otherwise identical except for heating and sparging the sample (see Figure 5) at 50 C, 75°Cand 100°C e



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Packed Column Peak Identification CC Area Counts (Thousand!) 1 2 3 4 5 β 7 8

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100°C

Figure 7: Effect of Purge Temperature on the concentration and resolvability of meat alcohols and carbonyls. The area under the peaks of the samples shown in Figure 6 are presented here. Fourteen (14) peaks were identified and are tabulated on the figure. Peak 9, written as '9/4', is plotted as l/4th the actual value so that all peaks would appear on the diagram.


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Figure 8: Packed column gas chromatographic profile with flame photometric detection (FPD) of cooked ground beef stored for 0, 1, 2, and 4 days at 40°F/4°C (as described in Figures 4 and 6). The samples were otherwise identical except for heating and sparging the sample (see Figure 5) at 50 °C, 75 °C and 100 °C.



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the production of flavor in meat (23). A significant proportion of the organic compounds that are known to produce meat-like flavor contain sulfur and a significant number of these are aldehydes. The specific aldehydes produced during the Maillard reaction and the specific amounts of aldehyde produced are controlled by the specific amino acid and sugars used in the reaction, respectively (22). It would be reasonable to expect that the production of MRPs can be controlled through control of reactants and reaction conditions such as temperature. There is an ever growing list of MRPs found in synthetic mixtures and in meat, but published quantitative correlations between the levels of MRPs and the sensory response of these compounds in meat are not to be found. Although much is known about the formation of MRPs in vitro, little is know of their formation and influence on the overall sensory perception and flavor of meat in vivo. The relative impact of sulfur-containing compounds to beef flavor cannot be understated (24). For that reason it is important to have methods available that will yield accurate assessment of the true composition of the flavor components of foods. Analysis of the sulfur compounds found in cooked ground beef has indicated that several of the herterocyclic compounds such as 4methylthiazole, 2-acetylthiazole, benzothiazole, and 2-furylmethanethiol are fairly stable (6). These latter compounds were identified in ground beef patties that had undergone a fairly strenuous 4 hour steam distillation-extraction protocol. On the other hand, use of milder methods having efficiency of recovery greater than 95% indicated that precooked ground beef did not contain any thiazoles at least at levels greater than 2.5 parts per billion (14). Other sulfur-containing compounds in meat are thought to be formed by Strecker degradation of cysteine, methionine and alanine and from hydrogen sulfide. Hydrogen sulfide is produced via several mechanisms including freeradical reactions (25). Hydrogen sulfide, which has been shown to be a product of the degradation of dimethyl trisulfide (26), can also react with several components of meat to give ethanedithiol, (methylthio)ethanethiol, and dimethyltrithiolane; the three latter have been shown to increase in meat during storage (6). The data in Figure 9 demonstrate clearly the increase in the tissue level of hydrogen sulfide during prolonged storage of precooked meat particularly at sparge temperatures above 75 ° C. This correlates well with the decline in dimethyl trisulfide. The abundance of free radicals during storage and the susceptibility of the sulfur amino acids to radical damage (26) would contribute to an increase in hydrogen sulfide content with storage. The content of dimethyl sulfide, dimethyl disulfide, and dimethyl trisulfide decreases with increased storage no matter what the sparging temperature (Figure 8 and Figure 9). Similar results were observed for dimethyl trisulfide in more harshly extracted samples (6). Dimethyl disulfide, an oxidation product of methanethiol, can react to form dimethyl trisulfide and dimethyl sulfide. Subsequently, dimethyl trisulfide may be degraded to hydrogen sulfide, carbonyl sulfide and methanethiol (25; 27).


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Lipids in Food Flavors - American Chemical Society

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