Consumer acceptance of canola oils during temperature-accelerated storage

Food Qualifyand pnfnncc 5 (1994) 237-243 0 1994 Elsevier Science Limited Printed in Great Britain. All tights reserved 0950%?93/94/$7.00

ELSEVIER

CONSUMERACCEPTANCEOFCANOLAOILSDURING TEMPERATURE-ACCELERATEDSTORAGE M. Vaisey-Genser,a L. J. Malcolmson,a D. Ryland,a R. Przybylski,a N . A . M. Eskina & 1. Armstrongb “Department of Food and Nutrition, %atistical AdvisoryService, University of Manitoba, Winnipeg, MB, Canada R3T 2N2 (Received 15 October 1993; accepted 15 February 1994)

Currently, differences in the stability of finished oils due to oil source, formulation and/or processing conditions are normally estimated from chemical tests and from assessments by trained sensory panels of samples subjected to accelerated storage at temperatures ranging from 60 to 14O’C. At the higher temperatures there is the risk that autoxidation proceeds by a different mechanism, yielding products which are not representative of shelf deterioration (Labuza, 1971). Storage at 60-70°C is recognized as having the fewest limitations, and results correlate well with evaluations of actual shelf life (Frankel, 1993). Reviews of stability studies performed on regular canola oil (RCO) suggest that trained sensory panels detected little or no change in odour intensity during the first two to four days of accelerated storage at 60-65°C or during 16 weeks of practical storage at ambient temperatures in the dark (Hawrysh, 1990; Malcolmson et aL, 1994). Early indications are that canola oil which has been genetically modified to be low in linolenic acid (LLCO) resists the oxidative changes which lead to rancidity for a substantially longer time than RCO. Przybylski et al. (1993) observed no change in the odour intensity of an experimental line of LLCO during 12 days of storage at 60°C. Labuza and Schmidl (1988) noted that, in the frozen food sector, practical shelf life had been shown to be 2-3 times longer for consumers compared to ‘just noticeable differences’ detected by laboratory panels. This reflects the distinction between judging sensory differences as opposed to assessing product acceptability. Consumer acceptance is an integrated judgment of the perceived sensory qualities of a product and its appre priateness for an intended use (Framingham, 1992). The present study was designed to provide a consumer acceptance baseline for the interpretation of stability assessments of canola oil during temperatureaccelerated storage. The availability of high and low linolenic acid canola oils provided the opportunity to contrast the acceptance of oils which were known to vary in stability, but were similar in genetic origin and in fatty acid composition apart from their contents of linolenic (18 : 3)) linoleic (18 : 2) and oleic (18 : 1) acids.

ABSTRACT Ninety-two consumersjudged the odour acceptability (yes or no) of regular (12.5 % l&3) and low linolenic acid (2.5 ‘7%18:3)

canola oils which had been stored at 60” C

for 21 and 42 days,respectively. For each storage day an average proportion of acceptance (APA) was calculated. Logistic regression analyses of storage days vs. APA yielded valid relationships

forboth oils.

These equations

permitted prediction of the number of days of accelerated storage for APA values porn to 0.5 APA,

the consumer

0.4 to 0.8. The storage time accqbtance

threshold,

12.5 days for regular canola oil and 34.3

was

days for low

linolenic acid canola oil, indicating substantially greater stability for the genetically modzfied cultivaz

The fre-

quency and character of free choice negative odour descriptors which were assigned to both oils at 0.5 APA were similar: @words:

Canolu

oil; consumer

acceptance;

sensory

stability; shelf lafe; oxidation; logistic regression.

INTRODUCTION Canola oil is a significant part of the Canadian diet accounting for 60% of domestic edible oil production in 1992 (Statistics Canada, 1993). It is widely used in salad oils, tablespreads, and in shortenings for baking and frying. Consumer acceptance dictates the success of a food product in the marketplace and logically should be the benchmark for quality standards. Establishing the tolerance of consumers to degrees of oxidation in canola oils is useful for the definition of the sensory limits of shelf life and may prove instructive for food safety. Anecdotal evidence that people like the taste of some oxidized lipids has raised concern in relation to uncertainty about the health implications of consuming lipid oxidation products (Haumann, 1993). 237

238

M. Vaisey-Cknseret al.

TABLE 1. Chemical Descriptors of Fresh Canola Oils oil

PV”

Iv*

Fattyacids (%)

@q/kg) RCO LLCO

0.50 0.59

122 110

16:0

16:l

l&O

l&l

l&2

18:3

2o:o

2O:l

22:l

4.5 4.3

0.3 0.2

1.5 1.6

55.8 59.3

22.7 29.6

12.5 2.5

0.5 0.5

1.4 1.3

0.2 0.1

TV = peroxide value, n = 2. w = iodine value calculated as triglycerides. “Other fatty acids each < 0.3.

MATERIALS

AND

METHODS

Oils Refined, bleached and deodorized canola oils containing no additives were obtained from a commercial processor (RCO) and a pilot plant (LLCO). Peroxide values (AOCS, 1990) showed that both oils were of good initial quality (Table 1). Fatty acid analyses confirmed that there were distinct differences in their content of polyunsaturates (PUFA) (Table 1). The 18 : 3 content was similar to the licensed varieties of Westar (RCO) and Stellar (LLCO) . Fatty acid composition was defined using the AOCS procedure (1990) as adapted by Przybylski et al. (1993). The lower iodine value calculated for LLCO as opposed to that for RCO (AOCS, 1990) reflected its higher proportion of monounsaturates (MUFA) (60.9% versus 57.7%) and fewer PUFAs (32.1% versus 35.2%) while maintaining a similarly low level of saturates (7%; Table 1). Oils were stored uncovered, in the dark, at 60°C in 90 g (100 ml) aliquots in 125 ml clear glass jars (49 mm i.d.). Accordingly, the ratio of sample surface area to volume was 0.188. Sufficient jars for consumer panelists were withdrawn randomly at 24 h intervals over 21 days for RCO and at 24 h intervals over 42 days for LLCO. Jars were removed, flushed with nitrogen, covered with teflon-lined plastic screw caps and stored at -18°C until required for analyses (maximum 12 weeks). Samples for odour evaluation consisted of 10 g of oil and 30 g of glass beads in 125 ml glass jars capped with teflon-lined plastic screw lids. Headspace was approximately 80%. Samples were equilibrated to 50°C for sensory testing (AOCS, 1990) by placing jars in a water bath on a Corning hotplate.

consumers prepared food in the home daily and all 92 of them used vegetable oil at least occasionally. They used oil in fried foods (87%), salad dressings (84%)) muffins or other baked goods (84%) and in marinated foods (64%). These people often stocked more than one kind of oil. Canola oil was usually purchased by 49% of the participants, olive oil by 45%, sunflower oil by 24%, corn oil by 23%, and other oils by 18%.

Sensory protocol Each panelist completed an orientation and two test sessions held on three successive days within a two-week period. To accommodate all panelists, there were six test slots on each day, two in the morning and four in the afternoon, each lasting 15-20 min. There were 45 participants in Week 1 and 47 in Week 2. Orientation involved the odour examination of six samples containing sequential dilutions (0,0.25,0.5, 1, 2 and 4% w/w) of a retail sample of boiled linseed oil (BLO) in fresh RCO. BLO, which is marketed as a wood protector, has a strong painty odour. The orientation was designed to acquaint panelists with the physical setting, the computer ballot and the protocol to be used in evaluating oil odour acceptability. The data presented in Fig. 1 show that these consumers characterized increases in BLO concentration as decreases in oil odour acceptability. In each of the two test sessions each panelist examined eight samples from one of the two oils. RCO was presented first during Week 1 and second during Week 2.

Panelists Ninety-two consumers (67 women and 25 men) were recruited to participate in the study by an invitation mailed to 720 university staff who were employed in areas other than food science or nutrition. The median age category was 31-50 years. Sixty-six percent of these

FIG. 1. Consumers’ percent acceptance dilutions of BLO in RCO; N= 92.

of odour for serial

Canolu Oil Accejdance

239

A minimum of 15 people evaluated the same eight samples which were selected to provide a range of storage times, but always included Day 0. Fewer days of storage for RCO than for LLCO (21 days versus 42 days) meant that all RCO storage days were repeated for Week 2. In total, for the RCO series, 91 judgments of Day 0 and 30-32 judgments for each of storage Days 1-21 were completed, whereas in the LLCO series there were 92 judgments of Day 0 and 15 or 16 judgments for each of storage days l-42. Odour examinations were carried out under red light in a sensory panel facility with eight computerequipped booths. For each sample, panelists were required to respond either ‘yes’ or ‘no’ to the following question: ‘Is this oil acceptable to use in preparing food?‘. Once this decision was made, panelists were invited to enter comments which described the sample’s odour. The repeated display of instructions for sample order and handling, excluding the time to record comments, assured at least 40 s between samples. The software program CSA Computerized Sensory Analysis System (Compusense Inc., Guelph, Canada) was used to input and record the sensory responses.

APA levels were permitted by the upper confidence limits for storage day estimates.

Data analysis

Use of logistic regression analysis

The average proportion of acceptance (APA = C(yes)/total observations) by storage day for each oil was transformed to logits, and logistic regression analysis was applied using the LOGISTIC procedure (SAS, 1990) from the following model (Hosmer & Lemeshow, 1989) :

Like SENSPRO (Norback, 1985) the LOGISTIC procedure (SAS, 1990) used in our study accommodates the analysis of binary data to provide estimates of the change in gradient of APA over time and of discrimination between different levels of consumer acceptance. Logistic regression analysis is a standard method for regression analysis of dichotomous data in many fields of the health sciences, particularly epidemiology (Pastides et aL, 1983; Hosmer 8c Lemeshow, 1989). To our knowledge, the use of logistic regression has not been demonstrated previously in the sensory measurement of food quality. However, the interpretation is based on maximum likelihood estimates which, in turn, have been used to assess sensory data from studies on the shelf life of ice cream and oat bran cereal (Wittinger & Smith, 1986; Labuza & Schmidl, 1988). Although the computation of logistic regression involves iterative methods to handle binary data, the likelihood estimates from the analysis have analogs in linear regression. For example, the deviance (-SlogL, where L represents the likelihood function) plays the same role as the residual sum of squares. A key parameter for the interpretation of results is the odds ratio, which is estimated from the estimated logistic regression coefficient and describes the strength of the association between dependent and independent variables (Hosmer & Lemeshow, 1989).

with the corresponding

logit transformed

model:

where = an intercept parameter in the logit scale = a gradient parameter in the logit scale P, X = a variable representing the day Z-(X) =theAPAatdayx. PO

Estimates of the storage days at specific APA levels for each oil were made as follows: (3)

= the estimated number of storage days at a given APA, level w, 8, = estimates of the model intercept and gradient parameters, respectively.

k

Maximum likelihood statistics were used to assess how well the model described the distribution of the APA data and to determine what distinctions between

RESULTS

AND

and lower

DISCUSSION

APA measurement The observed and predicted APA values for RCO and LLCO are provided in Fig. 2(a) and (b). Consumer APA as a measure of change in a food product during storage and processing has been defined by Lagrange and Norback (1987) as the acceptor set size and is considered to be a function of the perception of product attributes as well as overall acceptability. Norback (1985) has developed the statistical program SENSPRO which computes an acceptor set size gradient. The acceptor set size method was used successfully by Sharp et aZ. (1986) to determine the shelf life of refrigerated whitefish treated with various levels of potassium sorbate and different packaging conditions, and by Beausire et al. (1988) to optimize fresh turkey bratwurst formulations.

Fit of the logistic regression model The statistical analyses of APA data are summarized in Table 2. For both oil series, significant chi square (x2)

240

M. Vaisey-Genseret al.

a

RCQ

0.4.

a % P

O.J1‘L 0.2.

iz

0.1.

E a

0.0

k

. .,

(

.,

10

ti

..,

, ., .., DAYS

o.o\,, 0

.,.,

._ .., ( IO

,

.

......

r

40

50

40

50

JO

20

AT

6O’C

..,,,,,,,,,.,,,,,,,,,,,.,_r

20

JO

DAYSAT60V

FIG. 2. Effect of accelerated storage on consumers’ APA (a) of RCO odour, * = observed values (Day 0, N= 91; Days 1-21, N= 30-32) and (b) of LLCO odour, * = observed values (Day 0, N= 92; Days l-42, N= 15-16). P= predicted values; U,L = upper and

lower confidence limits.

statistics for the deviance (-2logL) confirmed that the logistic regression model provided an appropriate fit for the APA data and further, that adding the independent variable of storage days improved the values of predicted APA. The significance of both the intercepts (/I,,) and the gradient (&) in the logistic regression equations for each oil are reflected in their odds ratios, which serve as measures of association (Table 2). The regression lines in Fig. 2(a) and (b) , for RCO and LLCO, respectively, show the pattern of predicted consumer APA values during cumulative days of accelerated storage at 60°C. The shape of the upper and lower confidence limits indicates that AF’A predictions were subject to somewhat more variability at lower levels of acceptability, especially in the case of LLCO. The

length of storage time at which either oil is likely to exhibit a specific APA level may be interpolated from these graphs. Alternatively, such values may be calculated using eqn (3). Table 3 shows the storage days at 60°C which were calculated for APA levels ranging from O-8 to 0.4, together with their upper and lower 95% confidence limits. Three APA categories were distinct from one another in the RCO series: 0.7, O-6 and O-5 APA, while the confidence limits for the O-4 APA estimate overlapped with those at 0.5 APA. On the other hand, in the LLCO series, there were four distinct APA categories: 0.8,0.7,0.6 and 0.5 APA. The positive comments volunteered by consumer panelists to describe an oil sample after they had

Canolu Oil Accqbtance

241

TABLE 2. Maximum Likelihood Statistics from Logistic Regression Analyses of Consumer APAs of Stored Canola Oils RCO Source of variation

LLCO

Likelihood estimate

x2

Odds ratio

Likelihood estimate

x2

Odds ratio

945 1.136” -0.091’

63.0b 63.2’ 58*4b

3.113 0.913

846 1.871” -0.059

77.jh 125.6’ 69.6’

6.497 0.947

Deviance -210gC Intercept PO Gradient p,

-

“L = Likelihood function (Hosmer & Lemeshow, 1989)

“x2 significant at p= 0.0001. ‘q95% confidence intervals, respectively = (0.850, 1.421); (1.537, 2.205); (-0.115, -0.067); (-0.068, -0.042).

judged its acceptability were generally confined to the terms odourless, light or mild; buttery was cited occasionally (RCO, 3 times; LLCO, 5 times). The negative descriptors, such as those listed in Table 4, were more informative. The terms painty and fishy have been reported by others in association with oxidized RCO (Warner et aL, 1989). It is interesting that the character and frequency of negative descriptors for both oils were similar at the same APA level, as is illustrated for 0.5 APA in Table 4. This observation adds credibility to the ap propriateness of the APA system for defining levels of consumer acceptance of edible oils. It is worth noting that some panelists distinguished oil odour acceptability on the basis of intended use. An oil with a higher APA occasionally was referred to as suitable for use in salad dressing, whereas at lower APAs some oils were considered ‘OK’ for use in frying. This is consistent with the importance of appropriateness for function in the consumer’s choice of foods (Framingham, 1992).

Relative stability of RCO and LLCO As LLCO is not yet on the consumer market, RCO was the norm for the consumers in this study; 49% of them purchased it on a regular basis. This is a major change from 1981 when the vegetable oil buying patterns of 896 Canadian prairie consumers were surveyed by tele-

phone. In 1981, only 7% reported buying canola oil brands, while the more popular brands were those of soybean oil (27%) and corn oil (19%) (Shaykewich & Vaisey-Censer, 1982). In the present study, fresh LLCO appeared more acceptable to consumers than fresh RCO where predicted APAs at Day 0 were 0.86 and 0.76, respectively (Fig. 2(a) and (b)). The odds ratios for the intercepts (p,,) favoured the acceptance of LLCO since it was 6X as likely to be accepted as rejected at Day 0, while RCO was only 3X as likely to be accepted (Table 2). This observation was contrary to the results of an earlier study conducted in our laboratory where no difference in either odor intensity or odor pleasantness was found by a trained panel between fresh samples of RCO and LLCO (Pryzybylski et aL, 1993). To investigate this further, follow-up triangle tests were performed in the current study. They showed that the odours of Day 0 RCO and Day 0 LLCO were significantly different (N= 20; p = < O-01) even though their peroxide values (PV) were similarly low (Table 1). Although PV is considered reliable as a measure of initial oxidation products, it is recognized as less sensitive and precise than sensory 4. Frequency of Negative Free Choice Odour Descriptors Cited at 0.5 APA + Confidence Limits

TABLE

Descriptor TABLE 3. Distinctions in Storage Days Between Predicted

Consumer APA Categories APA (O-1)

LLCO

Pointestimate’(LCLUCL)*

Pointestimate’ (LCLUCL)*

(days) 0.8 0.7 0.6 0.5 0.4

Strong,

RCO

(days)

_c

-

3 8 12 17

(O-5) (6-10) (11-15) (15-21)

(days)

(days)

9 18 27 34 -

(4-12) (14-21) (24-30) (31-39) -

‘%alculated from eqn (3). b95% lower-upper confidence limits. T&mates unavailable as predicted APAwas not achieved.

heavy

RCO (Days 11-15) N= 153

LLCO (Days 31-39) N= 140

24

34

Oily, painty” Fishy, rancid Musty, staleb Sharp, sour, burnt” Used for frying

18 9 4 3 5

12 3 5 5 3

Total

63

s:,

“Includes: linseed oil, cod liver oil, machine oil, petroleum, turpentine. qncludes: tainted, off-odour, yeasty. qncludes: medicinal, chemical, aromatic, pungent, acidic, sulfurous, acrid, metallic, urine.

242

M. VaisTGenseretal.

measures as oxidation progresses (Frankel, 1993). The fact that RCO was commercially refined while LLCO was processed in a pilot plant may have contributed to the difference between these Day 0 oils. However, in prior work on freshly processed oils, RCO which had been bench-refined in the laboratories of the same pilot plant was found to be comparable to commercially refined RCO in both sensory and chemical measures (E&in et aZ., 1989). LLCO has been designed for enhanced keeping quality since 18 : 3 is the most rapidly oxidized of the common polyunsaturates (Labuza, 1971). From the consumer acceptance perspective, in the present study, LLCO appeared much more stable to accelerated storage than RCO. The odds ratios associated with the negative gradients (/3,) in the logistic regression equations forecast that LLCO was 95% as likely to be accepted as rejected with a one-day storage increment compared to 91% for RCO (Table 2). The practical significance of the differences in gradient of the RCO and LLCO logit regression lines is emphasized by the differences in storage time which were required to show comparable changes in consumer acceptance (Table 3). The consumer acceptance threshold, taken as the temperatureaccelerated storage time at O-5 APA, was 12 days for RCO and 34 days for LLCO (Table 3).

Shelf life implications LLCO appears to have an advantage over RCO in terms of shelf life stability. However, the comparison of the estimates of shelf life for the two oils must be viewed with some caution, given that the oils were processed under different conditions. Recognizing this limitation, LLCO may prove to be up to three times more stable than RCO. Evans et aL( 1973) reported that flavour scores for soybean oil aged for four days at 60°C were similar to scores for the same oils aged four months at 25°C. A similar relationship between temperatureaccelerated and ambient storage may be observed from Hawrysh’s work (1990) on canola oil, where no flavour change was observed during either 2-4 days at 60°C or 16 weeks at room temperature. Thus, assuming that the storage effect on canola oil of one day at 60°C is the equivalent of one month at room temperature, it may be projected from the present study that RCO would reach its threshold of shelf life acceptance (0.5 APA) at 12 months of storage at ambient temperatures and LLCO at 34 months.

CONCLUSIONS The primary value of the present work has been to identify the relationship between the storage days at 60°C and given levels of consumer acceptance of canola oil odour. Logistic regression analysis provided

a reliable tool to describe the change in consumer opinions of oil odour acceptance during temperatureaccelerated storage. Chemical and sensory definition of oil characteristics at distinctly different APA levels could offer benchmarks with consumer meaning for temperature-accelerated studies of canola oil quality. Such work is currently under way in our laboratories. In the course of the present investigation it has been confirmed that LLCO is more odour stable than its parent RCO.

ACKNOWLEDGEMENTS The cooperation of Car&nera Foods and of the Protein, Oilseeds and Starch (POS) Pilot Plant in supplying the refined, bleached and deodorized canola oils is much appreciated. The competent technical assistance of P. Lau is gratefully acknowledged. This research was made possible by funding from the Canola Council of Canada.

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