Measuring Flavor Interactions Using Fractional Omission Testing

Chapter 7

Downloaded by NATL UNIV OF SINGAPORE on March 25, 2017 | http://pubs.acs.org Publication Date (Web): June 15, 2015 | doi: 10.1021/bk-2015-1191.ch007

Measuring Flavor Interactions Using Fractional Omission Testing Neil Desforges,*,1 Kate O’Mahony,2 Perrine Delime,2 Joanne Hort,2 and Andrew J Taylor1 1WALTHAM ®

Centre for Pet Nutrition, Mars Petcare, Waltham-on-the-Wolds, LE14 4RT, United Kingdom 2Division of Food Sciences, University of Nottingham, LE12 5RD, United Kingdom *E-mail: [email protected].

Sensory omission is an experimental method used to identify key compounds contributing to a flavor. A novel method for omission testing involving the same-different test and a surety rating have been applied to a strawberry flavor. Results from separate omission experiments determined the orthonasal impact of removing different fractions of each individual volatile and the retronasal impact of different tastants on volatile omission. R-indices were calculated from the surety rating and were used to assess significant differences. All nine volatiles were significant on omission when tested orthonasally. Three volatiles remained significant when half of their concentration was removed or diluted in mineral water. Orthonasal testing was more sensitive than retronasal testing with a higher number of significant observations. This new approach utilizing the R-index indicates the relative contribution of a volatile to the overall perceived flavors.

© 2015 American Chemical Society Guthrie et al.; The Chemical Sensory Informatics of Food: Measurement, Analysis, Integration ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Introduction From a commercial perspective, it is important for industry to develop mixtures with the minimum number of volatiles necessary to represent the target flavors or fragrance. In the creation of food flavor models, the challenge is to determine which volatiles are needed to reproduce the flavors, as not all the volatiles in food contribute to sensory perception of flavors. Sensory omission testing involves omitting one volatile or a group of volatiles from a flavor mixture and comparing it to the original. It has several applications, such as the identification of key odorants in raw materials (1), but is often used in the assessment of the contribution of individual volatiles to the overall flavors and the elucidation of interactions between the volatiles (2). Grosch (3) reviewed sensory omission work published pre-2001 and indicated that although attribute rating (4) and similarity rating (5) have occasionally been used, the triangle test (ISO 4120:2004) was the most popular method used in sensory omission experiments (1, 4–6). The approach using the triangle test reported in the literature has often been limited statistically with assessor numbers too low (between 5 and 19) to accurately draw any significant conclusions, especially in terms of claims for similarity where power (1-β) and hence number of judges become critical. Furthermore, omission tests tend to focus on identifying ‘key’ volatiles and have not assessed the ‘relative’ contribution of volatiles to the overall flavors. Hence there is considerable scope to improve the approach used in such omission studies. The Same-Different test (ASTM E2139-05) is simple, efficient and intuitive for naïve assessors and has shown high sensitivity due to low memory requirements (7). The major limitation of the same-different test is that it can be subject to response bias when samples are similar and the same/different decision lies in the region of uncertainty. This can cause a certain level of cautiousness for the assessors who may place an emphasis on getting the answer correct (8). Response bias is a cognitive mechanism which is independent of the assessor’s sensitivity to the attributes within the sample; therefore it reduces the discriminatory power of the same-different test (9). To overcome the issue of response bias, a sureness rating can be added to the test so that the assessor can indicate the level of confidence they had in answering the “same or different?” question. The sureness rating can be analysed in conjunction with the “same” or “different” answer, by calculating the R-index statistic (9). Therefore, the R-index removes response bias and provides a discrimination index, which in turn provides a metric to compare the ‘relative’ contribution of volatiles to the overall flavors. This approach has not been used in this context previously. The aim of this study was to apply this methodology to a commercial strawberry flavor containing nine volatiles. The sensory test measured the relative importance of each individual volatile within the strawberry flavor when one volatile was completely removed and the relative importance of each individual volatile when the strawberry flavor was diluted into water. The strawberry flavor was assessed again when the relative importance of each individual volatile was determined when half the amount of one volatile was removed. The method was 78 Guthrie et al.; The Chemical Sensory Informatics of Food: Measurement, Analysis, Integration ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

also used to investigate cross-modal interactions between volatiles and tastants. The strawberry flavor was diluted into water and was assessed retronasally for the omission of volatiles in the absence and presence of sucrose and citric acid. The results from the orthonasal and retronasal testing were compared and discussed.

Materials and Methods


Materials A strawberry flavor, based on a commercially available product, and composed of nine volatiles (all Sigma Aldrich, UK) was used throughout this study (Table 1). Propylene glycol (PG) (Sigma Aldrich, UK) was used as a solvent as it is easily miscible with the related volatile compounds and works effectively as a volatile carrier (10). Other consumables included Evian™ mineral water (Danone™ Group, France) used as a palate cleanser and as a solvent. Plain, unsalted matzo crackers (Rakusens Limited, UK) were also used for palate cleansing.

Table 1. Strawberry Flavor Model Aroma

Volatile

Concentration (g/kg)

2,3-Butandione

buttery

0.00500

Butanoic acid

sweaty, rancid

0.920

Gamma-decalactone

fatty, peach like

1.33

Ethyl butanoate

fruity

5.00

Ethyl hexanoate

green, pineapple

3.36

Methyl dihydrojasmonate

jasmine

0.00300

4-Hydroxy-2,5-dimethyl-3-furanone

caramel

10.7

Methyl(E)-3-phenylprop-2-enoate

fruity

2.70

cis-3-Hexen-1-ol

leaf like

10.8

Preparation of the Original Strawberry Flavors in PG Samples were prepared by pipetting the volatiles (Table 1) into Duran® GL 45 laboratory glass bottles (SCHOTT, USA) using a calibrated balance (allowing a 5% error). Samples were then diluted in PG and mixed on a roller bed for 30 minutes. Samples were refrigerated at 4°C and used up to 8 days after preparation. All samples were removed from the refrigerator at least one hour prior to testing to ensure samples were at room temperature (20 ° C ± 2°C). 79 Guthrie et al.; The Chemical Sensory Informatics of Food: Measurement, Analysis, Integration ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Preparation of the Orthonasal Omission Samples in PG


Omission samples were prepared as described above. For n-1 samples, one volatile was removed from the original flavor model (n) and for the n-0.5 samples, half of the volatile was removed from the original flavor model (n). Nine omission samples were prepared for each test with each omission sample omitting either one volatile or half the volatile from the original flavor model. For sensory testing, samples were kept at room temperature (20 ± 2°C) and used within 1 week.

Preparation of the Orthonasal Omission Samples in Mineral Water

Omission samples (n-1) were prepared as described above, by omitting one volatile from the original flavors model (n). Nine omission samples were prepared, each omission sample omitting one volatile from the original flavor model. For sensory testing, strawberry flavor and omission samples in PG were diluted at 0.75% w/w with mineral water. Samples were kept at room temperature (20 ± 2°C) and used within 24 hours.

Preparation of the Retronasal Omission Samples in PG

Omission samples (n-1) were prepared as described above, by omitting one volatile from the original flavors model (n). Nine omission samples were prepared, each omission sample omitting one volatile from the original flavor model. For sensory testing, strawberry flavor and omission samples in PG were diluted at 0.75% w/w in mineral water; mineral water alone was used for the blank taste. Sucrose was added at 2% v/v for a sweet taste and citric acid was added at 0.05% v/v for an acidic taste. Samples were kept at room temperature (20 ± 2°C) and used within 24 hours.

Sensory Testing

Subjects

Naïve assessors (80% female, aged between 18 and 25) were recruited from students at the University of Nottingham. Assessors were recruited for the orthonasal and retronasal testing separately. Ethical approval for the use of human subjects were reviewed internally by Division of Food Sciences ethics committee. 80 Guthrie et al.; The Chemical Sensory Informatics of Food: Measurement, Analysis, Integration ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Sensory Sessions Six sensory sessions were carried out in isolated booths. Each session involved nine discrimination tests to compare each one of the nine omission samples (n-x) with the original mixture (n). Sessions one, two and three involved 100 assessors carrying out nine same-different tests on the flavors delivered orthonasally. Session one evaluated n-1 omission samples in PG, session two evaluated n-0.5 omission samples in PG and session three evaluated n-1 omission samples in mineral water. Sessions four, five and six involved 100 assessors carrying out nine same-different tests on the flavors delivered retronasally. Session four contained blank tastants, session five contained a sweet taste and session six contained an acid taste. The order of presentation for the samples was randomized over each session. FIZZ software (Biosystèmes, France) was used to provide a randomized balanced design for the same-different test within each sensory session. Assessors were instructed to fast (except water) at least one hour prior to the sessions. They were instructed to assess samples from left to right and were allowed to re-evaluate the samples if necessary. Within a session, after three and six tests, assessors were allocated a five minute break to limit sensory fatigue and carryover effects.

Orthonasal Delivery Screw-top 20 mL glass bottles containing 10 mL of sample were presented to the assessors. Assessors were instructed to sniff the samples and replace the lid immediately to prevent the volatiles dispersing throughout the test area.

Retronasal Delivery Assessors were instructed to sip from a 20 mL sample through the straw of a lidded pot (thus avoiding orthonasal detection). Mineral water and crackers were provided as a palate cleanser between samples to minimise carry over effect.

Same-Different Testing The protocol used in this study was an extension of the same-different test using a sureness rating (11). The same-different test with sureness rating can be regarded as a version of the degree of difference (DOD) test proposed by Aust (12). For each same-different test, the assessors assessed the two samples and stated whether they thought they were the same or different. Secondly, the assessors were asked to state the sureness level of their decision, represented by a four point surety scale (‘very unsure’, ‘unsure’, ‘sure’, ‘very sure’). A complete randomized balanced design was used for the sample presentation, with half of the assessors presented with a ‘same pair’ and the other half presented with a ‘different pair’. 81 Guthrie et al.; The Chemical Sensory Informatics of Food: Measurement, Analysis, Integration ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Data Analysis The R-index was calculated from the surety data for each omission test (9). According to the critical value tables published by Bi and O’Mahony (13), where there were fifty presentations of the same samples, an R-index of 59% or higher indicated a significant difference.

Results and Discussion


Orthonasal Testing The R-index values for the n-1 and n-0.5 orthonasal omission experiments are presented in Table 2. Each line of Table 2 corresponds to an omission test comparing the original flavor model with an omission sample omitting one volatile. The R-index can therefore be used as a measure of the relative importance of each individual volatile within the strawberry flavors. The results for n-1 omission testing in PG show that all of the volatiles contained within the strawberry flavors were significant with methyl(E)-3-phenylprop-2-enoate and 2,3-butandione reporting the highest and lowest R-indices respectively. These results show that the omission of 2,3-butandione was hardest to detect and the omission of methyl(E)-3-phenylprop-2-enoate was the easiest to discriminate.

Table 2. R-Index Values Associated with the Orthonasal Assessment of Each n-1 and n-0.5 Sample in Comparison to the Original Strawberry Flavors

a

Volatile

n-1 PG (%)

n-0.5 PG (%)

n-1 Water (%)

2,3-Butandione

62a

51

49

Butanoic acid

68a

52

48

Gamma-decalactone

69a

64a

53

Ethyl butanoate

76a

62a

54

Ethyl hexanoate

69a

69a

60a

Methyl dihydrojasmonate

69a

47

39

4-Hydroxy-2,5-dimethyl-3-furanone

72a

52

60a

Methyl(E)-3-phenylprop-2-enoate

79a

55

49

cis-3-Hexen-1-ol

76a

53

60a

R-index of 59% or higher indicated a significant difference, p-value

Measuring Flavor Interactions Using Fractional Omission Testing

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