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Sensory Evaluation of Frying Fat and Deep-Fried Products
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Sensory Evaluation of Frying Fat and Deep-Fried Products Sharon L. Melton
Retired, University of Tennessee, Department of Food Science and Technology, Knoxville, TN
Sensory evaluation is defined as “a scientific discipline used to invoke, measure, analyze, and interpret reactions to characteristics of foods and materials as they are perceived by the senses of sight, smell, taste, touch, and hearing” (1). According to Mounts and Warner (2), sensory evaluation is a necessary part of both product development and quality control. Penfield and Campbell (3) suggested that sensory evaluation could also be used in research and shelf life studies and that many sensory tests are used in food-science research. Sensory tests are classified into two basic types: affective and analytical (4). In affective testing (which includes consumer testing), preference, acceptance, or opinions on food products are determined. Usually 50–100 untrained, randomly selected people are asked which one, two, or more products they prefer. Affective tests are important to market-development studies but are not as relevant to flavor research investigations (4). A detailed discussion of consumer sensory evaluation was published by the American Society for Testing Materials (ASTM) (5). Analytical tests evaluate differences between products and rate the quality or intensity of odor or flavor characteristics (4). According to Penfield and Campbell (3), analytical tests discriminate between or among samples (difference tests) or describe or score the quality of a product (descriptive tests). The discriminator tests also include sensitivity tests. Several tests are used in affective or consumer testing. The most commonly used evaluation technique for measuring acceptability or preference is the hedonic scale (3). Hedonic scales generally have 5–9 points, with descriptive terms denoting likability of product or product characteristic, as shown in Fig. 20.1. Other scales have been used to minimize misinterpretation of the terms in Fig. 20.1. The facial hedonic scale illustrated in Fig. 20.2 (6) is particularly useful for this effort. A 9-point hedonic scale was used by Fuller et al. (7) with a panel of 20–26 individuals to estimate the acceptability of potato chips fried in different oils, with and without the antioxidant propyl gallate (PG), and stored under different light intensities for up to 20 days. Fig. 20.3 shows that potato chips fried in cottonseed • 359 •
360 l S.L. Melton
Fig.20.1. Hedonic score card for consumer acceptability of products.
Fig. 20.2. Facial hedonic scale used by the Continental Can Company (6).
Sensory Evaluation of Frying Fat and Deep-Fried Products l 361
Fig. 20.3. Average hedonic ratings of potato chips fried in different oils with and without antioxidant and stored under light with 13 foot-candles intensity (HVO is hydrogenated vegetable oil, and PG is the antioxidant corresponding to 0.01% propyl gallate and 0.005% citric acid in the oil) (7).
oil, with and without PG, received lower scores, initially and throughout the 20-day storage period, than chips fried in other oils. A consumer panel (n = 119) used an 8-point hedonic scale (1 = dislike extremely to 8 = like extremely) to evaluate flavor and overall likability of fresh potato chips fried in canola and/or cottonseed oils (8). The mean sensory scores for flavor and overall likability did not differ (P > 0.05) among the oil types used for chip production and were between “like slightly” and “like moderately” (5.2–5.5). Consumer-type panels (n = 77), however, using likability hedonic scales showed different degrees of likability for the flavor of codfish fried in fresh vs. used (discarded) commercial frying fat (9). Part of the panel (n = 29) liked the flavor of codfish fried in fresh fat more than that of codfish fried in used fat; part of the panel (n = 24) reversed that trend by liking the flavor of codfish fried in used fat. The rest of the panel (n = 24) liked the flavor of codfish fried in fresh or used fat equally well. Behavior exhibited by the consumer panel evaluating fried codfish also extended to consumer evaluation of food fried in different oils (9). Actual flavor differences due to types of frying oil certainly exist; however, the degree of likability for individual panelists is dependent on backgrounds and life experiences. Several specific methods are included in analytical sensory difference tests. The triangle, duo-trio, paired comparison, and ranking or rating difference from a control are examples of such tests. The premise of these tests is to detect differences among samples based on one attribute; if a flavor difference is being evaluated, the test is invalid if panelists differentiate by color, texture, or temperature instead. The triangle
362 l S.L. Melton
Sample provided
Odd sample
Difference observed: Fig. 20.4. Typical questionnaire for the triangle test (3).
Fig. 20.5. Correct identification in the triangle test of identical samples of potato chips fried in different oils with and without antioxidant added and stored in air up to 20 days in 13 foot-candles intensity light (HVO is hydrogenated vegetable oil, and PG is the antioxidant corresponding to 0.01% propyl gallate and 0.005% citric acid in the oil) (7).
test is perhaps the easiest analytical test for inexperienced panelists. In this test, the panelist is asked to determine which two of the three samples presented are identical and which one is different. A typical questionnaire for a triangle test is presented in Fig. 20.4. Data from triangle tests can be analyzed statistically with significant results. Tables for determining the significance have been published elsewhere (3,8). The triangle test was used by Fuller et al. (7) to determine the time required to cause significant odor changes in potato chips fried in different oils and held under light intensities of 13 and 100 foot-candles. The control for the triangle test was nitrogenpacked potato chips stored in the dark. During sensory evaluation, the control and
Sensory Evaluation of Frying Fat and Deep-Fried Products l 363
treated chips were presented as the odd sample an equal number of times. The results of the triangle tests are given in Fig. 20.5 for potato chips fried in different oils, with and without antioxidant, and stored under 13 or 100 foot-candles of light for up to 20 days. Figure 20.5 shows that after only 3 days of storage at 13 foot-candles of light, the panel differentiated between chips fried in cottonseed oil and stored in light and those stored in the dark under nitrogen. By 12 days, the panel differentiated between chips fried in hydrogenated vegetable oil or high oleic sunflower oil and stored in light and chips fried in the same oils but stored in the dark. The triangle test was also used by Maga (10) to determine if blindfolded, experienced panelists could differentiate between the flavors of light- and dark-colored potato chips during storage. The blindfolded panel could not differentiate flavor or texture between light- and dark-colored potato chips stored up to 2 wk. The other analytical sensory tests (duo-trio, paired comparison, and ranking or rating difference from a control) are also applicable to fried foods. Details on the use of these tests are in Warner (4) and in ASTM manuals (11). Except for ranking, these tests, including the triangle, can be used for product matching, product improvement, process improvement, and quality control (2). The triangle and duo-trio tests are also used as measures of panelists’ discriminatory ability in selection for a trained panel. Analytical descriptive tests require panelists with more experience and training than do simpler difference tests (4). Panelists must not only be able to detect differences among samples, but they must have a vocabulary to describe their perceptions. The American Oil Chemists’ Society (AOCS) has adopted, as official methods, two variations of quality and intensity rating scales for flavor of oil, as shown in Fig. 20.6 and 20.7. The usability of such scales as those listed in Fig. 20.6 and 20.7 depends on panelists being very well trained in identifying various flavors and scoring their intensity. According to Mounts and Warner (2), analytical descriptive tests can be applied to matching products, product improvement, process improvement, cost reduction, and/or selection of new sources of supply, quality control, and correlation of sensory with chemical/physical measures. Mounts et al. (12) used a trained 15-member sensory panel to evaluate fresh and stored potatoes fried in low-linolenic acid, regular, and partially hydrogenated soybean oil. The panel evaluated the fried potato flavor on overall intensity and for intensity of individual flavors on a 10-point intensity scale, where 10 = bland and 1 = strong. The panel was experienced in testing vegetable oil according to AOCS Recommended Practice CG 2-83 (13). Fishy flavors were significantly lower in potatoes fried in soybean oil with lower linolenic acid content than those fried in regular soybean oil (6.2% linolenic acid) (12). Potatoes fried in low-linolenic acid soybean oil also lacked the hydrogenated flavors of potatoes fried in hydrogenated oil. Robertson and Morrison (14) used analytical descriptive tests to evaluate the flavor of potato chips fried in different oils and stored under accelerated conditions at 31°C for up to 10 wk. A 10-member trained panel used the following defined flavor descriptors: fresh = good chip flavor, no off flavors; heated = good quality but flavor
364 l S.L. Melton
Fig. 20.6. AOCS flavor-quality scale (4).
Fig. 20.7. AOCS flavor-intensity scale (4).
Sensory Evaluation of Frying Fat and Deep-Fried Products l 365
indicating some storage; stale = fair, not fresh, flat, or tasteless for aging; off flavor = poor undesirable flavor, such as, grassy; and rancid = bad, old oil flavor. The panel was trained to detect the stages of flavor deterioration using chips that were fresh or heated in a 60°C oven for 3, 6, or 9 days. The descriptive quality ratings were transformed into numerical scores where 1 = fresh to 5 = rancid, and the data were analyzed statistically. After the panelists were trained to detect stages of flavor deterioration, they were also trained to evaluate intensities of each odor–flavor descriptor as 1 = weak, 2 = moderate, and 3 = strong. These intensity ratings were expressed as flavor intensity values (FIV), where FIV is calculated from the following equation: FIV = [Nw + 2(Nm ) + 3(Ns )]/N where Nw = number of weak responses, Nm = number of medium responses, Ns = number of strong responses, and N = number of tasters (14). An average FIV was determined for each descriptor used by the panel to describe the flavor of the chips: fresh, heated, stale, off flavor, and rancid. The trained panel was used to evaluate the flavor of potato chips fried in cottonseed, sunflower, or palm oil and then stored for up to 10 wk. A coded reference chip also was evaluated at the same time as the stored chips. The reference chip was fresh and was fried in cottonseed oil. The analysis of variance for descriptive quality ratings of potato chips fried in different oils and stored up to 10 wk is shown in Table 20.1. The sensory scores of the potato chips are shown in Fig. 20.8, and FIV for fresh (0-wk storage time) and 10-wk-stored chips fried in different oils are shown in Table 20.2. The trained panel scored the flavor of fresh chips fried in cottonseed oil between very good, fresh (or a value of 1.0) and good, heated (or a value of 2.0). The average sensory score for the flavor of fresh chips fried in palm and sunflower oils was between 2.0 and 3.0. Of all flavor descriptors, fresh received the maximum FIV in 0-wk chips fried in cottonseed or sunflower oil, while the descriptor receiving the maximum FIV for 0-wk chips fried in palm oil was off flavor (Table 20.2). After 10 wk of storage, the stale flavor description received the maximum FIV for chips fried in cottonseed oil, but the rancid flavor had the highest FIV in chips fried in sunflower oil. Chips fried in palm oil and stored 10 wk had the greatest FIV for the rancid and heated descriptors (Table 20.2). Two other descriptive methods for testing the quality of fried food are the flavor profile method and the quantitative descriptive analysis. In the flavor profile method, perceived factors are called “character notes.” A list of these notes is made by each trained panel member during preliminary evaluation of the food being investigated. The lists are then compared by the entire panel, and agreement is reached on which notes will be used for further analysis. Then each panelist evaluates the sample again for the intensity of each character note and the amplitude of overall aroma and taste. The panel then reaches a consensus of opinion on the intensity of each character note
366 l S.L. Melton TABLE 20.1 Analysis of Variance of Descriptive Quality Ratings for Stored Potato Chips Fried in Different Oilsa Source of variation
Dfb
Sum of squares
Mean squares
F-value
Time
5
147.364
29.462
21.14c
Samples
2
22.149
11.079
7.95c
Time × samples
10
26.440
2.644
1.90
Reps (time × samples)
36
50.198
1.394
461.500
0.949
Panelists (rep × samples) 486
Source: Robertson, J.A., and W.H. Morrison, III, J. Food Sci. 43:420 (1978). Reference samples were omitted from the analysis of variance. b Degrees of freedom. c Significant at the P < 0.05 level. a
TABLE 20.2 Flavor Intensity Values (FIV) of Stored Potato Chipsa Storage time (wk)
Flavor description
Reference cottonseed
Cottonseed
Sunflower
Palm
0
Fresh
1.27
1.80
0.83
0.53
Heated
0.40
—
0.20
0.13
Stale
1.10
0.13
0.07
0.30
Off-flavor
—
—
0.37
0.87
Rancid
0.17
0.10
0.30
0.20
Fresh
2.37
—
—
—
Heated
0.17
0.20
0.03
0.07
10
Stale
—
1.20
0.73
0.90
Off-flavor
0.07
0.43
0.70
0.67
Rancid
—
0.53
0.87
0.87
Source as in Table 20.1. a Chips fried in oils except reference cottonseed oil were stored at 31°C; those fried in reference cottonseed oil were stored at -20°C.
Sensory Evaluation of Frying Fat and Deep-Fried Products l 367
Fig. 20.8. Effect of storage on mean (n = 30) sensory scores of potato chips fried in different frying oils (14).
and the amplitude of the overall aroma, taste, and aftertaste. In the original flavor profile method, the results were not subjected to statistical analysis. Although flavor profiling was successful for comparing products, for quality control, and for product development, it was not used extensively in sensory evaluation of fried food or oil. Berry et al. (16) gave a classic application of flavor profiling. In quantitative descriptive analysis (QDA), the sensory characteristics of a product are identified and quantified by a panel under the guidance of a panel leader (17). After the identification of characteristics to be evaluated by the panel as a group, each panelist individually rates the intensity of each characteristic on a 150-cm horizontal linear scale anchored at the ends by the terms “weak” and “intense.” The results of QDA are analyzed statistically using analysis of variance, factor, or regression analysis (2). Quantitative descriptive analysis was used by researchers analyzing the sensory characteristics of fried food. Stevenson et al. (18) used an eight-member trained QDA panel to assess exterior crispness, interior dryness, intensity of oil flavor, oil off flavor, oily mouthcoat, potato flavor, color, and overall quality of french-fried potatoes cooked in canola oil and soybean fat. Results reported by Stevenson et al. (18) are illustrated in Fig. 20.9, which shows the scores they reported in tabular form. In Fig. 20.9, each line radiating from the center of the diagram represents a sensory characteristic; the distance from the center outward on each line represents the intensity of that characteristic. The point at which the product line intersects each characteristic line is the relative intensity of that characteristic in the product.
368 l S.L. Melton
Fig. 20.9. Average scores from sensory evaluation (QDA) of french-fried potatoes cooked in canola oil or soybean oil as prepared from data presented by Stevenson et al. (18).
When there is a significant difference in intensity of a characteristic between the two products, there is a difference between the intersections of the product lines with the characteristic line. For example, french-fried potatoes cooked in soybean oil were darker in color, had greater exterior crispness and interior dryness, but had less overall quality than french-fried potatoes cooked in canola oil. French-fried potatoes cooked in either oil had equal intensities of potato and oil flavors and the same level of oily mouth feel (18). Several sources give extensive details on panel selection and training (19), presentation of samples (20), and sensory evaluation facilities (5,21–23). Warner (4) reported that the qualifications for analytical panelists include an interest in testing oil-containing foods, normal sensory acuity, availability to participate in regularly conducted panels, ability to discriminate known differences, and consistency of response for the same samples. A minimum of 10–12 panelists is recommended for analytical panels if the results will be analyzed statistically (24). Only five highly trained panelists, however, are needed for a flavor profile panel (4); their results are not usually analyzed statistically. The triangle test is often used to select potential candidates for analytical panels based on their ability to detect flavor differences. To be selected for the panel, a candidate must score at least 60% correct of a total of 24 responses (15). According to Warner (4), attribute scoring or descriptive tests are unsuitable for screening panelists because definite correct responses cannot be established.
Sensory Evaluation of Frying Fat and Deep-Fried Products l 369
In training, panelists should be familiarized with panel room operation, test procedure and products, and scoring or rating methods used. Panelists should then be trained to identify sensory characteristics and to develop a common vocabulary of descriptions for those characteristics. Reference standards for typical odors/flavors are necessary in training a panel, according to Warner (4), who listed reference standards for flavors in oils and oil-containing food. The reference standards also should be characterized by expert judges prior to being used in training. Other chemical standards of odors in food were published by Harper et al. (25). For presentation to panelists, all samples should be of uniform portion and temperature and should be coded properly. Freshly fried food should be served to panelists at 50°C in covered glass containers (4). The sample size of the fried food should be one-inch cubes, and no more than 2–4 samples should be presented to panelists at any one session (4). If the fried food is a snack food, it may not be possible to have a one-inch cube and it may also be inappropriate to serve it at 50°C. Specific sample recommendations are also given by the ASTM (16). Three-digit random numbers are the most commonly used sample codes for sensory analysis and can be selected from a random number table in a statistics book or sensory evaluation reference book (17). To avoid bias in sensory analysis experiments with a small number of samples (3 or fewer), all possible orders of sample presentation should be used an equal number of times (3). When there is a large number of samples to be analyzed in an experiment, the order of presentation should be randomized to minimize psychological effects (23). The environment also is an important factor in the sensory evaluation of food and should be without distractions so that the judges can concentrate on the testing at hand and do so independently (3). The judges should be separated, with each judge in an individual compartment if possible. Detailed information on the requirements for a sensory evaluation laboratory were published by the ASTM (26). References 1. 2. 3. 4. 5. 6. 7.
Institute of Food Technologists. Food Technol. 1981, 35, 50. Mounts, T.L.; and K. Warner. In Handbook of Soy Oil Processing and Utilization; D.R. Erickson, E.H. Pryde, O.L. Brekke, T.L. Mounts, and R.A. Falb, Eds.; American Oil Chemists’ Society: Champaign, IL, 1980; pp. 245–266. Penfield, M.P.; and A.M. Campbell. In Experimental Food Science, 3rd ed.; Academic Press: San Diego, CA, 1990; pp. 51–77. Warner, K. In Flavor Chemistry of Fats and Oils; D.B. Min and T.H. Smouse, Eds.; American Oil Chemists’ Society: Champaign, IL, 1985; pp. 207–221. American Society for Testing Materials (ASTM). STP 682. In Manual on Consumer Sensory Evaluation; E.E. Schaefer, Ed.; ASTM: Philadelphia, PA, 1979. Ellis, B.H. Food Technol. 1968, 22, 583. Fuller, G.; D.G. Guadagni; M.L. Weaver; G. Notter; and R.J. Horvat. J. Food Sci. 1971, 36, 43.
370 l S.L. Melton 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
Melton, S.L.; M.K. Trigiano; M.P. Penfield; and R. Yang. Potato Chips Fried in Canola and/or Cottonseed Oil Maintain High Quality. J. Food Sci. 1993, 58, 1079–1083. Melton, S.L.; S. Jafar; D. Sykes; and M.K. Trigiano. J. Am. Oil Chem. Soc. 1994, 71, 1301. Maga, J.A. J. Food Sci. 1973, 38, 1251. American Society for Testing and Materials (ASTM). STP 434. In Manual on Consumer Sensory Testing Methods; ASTM: Philadelphia, PA, 1968. Mounts, T.L.; K. Warner; G.R. List; W.E. Neff; and R.F. Wilson. J. Am. Oil Chem. Soc. 1994, 71, 495. American Oil Chemists’ Society (AOCS). In Official Methods and Recommended Practices of the American Oil Chemists’ Society, 4th ed.; D. Firestone, Ed.; American Oil Chemists’ Society: Champaign, IL, 1990. Robertson, J.A.; and W.H. Morrison, III. J. Food Sci. 1978, 43, 420. Caul, J.F. Adv. Food Res. 1957, 7, 1. Berry, B.W.; J.A. Maga; C.R. Calkins; L.H. Wells; Z.L. Carpenter; and H.R. Cross. J. Food Sci. 1980, 45, 1113. Stone, H.; J. Sidel; A. Woolsey; and R.C. Singleton. Food Technol. 1974, 28, 24. Stevenson, S.G.; L. Jeffery; M. Vaisey-Genser; B. Fyfe; F.W. Hougen; and N.A.M. Eskin. Can. Inst. Food Sci. Technol. J. 1984, 17, 187. American Society for Testing Materials (ASTM). STP 758. In Guidelines for the Selection and Training of Sensory Panel Members; ASTM: Philadelphia, PA, 1981. American Society for Testing Materials (ASTM). End Use Products. In Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1989; Vol. 15.07. Larmond, E. Laboratory Methods for Sensory Evaluation of Food, Publication 1637; Canadian Department of Agriculture: Ottawa, Ontario, Canada, 1977. Amerine, M.A.; R.M. Pangborn; and E.B. Rossler. In Principles of Sensory Evaluation of Food; Academic Press: New York, 1965. Larmond, E., Food Technol. 1973, 27, 28. Sidel, J.L.; and H. Stone. Food Technol. 1976, 30, 32. Harper, R.; D.G. Land; N.M. Griffiths; and E.C. Bate-Smith. Br. J. Psychol. 1968, 49, 231. American Society for Testing Materials (ASTM). In Physical Requirement Guidelines for Sensory Evaluation Laboratories, Publication 913; J. Eggert and K. Zook, Eds.; ASTM: Philadelphia, 1986.
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Sensory Evaluation of Frying Fat and Deep-Fried Products Sharon L. Melton
Retired, University of Tennessee, Department of Food Science and Technology, Knoxville, TN
Sensory evaluation is defined as “a scientific discipline used to invoke, measure, analyze, and interpret reactions to characteristics of foods and materials as they are perceived by the senses of sight, smell, taste, touch, and hearing” (1). According to Mounts and Warner (2), sensory evaluation is a necessary part of both product development and quality control. Penfield and Campbell (3) suggested that sensory evaluation could also be used in research and shelf life studies and that many sensory tests are used in food-science research. Sensory tests are classified into two basic types: affective and analytical (4). In affective testing (which includes consumer testing), preference, acceptance, or opinions on food products are determined. Usually 50–100 untrained, randomly selected people are asked which one, two, or more products they prefer. Affective tests are important to market-development studies but are not as relevant to flavor research investigations (4). A detailed discussion of consumer sensory evaluation was published by the American Society for Testing Materials (ASTM) (5). Analytical tests evaluate differences between products and rate the quality or intensity of odor or flavor characteristics (4). According to Penfield and Campbell (3), analytical tests discriminate between or among samples (difference tests) or describe or score the quality of a product (descriptive tests). The discriminator tests also include sensitivity tests. Several tests are used in affective or consumer testing. The most commonly used evaluation technique for measuring acceptability or preference is the hedonic scale (3). Hedonic scales generally have 5–9 points, with descriptive terms denoting likability of product or product characteristic, as shown in Fig. 20.1. Other scales have been used to minimize misinterpretation of the terms in Fig. 20.1. The facial hedonic scale illustrated in Fig. 20.2 (6) is particularly useful for this effort. A 9-point hedonic scale was used by Fuller et al. (7) with a panel of 20–26 individuals to estimate the acceptability of potato chips fried in different oils, with and without the antioxidant propyl gallate (PG), and stored under different light intensities for up to 20 days. Fig. 20.3 shows that potato chips fried in cottonseed • 359 •
360 l S.L. Melton
Fig.20.1. Hedonic score card for consumer acceptability of products.
Fig. 20.2. Facial hedonic scale used by the Continental Can Company (6).
Sensory Evaluation of Frying Fat and Deep-Fried Products l 361
Fig. 20.3. Average hedonic ratings of potato chips fried in different oils with and without antioxidant and stored under light with 13 foot-candles intensity (HVO is hydrogenated vegetable oil, and PG is the antioxidant corresponding to 0.01% propyl gallate and 0.005% citric acid in the oil) (7).
oil, with and without PG, received lower scores, initially and throughout the 20-day storage period, than chips fried in other oils. A consumer panel (n = 119) used an 8-point hedonic scale (1 = dislike extremely to 8 = like extremely) to evaluate flavor and overall likability of fresh potato chips fried in canola and/or cottonseed oils (8). The mean sensory scores for flavor and overall likability did not differ (P > 0.05) among the oil types used for chip production and were between “like slightly” and “like moderately” (5.2–5.5). Consumer-type panels (n = 77), however, using likability hedonic scales showed different degrees of likability for the flavor of codfish fried in fresh vs. used (discarded) commercial frying fat (9). Part of the panel (n = 29) liked the flavor of codfish fried in fresh fat more than that of codfish fried in used fat; part of the panel (n = 24) reversed that trend by liking the flavor of codfish fried in used fat. The rest of the panel (n = 24) liked the flavor of codfish fried in fresh or used fat equally well. Behavior exhibited by the consumer panel evaluating fried codfish also extended to consumer evaluation of food fried in different oils (9). Actual flavor differences due to types of frying oil certainly exist; however, the degree of likability for individual panelists is dependent on backgrounds and life experiences. Several specific methods are included in analytical sensory difference tests. The triangle, duo-trio, paired comparison, and ranking or rating difference from a control are examples of such tests. The premise of these tests is to detect differences among samples based on one attribute; if a flavor difference is being evaluated, the test is invalid if panelists differentiate by color, texture, or temperature instead. The triangle
362 l S.L. Melton
Sample provided
Odd sample
Difference observed: Fig. 20.4. Typical questionnaire for the triangle test (3).
Fig. 20.5. Correct identification in the triangle test of identical samples of potato chips fried in different oils with and without antioxidant added and stored in air up to 20 days in 13 foot-candles intensity light (HVO is hydrogenated vegetable oil, and PG is the antioxidant corresponding to 0.01% propyl gallate and 0.005% citric acid in the oil) (7).
test is perhaps the easiest analytical test for inexperienced panelists. In this test, the panelist is asked to determine which two of the three samples presented are identical and which one is different. A typical questionnaire for a triangle test is presented in Fig. 20.4. Data from triangle tests can be analyzed statistically with significant results. Tables for determining the significance have been published elsewhere (3,8). The triangle test was used by Fuller et al. (7) to determine the time required to cause significant odor changes in potato chips fried in different oils and held under light intensities of 13 and 100 foot-candles. The control for the triangle test was nitrogenpacked potato chips stored in the dark. During sensory evaluation, the control and
Sensory Evaluation of Frying Fat and Deep-Fried Products l 363
treated chips were presented as the odd sample an equal number of times. The results of the triangle tests are given in Fig. 20.5 for potato chips fried in different oils, with and without antioxidant, and stored under 13 or 100 foot-candles of light for up to 20 days. Figure 20.5 shows that after only 3 days of storage at 13 foot-candles of light, the panel differentiated between chips fried in cottonseed oil and stored in light and those stored in the dark under nitrogen. By 12 days, the panel differentiated between chips fried in hydrogenated vegetable oil or high oleic sunflower oil and stored in light and chips fried in the same oils but stored in the dark. The triangle test was also used by Maga (10) to determine if blindfolded, experienced panelists could differentiate between the flavors of light- and dark-colored potato chips during storage. The blindfolded panel could not differentiate flavor or texture between light- and dark-colored potato chips stored up to 2 wk. The other analytical sensory tests (duo-trio, paired comparison, and ranking or rating difference from a control) are also applicable to fried foods. Details on the use of these tests are in Warner (4) and in ASTM manuals (11). Except for ranking, these tests, including the triangle, can be used for product matching, product improvement, process improvement, and quality control (2). The triangle and duo-trio tests are also used as measures of panelists’ discriminatory ability in selection for a trained panel. Analytical descriptive tests require panelists with more experience and training than do simpler difference tests (4). Panelists must not only be able to detect differences among samples, but they must have a vocabulary to describe their perceptions. The American Oil Chemists’ Society (AOCS) has adopted, as official methods, two variations of quality and intensity rating scales for flavor of oil, as shown in Fig. 20.6 and 20.7. The usability of such scales as those listed in Fig. 20.6 and 20.7 depends on panelists being very well trained in identifying various flavors and scoring their intensity. According to Mounts and Warner (2), analytical descriptive tests can be applied to matching products, product improvement, process improvement, cost reduction, and/or selection of new sources of supply, quality control, and correlation of sensory with chemical/physical measures. Mounts et al. (12) used a trained 15-member sensory panel to evaluate fresh and stored potatoes fried in low-linolenic acid, regular, and partially hydrogenated soybean oil. The panel evaluated the fried potato flavor on overall intensity and for intensity of individual flavors on a 10-point intensity scale, where 10 = bland and 1 = strong. The panel was experienced in testing vegetable oil according to AOCS Recommended Practice CG 2-83 (13). Fishy flavors were significantly lower in potatoes fried in soybean oil with lower linolenic acid content than those fried in regular soybean oil (6.2% linolenic acid) (12). Potatoes fried in low-linolenic acid soybean oil also lacked the hydrogenated flavors of potatoes fried in hydrogenated oil. Robertson and Morrison (14) used analytical descriptive tests to evaluate the flavor of potato chips fried in different oils and stored under accelerated conditions at 31°C for up to 10 wk. A 10-member trained panel used the following defined flavor descriptors: fresh = good chip flavor, no off flavors; heated = good quality but flavor
364 l S.L. Melton
Fig. 20.6. AOCS flavor-quality scale (4).
Fig. 20.7. AOCS flavor-intensity scale (4).
Sensory Evaluation of Frying Fat and Deep-Fried Products l 365
indicating some storage; stale = fair, not fresh, flat, or tasteless for aging; off flavor = poor undesirable flavor, such as, grassy; and rancid = bad, old oil flavor. The panel was trained to detect the stages of flavor deterioration using chips that were fresh or heated in a 60°C oven for 3, 6, or 9 days. The descriptive quality ratings were transformed into numerical scores where 1 = fresh to 5 = rancid, and the data were analyzed statistically. After the panelists were trained to detect stages of flavor deterioration, they were also trained to evaluate intensities of each odor–flavor descriptor as 1 = weak, 2 = moderate, and 3 = strong. These intensity ratings were expressed as flavor intensity values (FIV), where FIV is calculated from the following equation: FIV = [Nw + 2(Nm ) + 3(Ns )]/N where Nw = number of weak responses, Nm = number of medium responses, Ns = number of strong responses, and N = number of tasters (14). An average FIV was determined for each descriptor used by the panel to describe the flavor of the chips: fresh, heated, stale, off flavor, and rancid. The trained panel was used to evaluate the flavor of potato chips fried in cottonseed, sunflower, or palm oil and then stored for up to 10 wk. A coded reference chip also was evaluated at the same time as the stored chips. The reference chip was fresh and was fried in cottonseed oil. The analysis of variance for descriptive quality ratings of potato chips fried in different oils and stored up to 10 wk is shown in Table 20.1. The sensory scores of the potato chips are shown in Fig. 20.8, and FIV for fresh (0-wk storage time) and 10-wk-stored chips fried in different oils are shown in Table 20.2. The trained panel scored the flavor of fresh chips fried in cottonseed oil between very good, fresh (or a value of 1.0) and good, heated (or a value of 2.0). The average sensory score for the flavor of fresh chips fried in palm and sunflower oils was between 2.0 and 3.0. Of all flavor descriptors, fresh received the maximum FIV in 0-wk chips fried in cottonseed or sunflower oil, while the descriptor receiving the maximum FIV for 0-wk chips fried in palm oil was off flavor (Table 20.2). After 10 wk of storage, the stale flavor description received the maximum FIV for chips fried in cottonseed oil, but the rancid flavor had the highest FIV in chips fried in sunflower oil. Chips fried in palm oil and stored 10 wk had the greatest FIV for the rancid and heated descriptors (Table 20.2). Two other descriptive methods for testing the quality of fried food are the flavor profile method and the quantitative descriptive analysis. In the flavor profile method, perceived factors are called “character notes.” A list of these notes is made by each trained panel member during preliminary evaluation of the food being investigated. The lists are then compared by the entire panel, and agreement is reached on which notes will be used for further analysis. Then each panelist evaluates the sample again for the intensity of each character note and the amplitude of overall aroma and taste. The panel then reaches a consensus of opinion on the intensity of each character note
366 l S.L. Melton TABLE 20.1 Analysis of Variance of Descriptive Quality Ratings for Stored Potato Chips Fried in Different Oilsa Source of variation
Dfb
Sum of squares
Mean squares
F-value
Time
5
147.364
29.462
21.14c
Samples
2
22.149
11.079
7.95c
Time × samples
10
26.440
2.644
1.90
Reps (time × samples)
36
50.198
1.394
461.500
0.949
Panelists (rep × samples) 486
Source: Robertson, J.A., and W.H. Morrison, III, J. Food Sci. 43:420 (1978). Reference samples were omitted from the analysis of variance. b Degrees of freedom. c Significant at the P < 0.05 level. a
TABLE 20.2 Flavor Intensity Values (FIV) of Stored Potato Chipsa Storage time (wk)
Flavor description
Reference cottonseed
Cottonseed
Sunflower
Palm
0
Fresh
1.27
1.80
0.83
0.53
Heated
0.40
—
0.20
0.13
Stale
1.10
0.13
0.07
0.30
Off-flavor
—
—
0.37
0.87
Rancid
0.17
0.10
0.30
0.20
Fresh
2.37
—
—
—
Heated
0.17
0.20
0.03
0.07
10
Stale
—
1.20
0.73
0.90
Off-flavor
0.07
0.43
0.70
0.67
Rancid
—
0.53
0.87
0.87
Source as in Table 20.1. a Chips fried in oils except reference cottonseed oil were stored at 31°C; those fried in reference cottonseed oil were stored at -20°C.
Sensory Evaluation of Frying Fat and Deep-Fried Products l 367
Fig. 20.8. Effect of storage on mean (n = 30) sensory scores of potato chips fried in different frying oils (14).
and the amplitude of the overall aroma, taste, and aftertaste. In the original flavor profile method, the results were not subjected to statistical analysis. Although flavor profiling was successful for comparing products, for quality control, and for product development, it was not used extensively in sensory evaluation of fried food or oil. Berry et al. (16) gave a classic application of flavor profiling. In quantitative descriptive analysis (QDA), the sensory characteristics of a product are identified and quantified by a panel under the guidance of a panel leader (17). After the identification of characteristics to be evaluated by the panel as a group, each panelist individually rates the intensity of each characteristic on a 150-cm horizontal linear scale anchored at the ends by the terms “weak” and “intense.” The results of QDA are analyzed statistically using analysis of variance, factor, or regression analysis (2). Quantitative descriptive analysis was used by researchers analyzing the sensory characteristics of fried food. Stevenson et al. (18) used an eight-member trained QDA panel to assess exterior crispness, interior dryness, intensity of oil flavor, oil off flavor, oily mouthcoat, potato flavor, color, and overall quality of french-fried potatoes cooked in canola oil and soybean fat. Results reported by Stevenson et al. (18) are illustrated in Fig. 20.9, which shows the scores they reported in tabular form. In Fig. 20.9, each line radiating from the center of the diagram represents a sensory characteristic; the distance from the center outward on each line represents the intensity of that characteristic. The point at which the product line intersects each characteristic line is the relative intensity of that characteristic in the product.
368 l S.L. Melton
Fig. 20.9. Average scores from sensory evaluation (QDA) of french-fried potatoes cooked in canola oil or soybean oil as prepared from data presented by Stevenson et al. (18).
When there is a significant difference in intensity of a characteristic between the two products, there is a difference between the intersections of the product lines with the characteristic line. For example, french-fried potatoes cooked in soybean oil were darker in color, had greater exterior crispness and interior dryness, but had less overall quality than french-fried potatoes cooked in canola oil. French-fried potatoes cooked in either oil had equal intensities of potato and oil flavors and the same level of oily mouth feel (18). Several sources give extensive details on panel selection and training (19), presentation of samples (20), and sensory evaluation facilities (5,21–23). Warner (4) reported that the qualifications for analytical panelists include an interest in testing oil-containing foods, normal sensory acuity, availability to participate in regularly conducted panels, ability to discriminate known differences, and consistency of response for the same samples. A minimum of 10–12 panelists is recommended for analytical panels if the results will be analyzed statistically (24). Only five highly trained panelists, however, are needed for a flavor profile panel (4); their results are not usually analyzed statistically. The triangle test is often used to select potential candidates for analytical panels based on their ability to detect flavor differences. To be selected for the panel, a candidate must score at least 60% correct of a total of 24 responses (15). According to Warner (4), attribute scoring or descriptive tests are unsuitable for screening panelists because definite correct responses cannot be established.
Sensory Evaluation of Frying Fat and Deep-Fried Products l 369
In training, panelists should be familiarized with panel room operation, test procedure and products, and scoring or rating methods used. Panelists should then be trained to identify sensory characteristics and to develop a common vocabulary of descriptions for those characteristics. Reference standards for typical odors/flavors are necessary in training a panel, according to Warner (4), who listed reference standards for flavors in oils and oil-containing food. The reference standards also should be characterized by expert judges prior to being used in training. Other chemical standards of odors in food were published by Harper et al. (25). For presentation to panelists, all samples should be of uniform portion and temperature and should be coded properly. Freshly fried food should be served to panelists at 50°C in covered glass containers (4). The sample size of the fried food should be one-inch cubes, and no more than 2–4 samples should be presented to panelists at any one session (4). If the fried food is a snack food, it may not be possible to have a one-inch cube and it may also be inappropriate to serve it at 50°C. Specific sample recommendations are also given by the ASTM (16). Three-digit random numbers are the most commonly used sample codes for sensory analysis and can be selected from a random number table in a statistics book or sensory evaluation reference book (17). To avoid bias in sensory analysis experiments with a small number of samples (3 or fewer), all possible orders of sample presentation should be used an equal number of times (3). When there is a large number of samples to be analyzed in an experiment, the order of presentation should be randomized to minimize psychological effects (23). The environment also is an important factor in the sensory evaluation of food and should be without distractions so that the judges can concentrate on the testing at hand and do so independently (3). The judges should be separated, with each judge in an individual compartment if possible. Detailed information on the requirements for a sensory evaluation laboratory were published by the ASTM (26). References 1. 2. 3. 4. 5. 6. 7.
Institute of Food Technologists. Food Technol. 1981, 35, 50. Mounts, T.L.; and K. Warner. In Handbook of Soy Oil Processing and Utilization; D.R. Erickson, E.H. Pryde, O.L. Brekke, T.L. Mounts, and R.A. Falb, Eds.; American Oil Chemists’ Society: Champaign, IL, 1980; pp. 245–266. Penfield, M.P.; and A.M. Campbell. In Experimental Food Science, 3rd ed.; Academic Press: San Diego, CA, 1990; pp. 51–77. Warner, K. In Flavor Chemistry of Fats and Oils; D.B. Min and T.H. Smouse, Eds.; American Oil Chemists’ Society: Champaign, IL, 1985; pp. 207–221. American Society for Testing Materials (ASTM). STP 682. In Manual on Consumer Sensory Evaluation; E.E. Schaefer, Ed.; ASTM: Philadelphia, PA, 1979. Ellis, B.H. Food Technol. 1968, 22, 583. Fuller, G.; D.G. Guadagni; M.L. Weaver; G. Notter; and R.J. Horvat. J. Food Sci. 1971, 36, 43.
370 l S.L. Melton 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
Melton, S.L.; M.K. Trigiano; M.P. Penfield; and R. Yang. Potato Chips Fried in Canola and/or Cottonseed Oil Maintain High Quality. J. Food Sci. 1993, 58, 1079–1083. Melton, S.L.; S. Jafar; D. Sykes; and M.K. Trigiano. J. Am. Oil Chem. Soc. 1994, 71, 1301. Maga, J.A. J. Food Sci. 1973, 38, 1251. American Society for Testing and Materials (ASTM). STP 434. In Manual on Consumer Sensory Testing Methods; ASTM: Philadelphia, PA, 1968. Mounts, T.L.; K. Warner; G.R. List; W.E. Neff; and R.F. Wilson. J. Am. Oil Chem. Soc. 1994, 71, 495. American Oil Chemists’ Society (AOCS). In Official Methods and Recommended Practices of the American Oil Chemists’ Society, 4th ed.; D. Firestone, Ed.; American Oil Chemists’ Society: Champaign, IL, 1990. Robertson, J.A.; and W.H. Morrison, III. J. Food Sci. 1978, 43, 420. Caul, J.F. Adv. Food Res. 1957, 7, 1. Berry, B.W.; J.A. Maga; C.R. Calkins; L.H. Wells; Z.L. Carpenter; and H.R. Cross. J. Food Sci. 1980, 45, 1113. Stone, H.; J. Sidel; A. Woolsey; and R.C. Singleton. Food Technol. 1974, 28, 24. Stevenson, S.G.; L. Jeffery; M. Vaisey-Genser; B. Fyfe; F.W. Hougen; and N.A.M. Eskin. Can. Inst. Food Sci. Technol. J. 1984, 17, 187. American Society for Testing Materials (ASTM). STP 758. In Guidelines for the Selection and Training of Sensory Panel Members; ASTM: Philadelphia, PA, 1981. American Society for Testing Materials (ASTM). End Use Products. In Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1989; Vol. 15.07. Larmond, E. Laboratory Methods for Sensory Evaluation of Food, Publication 1637; Canadian Department of Agriculture: Ottawa, Ontario, Canada, 1977. Amerine, M.A.; R.M. Pangborn; and E.B. Rossler. In Principles of Sensory Evaluation of Food; Academic Press: New York, 1965. Larmond, E., Food Technol. 1973, 27, 28. Sidel, J.L.; and H. Stone. Food Technol. 1976, 30, 32. Harper, R.; D.G. Land; N.M. Griffiths; and E.C. Bate-Smith. Br. J. Psychol. 1968, 49, 231. American Society for Testing Materials (ASTM). In Physical Requirement Guidelines for Sensory Evaluation Laboratories, Publication 913; J. Eggert and K. Zook, Eds.; ASTM: Philadelphia, 1986.