Thermal Generation of Aromas - American Chemical Society

Chapter 33

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 29, 2018 | https://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch033

Flavor Constituents of Roasted Cashew Nuts A. Jayalekshmy and C. S. Narayanan Regional Research Laboratory, Commonwealth Scientific and Industrial Research Organisation, Trivandrum-695019, India

The flavor constituents of plain and roasted cashew nuts have not been previously reported in the liter ature. In the present study, aroma compounds have been isolated from plain, oven-roasted and oil-roasted cashew nuts by simultaneous distillation extraction and by steam distillation followed by selective extrac tion, after pH adjustment. Compound identification was carried out by GC and GC-MS analyses. Esters and lactones are present in plain cashews whereas roasted samples also contain pyrazines.

The cashew tree (Anacardium occidentale, Linn.) was introduced into India from Brazil by the Portugese some 400 years ago and became established on the west coast. The tree i s very much valued for the tasty and nutritious cashew nut kernels and, the cashew apple finds limited use in making fermented juice products like 'Feni'. India has been one of the foremost exporters of cashew nuts and during 1987 nearly 12,942 tons were exported to United States (I). In the traditional way of processing, the fully mature nuts in shells are kept on wire trays and roasted for 80 to 90 seconds in a bath of cashew nut shell l i q u i d (CNSL) main tained at a temperature of 180 C. Due to the high temperature and presence of moisture inside the kernel, the nuts are roasted to a desired degree and can be easily broken and manually shelled. After t h i s , s k i l l e d labourers remove the brown testa without damaging the kernel and the nuts are graded according to market standard. Usually, export kernels are packed in vacuum with or without carbon dioxide.

Cashew nut shell l i q u i d (CNSL) i s the exudate from the cashew nut shell. It i s mainly phenolic in nature. Industrially used f o r making polymeric resins etc. A bath of the CNSL is tradition ally used for heating the whole nuts with s h e l l s . 0097-6156/89/0409-0355$06.00/0 ο 1989 American Chemical Society Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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THERMAL GENERATION OF AROMAS

Cashew kernels possess pleasant taste and flavor and are eaten either raw or roasted with salt. The raw nuts, which are packed in flexible packages, are usually marketed as 'plain' cashews. The 'roasted' nuts are usually f r i e d i n vegetable o i l to light brown colour, salted and packed in cans. The flavor characteristics of plain cashew nuts are enhanced as a result of o i l roasting. A literature review revealed that the flavor constituents of plain or roasted cashew nuts have not been i n v e s t i gated previously. In the present study, the authors have attempted to isolate the flavor compounds by steam distillation and extraction and to identify them by GC and GC-MS techniques. T h i r t y s i x compounds have been identified for the f i r s t time. Experimental Commercial cashew nuts available in the market were purchased. Proximate analysis of the plain cashews was carried out in t r i plicate according to AOAC methods (2^). Flavor studies Commercial samples used in the experiment were both plain and oil-roasted cashew nuts. In order to determine the volatiles formed from cashew nuts without the interference of the vegetable oil medium, plain cashew nuts in the form of small bits (1 cm length) were roasted in an a i r oven at 150 C f o r 10 min. The flavor extracts of plain, oven-roasted and oil-roasted nuts were isolated by two different methods: (a) simultaneous distillation and extraction in a simple SDE apparatus (3_) and (b) by steam distillation and selective extraction of basic, neutral and acidic components following classical pH adjustment procedure. Distilled methylene chloride (DCM) was used as the extraction solvent in the f i r s t method and the distillation was carried out at atmospheric pressure. Two hundred grams of powdered cashew nuts were used f o r extraction and the methylene chloride removed at low temperature ( ^ 5 0 ° C ) over a water-bath using a Vigreaux column of 30 cm length and 5.0 cm od. The concentrated extract (0.5 ml) was sealed and kept frozen (-10 C ) . In the case of selective extraction of compounds from steam distillate after pH adjustment, about 1.5 L distillate were collected by co-distilling 200 g of powdered nuts with 2.5 L of water. The basic, neutral and acidic compounds were extracted using either methylene chloride or ether and the solvent was removed by d i s t i l l a t i o n . In the case of the acidic fraction, the ether extract was methylated by refluxing with methanol-sulphuric acid (50:1) reagent for 2 hours. The methylated samples were washed free of acid and extracted with r e d i s t i l l e d hexane, d r i e d and stored frozen. Analytical methods GC analysis: Preliminary GC analysis of total extracts (SDE method) of a l l the three samples were carried out in a Hewlett Packard

Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

33.

JAYALEKSHMY AND NARAYANAN

Flavor Constituents of Cashew Nuts


5840A gas c h r o m a t o g r a p h equipped w i t h a flame ionization detec tor. N i t r o g e n was used as t h e c a r r i e r gas at a f l o w r a t e of 20 m l / m i n . A p a c k e d s . s . column of 3 p e r c e n t OV-17 (1.83 m χ 3 mm i d ) was used u n d e r t h e c o n d i t i o n s , i n i t i a l temperature 80 C, t h e n p r o g r a m m e d at 5 /min r a t e t o 225 C and h e l d at t h a t t e m p e r a t u r e f o r 15 m i n . The e x p e r i m e n t s were r e p e a t e d i n t h e same column u n d e r same c o n d i t i o n s u s i n g TCD d e t e c t o r and h e l i u m as c a r r i e r gas. The e f f l u e n t was ' s n i f f e d ' b y a p a n e l of judges and t h e consensus of t h e i r independent d e s c r i p t i o n t a k e n . GC Retention time indices ( I g ) : The I g v a l u e s were c a l c u l a t e d b y l i n e a r i n t e r p o l a t i o n of t h e unknown between r e t e n t i o n t i m e s of a s e r i e s of m e t h y l e s t e r s of n o r m a l c a r b o x y l i c a c i d s u n d e r t h e GC c o n d i t i o n s c i t e d f o r OV-17 column and n i t r o g e n as c a r r i e r gas. The method was s i m i l a r to t h e one suggested b y Van den Dool and K r a t z ( 4 ) . The I„ v a l u e of each s t a n d a r d was arbi t r a r i l y g i v e n t h e c a r b o n number of t h e a c i d of t h e e s t e r ; t h u s hexanoate was g i v e n a v a l u e of 6.0. The I„ v a l u e s of t h e v a r i o u s p e a k s i n t h e t o t a l e x t r a c t s (SDE method) o f a l l t h e t h r e e s a m p l e s and of v a r i o u s a u t h e n t i c compounds w e r e d e t e r m i n e d under c o r r e s p o n d i n g GC c o n d i t i o n s u s i n g t h e same column. C a p i l l a r y GC and GC-MS analysis: The f l a v o r e x t r a c t s i s o l a t e d b y SDE and s e l e c t i v e e x t r a c t i o n methods of p l a i n , o v e n - r o a s t e d and o i l - r o a s t e d c a s h e w nuts w e r e a n a l y z e d u s i n g a c r o s s - l i n k e d m e t h y l s i l i c o n column (60 m χ 0.2 mm i d ) made of f u s e d s i l i c a c o u p l e d to a H e w l e t t P a c k a r d 5995B m o d e l q u a d r u p o l e mass s p e c trometer. The i o n i s a t i o n v o l t a g e was 70 eV and e l e c t r o n m u l t i p l i e r v o l t a g e was 1600V. The a n a l y s i s was done i n s p l i t mode w i t h a r a t i o of 1:75 and t h e c a r r i e r gas was h e l i u m . Following GC c o n d i t i o n s w e r e found t o be r e a s o n a b l y good f o r t h e v a r i o u s f l a v o r e x t r a c t s ; i s o t h e r m a l at 90° f o r 5 m i n , t h e n t e m p e r a t u r e p r o g r a m m e d to 225 at t h e r a t e of 5 /min w i t h a f i n a l t e m p e r a t u r e h o l d f o r 25 m i n . The b a s i c and n e u t r a l compounds w e r e a n a l y z e d i n t h i s column u n d e r t h e same c o n d i t i o n s . The a c i d i c f r a c t i o n s were a n a l y z e d , as m e t h y l e s t e r s , on a m e t h y l s i l i c o n column (12 m χ 0.2 mm i d ) under the above s a i d program conditions. In a l l t h e e x p e r i m e n t s , t h e i n s t r u m e n t was tuned and c a l i b r a t e d w i t h PFTBA (Per Fluoro T r i Butyl Amine). Methyl heptanoate was used as t h e i n t e r n a l s t a n d a r d . The mass s p e c t r a l d a t a of t h e v a r i o u s f r a c t i o n s w e r e m a t c h e d w i t h t h e NBS l i b r a r y of F l a v o r and F r a g r a n c e s i n t h e d a t a b a s e s y s t e m p r o v i d e d w i t h t h e i n s t r u ment. Mass s p e c t r a of compounds w h i c h w e r e not c o v e r e d u n d e r t h i s l i b r a r y were compared w i t h p u b l i s h e d s p e c t r a l data (5), (6).

RESULTS The p r o x i m a t e Table I.

composition

of

plain

cashew

kernels i s given in


357

358

THERMAL GENERATION OF AROMAS Table I. Proximate composition

(%)

of plain cashews 5.00 45.40 22.58 9.79 25.83


Moisture Fat Protein Soluble sugars Total carbohydrates (by difference) Ash

2.19

The pH of the steam distillate was 6.2 in a l l cases. The total flavor isolate (by SDE) of plain cashews had, on the whole, a strong pungent and green aroma, reminiscent of the cashew nut testa and cashew s h e l l , whereas the flavor isolates from roasted samples had the characteristic mildly nutty aroma also. The flavor fractions obtained by selective extraction method gave some information about the chemical nature of compounds responsible for the characteristic flavor notes. Accordingly, the basic fraction of roasted nuts, (both oven-and oil-roasted), had the t y p i c a l nutty aroma associated with pyrazine. compounds. The basic fraction of plain cashews d i d not have any characteristic flavor in particular. The neutral fraction of plain cashews had a mildly estery, fragrant, green flavor, t y p i c a l l y reminiscent of freshly plucked cashew nuts. The neutral fraction of roasted nuts had an estery, nutty and slightly pungent, heavy aroma. The acidic fractions of plain and roasted samples had the t y p i c a l , short chain fatty acid note. The quantitative distribution of various fractions is shown inTable II. Table II. Quantitative distribution of flavor Nature of extraction Simultaneous distillation & extraction (SDE) (Total extract) Steam distillation and fractional extraction by pH adjustment (selective extraction) Basic compounds Neutral Acidic

extract

Quantity of aroma from cashew nuts (mg/kg) Oil roasted Oven roasted Plain 100

250

273

105

300

309

100 115 85

112 107 90

3 84 18

The authenticity of the peaks in GC and GC-MS analyses were tested by the relative retention time of the standards with reference to methyl heptanoate. In addition to t h i s , the I values were calculated on OV-17 column, for the standards ana sample peaks. Compounds for which standards were not there, but had very good mass spectral agreement (correlation ratio ^0.95) were also considered as present p o s i t i v e l y . A few peaks for which £


33.




1

spectral agreement was less, have been described as 'tentative . Relative concentrations of compounds of the different extracts and flavor fractions were calculated from the electronically integrated peak areas of the respective peaks as against the total area of a l l the peaks in that sample. From these values and the quantitative distribution data, the actual concentration of each compound was calculated. The results are shown in Table III. Preliminary GC analysis revealed that there are more number of peaks in roasted samples compared to plain cashews. Also selective extraction method was found to be slightly superior to the SDE method under the conditions of the experiment adopted in this study. However, the compounds in oven-roasted and oil-roasted samples d i d not differ much, qualitatively and quantitatively. In total, 26 compounds have been identified in plain cashews and 36 compounds in roasted samples. The identified peaks constituted 70 percent of the total peaks registered in GC analysis of the individual samples. The descriptive flavor profile of the eluting peaks of the plain and roasted samples were studied. Since the flavor isolate from oven roasted cashew nuts contained the flavor components of plain cashews also and since analysis showed that there was not much difference between the flavor constituents of oven-roasted and oil-roasted samples, the aromagram of the oven-roasted cashew nuts (SDE) was taken as representative. F i g . l gives the GC profile of oven roasted cashew nuts and the sensory properties of the numbered peaks are included in Table III. Discussion Of the 26 compounds identified in plain cashews, 5-heptene-2one was found to be most abundant (30 percent). The eluting peak had an intense green aroma. Another important compound was found to be 1,3-propanediol diacetate, the actual nasal impact of the corresponding peak being fresh green, cashew nut-like aroma. Butyl acetate, methyl and ethyl esters of higher fatty acids and the lactones may add to the estery, oily and nutty flavor of plain cashews. The green, mango-like aroma, identical with peak No.5, could be possibly due to myrcene. The phenolic compound (peak No.36) had the typical aroma of cashew nut s h e l l . The acidic fraction contained fatty acids (C^ to C ^ ) . The analysis of roasted samples (both oven- and oil-roasted) reveal the presence of 7 pyrazines, on an average, contributing 30% of total aroma. Among the pyrazines, 2,6-dimethyl pyrazine occurs in large concentration and has a green, nutty odour. The peaks corresponding to methyl pyrazine, 2-ethyl-6-methyl pyrazine and methyl-pyrrolo (1,2-c*) pyrazine registered a mild roasted, sensory impact. The last one, a b i c y c l i c pyrazine, has been reported in model system studies, related to coffee roasting (60. As Maga reviewed, the pyrazines are mostly r e s ponsible for the roasted flavors of many food systems (7). Analysis of the neutral fraction of flavor isolates revealed that most of the neutral compounds l i k e esters, lactones and carbonyls present in plain cashews were found in roasted samples also. In addition, three furans have been detected in the roasted


359


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THERMAL GENERATION OF AROMAS


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samples. Of these, furfural and 2-pentyl furan have been r e p o r ted in most systems (8). The latter was identified to have a green, grassy flavor note. The t h i r d furan derivative is (2furanyl m e t h y l ) - 5-methyl— 2-(5H) furanone which is noticed to have a strong, toasted cereal - l i k e aroma. This is formed in considerable amount during roasting. Furans are formed by the thermal degradation of sugars (&). The compound 2,4-undecadienial found only in roasted cashew nuts must be derived from l i p i d s of cashew nuts. The corresponding peak had a raw green aroma. The formation of benzaldehyde during thermal degradation of l i p i d s is suggested; however, in cashew nuts benzaldehyde could be detected in plain cashews also, which had not undergone much of heat treatment. The short chain fatty acids (C^ to detected in plain cashews could be detected in roasted samples' also, in higher amounts. Dodecenoic acid was the only unsaturated fatty acid among them. These fatty acids are formed by the degradation of glycerides by mild enzymatic action or h y d r o l y s i s . To sum u p , the flavor constituents of plain and roasted cashew nuts are reported here for the first time. The mild flavor of cashew nuts can be attributed to the carbonyls, esters and lactones, especially to 5-heptene-2-one and 1,3-propanediol diacetate. Upon roasting, 2,6-dimethyl pyrazine, 2,6-diethyl pyrazine and the furanone are formed in larger amounts and from flavor profile also these compounds are l i k e l y to play a s i g n i ficant role in the characteristic aroma of roasted cashew nuts. Acknowledgment The authors wish to express their sincere thanks to M s . K . P . Padmakumari and M s . Beena Symon for their technical assistance. The constant support and encouragement by D r . A . D . Damodaran, Director, Regional Research Laboratory, Trivandrum i s gratefully acknowledged. Thanks are also due to Prof. Joseph Maga, Colorado Uni versity for most of the authentic compounds. Literature cited 1. Cashew Exports in 1987. Cashew Bulletin. 1988, 35(3), 3-4. 2. Official Methods of Analysis. Association of Official Analytical Chemists (AOAC), 1975, 12th Ed. 3. Schultz, Τ., Flath, R., Mon, R., Eggling, S., Teranishi, R. J. Agric. Food Chem. 1977, 25, 446. 4. Vanden Dool, H. and Kratz, P.D. J. Chromatogr. 1963, 11, 463-471. 5. Atlas of Spectral Data and physical Constants for Organic Compounds. Vols.I-V. Grasselli, J.G., Ritchey, W.M., Eds., CRC Press: 2nd Edn.


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Flavor Constituents ofCashew Nuts

6. Baltes, W. and Bochmann, G. Z. Lebensm Unters Forsch. 1987, 184, 478-484. 7. Maga, J.A., CRC Critical Reviews in Food Science and Nutrition. 1982, 16, 1-48. 8. Maga, J.A. CRC Critical Reviews in Food Science and Nutrition. 1979, 11, 355-400. 1989


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Thermal Generation of Aromas - American Chemical Society

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