Variation in Oat Groats Due to Variety, Storage and Heat Treatment. I: Phenolic Compounds

Journal of Cereal Science 24 (1996) 263–272

Variation in Oat Groats Due to Variety, Storage and Heat Treatment. I: Phenolic Compounds L. H. Dimberg∗, E. L. Molteberg†‡, R. Solheim† and W. Frølich†§ ∗Swedish University of Agricultural Sciences, Department of Food Science, P.O. Box 7051, S-750 07 Uppsala, Sweden, †MATFORSK-Norwegian Food Research Institute, Osloveien 1, N-1430 A˚s, Norway and ‡Department of Food Science, Agricultural University of Norway, P.O. Box 5036, N-1432 A˚s, Norway Received 27 July 1995

ABSTRACT Low molecular weight phenolic compounds present in heat processed oats (Avena sativa L) were analysed. The oat grains were of three varieties (Kapp, Mustang and Svea), stored at different relative humidities (30, 55 or 80%) and periods (3·5 or 15·5 months) and processed with or without hulls. Eleven UV-absorbing compounds detected by High Performance Liquid Chromatography were subjected to univariate and multivariate statistical analysis. The selected compounds included caffeic acid, p-coumaric acid, ferulic acid, vanillic acid, p-hydroxybenzaldehyde, vanillin, coniferyl alcohol, three avenanthramides and one unidentified substance. The levels of vanillic acid, vanillin and, especially, p-coumaric acid, p-hydroxybenzaldehyde and coniferyl alcohol increased significantly in samples processed with hulls, but not in samples processed without hulls. Ferulic acid increased in both processes, while caffeic acid and the avenanthramides were found to decrease during processing. Storage of unprocessed samples for 1 year generally increased the levels of phenolic acids and aldehydes. For the phenolic acids (except ferulic acid), this increase was most pronounced after storage at high relative humidity (80%). The avenanthramides were present at their highest levels in Mustang, caffeic acid in Svea and Mustang, the unidentified compound in Svea, while all the other compounds studied were present predominantly in the variety Kapp.  1996 Academic Press Limited

Keywords: antioxidant, avenanthramide, flavour.

the characteristic aroma of toasted oats. A relatively high lipid content, a large portion of unsaturated fatty acids and active lipolytic enzymes contribute to rapid development of rancidity in unstabilised oat flour1. Therefore, a typical commercial heat process for oats includes steam stabilisation, which inactivates most of the enzymes2,3. However, oxidative rancidity may also occur in heat-treated oats where lipolytic enzymes are inactivated1. Although oats have a potent antioxidative system4–7, thermal processing may cause antioxidant destruction and, therefore, result in an increasing oxidation rate with rancid flavour as a consequence. Accordingly, heat processing changes the chemical composition of the oat grains and thereby influences the quality of oat products.

INTRODUCTION Oats are heat processed to enhance the stability of the products derived from them and to develop  : HPLC=High Performance Liquid Chromatography; rh=relative humidity; NMR=Nuclear Magnetic Resonance; PCA=Principal Component Analysis; FFA=free fatty acid; V=vanillin; VA=vanillic acid; CA=caffeic acid; FA=ferulic acid; pCA=p-coumaric acid; pHBA=p-hydroxybenzaldehyde; CAL=coniferyl alcohol; AV=avenanthramide; U=unidentified. § Current address: Skarland Press, P.O. 5042, Majorstuen, N-0301, Oslo, Norway. Corresponding author: L. H. Dimberg.

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Most of the oat flavour components, such as terpenes, alkylbenzenes, aldehydes, alcohols and heterocycles, are volatile and require a heat process (kiln-drying)2 to develop8,9. However, non-volatile compounds may also contribute to the flavour. Oat grains contain a rich variety of non-volatile, low molecular weight, phenolic compounds which may contribute to sensory properties10. One group of low molecular weight phenolic compounds, the free phenolic acids, are minor compounds in cereals, together rarely exceeding 200 mg/kg dry weight of whole, mature, undamaged kernels11. Even at low levels (10–90 mg/kg), however, free phenolic acids may contribute significantly to objectionable flavours12,13. Phenolic aldehydes may also contribute to the flavour of foods. Vanillin is an antioxidant14, and also one of the most common flavouring agents for sweet foods. A group of phenolic compounds in oat grains, commonly known as avenanthramides7,15,16, includes a group of hydroxy and/or methoxy substituted cinnamoylanthranilic acids, which together constitute 200–800 mg/kg dehulled oats11. Two of the avenanthramides, N-(4′-hydroxy-3′-methoxy-(E)cinnamoyl)-5-hydroxy-anthranilic acid and N-(4′hydroxy-3′-methoxy-(E)-cinnamoyl)-5-hydroxy-4methoxy-anthranilic acid, are heat-stable antioxidants7 and may, thus, contribute to the oxidative stability of heat-treated oats. The variety, quality of the raw material, storage conditions and nature of the stabilising and drying procedures all contribute to the quality of the processed oats. In addition, the levels of inherent and/or produced flavour and antioxidative components throughout storage and processing are important. Free fatty acids and phenolic compounds are two groups of non-volatile compounds which may directly or indirectly influence the flavour. FFAs are the basis for development of rancid flavour, while phenolic acids are known to contribute to flavour10,12,13 and to act as antioxidants4,7,14. In a previous study the content of FFA was found to be greatly affected by storage and processing17. The objective of the present study was to investigate the effects of storage and heat treatment of three oat varieties on the levels of free, low molecular weight, phenolic compounds. In a follow-up study to the present one the sensory properties of oats influenced by variety, storage and heat treatment were investigated18. The contribution of low molecular weight phenolic compounds to the sensory quality was also discussed.

EXPERIMENTAL Design A complete factorial design was used. The samples—grain of three oat varieties (Kapp, Mustang and Svea), stored at three levels of relative humidity (30, 55 or 80% rh) for 3·5 or 15·5 months—were the same as used previously17. The storage temperature was 20 °C for the first 8 months and 15 °C for the last 7·5 months. When the storage study was initiated the moisture content of the grains was ca 11·2%. After storage the water contents were 9·3, 12·4 and 14·7%, respectively, for samples stored at 30, 55 or 80% rh. Processing resulted in water contents in the range of 9·2– 11·7% for samples heat-treated with hulls and in the range of 6·2–9·2% for samples heat-treated without hulls17. Heat treatment was either performed directly (with hulls) or after dehulling (without hulls). Unprocessed samples were used as controls. Processing A pilot process was used that was developed17 to simulate a typical commercial process. Before heating, which was performed by steaming in a baking oven (100 °C) for 10 min, the oats were soaked in water for 2 min. The samples, with or without hulls, were dried at 100 °C for 4 and 3·5 h, respectively. After drying, the samples with hulls were dehulled. All samples were stored under nitrogen at −20 °C until milling, which was performed on a Retsch ultra centrifugal mill with a 0·5 mm screen. Extraction of phenolic compounds for analysis Duplicate flour samples (10·0 g) were extracted twice with 80% (v/v) aqueous ethanol (100 mL) at room temperature as described previously7, except that the use of an ultrasonic bath was replaced by magnetic stirring. The ethanol extracted compounds were suspended in 1·0 mL methanol and then centrifuged. The samples (supernatants) were stored under nitrogen at −20 °C until analysis by High Performance Liquid Chromatography (HPLC). Isolation and identification of phenolic compounds Oat groats (500 g) processed with hulls were extracted with 80% (v/v) aqueous ethanol (2·5 L) in

Phenol in processed oats

an Ultra Turrax homogeniser at room temperature. The extract was filtered and the solvent evaporated at 40 °C. Repeated column chromatography and preparative thin layer chromatography (CHCl3–methanol–H2O (140:15:1 or 10:5:1) as mobile phases) yielded p-hydroxybenzaldehyde (pHBA), trans-coniferyl alcohol (CAL) and two avenanthramides (AV3 and AV4) in small amounts (1–3 mg). Nuclear magnetic resonance (1H-NMR) spectra of the four isolated substances were recorded on a Varian VXR 400 spectrometer at 400 Hz with CD3OD as solvent. The spectra of pHBA and CAL were identical with the spectra of authentic samples. The spectra of the other two compounds corresponded closely to the spectra reported by Collins16 for N-(3′,4′-dihydroxy(E)-cinnamoyl)-5hydroxyanthranilic acid (AV3) and N-(4′-hydroxy(E)-cinnamoyl)-5-hydroxy-anthranilic acid (AV4). The HPLC chromatographic retention times and the UV spectral properties of the two isolated avenanthramides were also in agreement with those reported by Collins and Mullin15. HPLC analysis HPLC was performed on a Merck Hitachi instrument equipped with a diode-array detector, according to the method described previously7. The components were separated on a reversed phase C-18 column with a gradient of acetonitrile (0–30%, v/v, in 60 min) as mobile phase. Compounds were detected at 250 nm (with a bandwidth of 100 nm) and peak retention times and areas were monitored and integrated automatically. Six chromatographic peaks (Fig. 1) have been identified previously7 and four (pHBA, CAL, AV3 and AV4) were identified in this paper (see above). One peak (U) is still unidentified, but the UVspectrum very much resembled those of flavonoids (not shown). Quantification of all the compounds, except U and the avenanthramides, was performed using commercial standards, all with a purity grade of at least 97%. For quantification of the avenanthramides, N-(4′-hydroxy-3′-methoxy-(E)cinnamoyl)-5-hydroxyanthranilic acid (AV1), isolated from oats7, was used as standard. Statistical analysis Averaged peak areas from duplicate extractions were statistically analysed by ANOVA. Tukey’s

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Studentitised range test (SAS, version 6·04, SAS Institute, Cary, NC, U.S.A.) was used to find significant differences between samples. Principal Component Analysis (PCA)19 (Unscrambler-Extended version 5·5, Camo a/s, Trondheim, Norway), was used to study the main variation in the material and the relationship between the various samples and the content of phenolic compounds. PCA models the data in terms of principal components (factors). Each factor can be characterised graphically by a score plot, which shows how samples are related to each other, and a loading plot, which visualises the pattern of variation among the variables. Samples and variables that are located in the same area of the respective plots are closely related to each other. All variables were mean centred and scaled to unit variance prior to PCA. The calibration model was validated by full scale cross validation.

RESULTS Sixty eight chromatographic peaks were detected by HPLC analysis (Fig. 1). Eleven peaks were subjected to subsequent statistical analysis. These peaks included the compounds of main interest, i.e. vanillic acid (VA), caffeic acid (CA), p-coumaric acid (pCA), ferulic acid (FA), p-hydroxybenzaldehyde (pHBA), vanillin (V), coniferyl alcohol (CAL) and three avenanthramides (AV1, AV3, AV4). In addition, one peak (U), which varied systematically in the chromatographic patterns among samples, was included. Results from Tukey’s test (a=0·05) showed that the content of various phenolic compounds differed significantly among the varieties and between samples heat-treated in different ways while differences among the storage variables were less pronounced (Table I). VA, pCA, FA, V, pHBA and CAL were all strongly connected to the variety Kapp, while the contents of CA and AV4 were lowest in Kapp (Table I). AV1, AV3 and AV4 had all the highest levels in Mustang and U was most pronounced in Svea. V, pHBA, CAL and the phenolic acids, except CA, increased considerably compared with unprocessed samples when the heat treatment was performed on samples with hulls. This increase varied from an average of 25% to 900% (Fig. 2; Table I). The levels of FA also increased in samples processed without hulls and this increase was about 20% (Table I). On the other hand, the content of CA, the av-

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60

50

AV1

U

10

0

AV4

AV3 FA

VA

20

CAL pCA

30

CA V

pHBA

mAU

40

10

0

20

30 Time (min)

40

50

60

Figure 1 UV-absorbing (250±50 nm) compounds in an 80% ethanol extract of oats. The extract is from grains of the variety Mustang, stored at 80% rh for 15·5 months and processed with hulls. The peaks are p-hydroxybenzaldehyde (pHBA), vanillic acid (VA), caffeic acid (CA), vanillin (V), coniferyl alcohol (CAL), p-coumaric acid (pCA), ferulic acid (FA), unidentified (U), avenanthramide 3 (AV3), avenanthramide 4 (AV4) and avenanthramide 1 (AV1).

Table I

The contents (mean values for all samples within the same category) of various phenolic compounds of different varieties, storage humidities, storage times and heat treatments

Compound§

VA∗ CA∗ pCA∗ FA∗ V∗ pHBA∗ CAL∗ AV1∗ AV3∗ AV4∗ U†

Variety

Relative humidity (% rh)

Storage time (months)

Mustang

Svea

Kapp

30

55

80

3·5

15·5

1·2 2·4 1·5 2·2 2·1 0·8 0·7 >43 >62 >47 98

1·2 2·4 1·3 2·0 2·1 0·7 0·8 22 33 34 >152

>1·3 <1·8 >2·0 >2·7 >2·6 >1·1 >1·0 21 28 <25 99

1·2 <1·6 1·3 2·0 2·3 0·9 >1·0 31 44 38 123

1·0 2·4 1·2 2·3 2·1 0·9 0·8 28 42 37 113

>1·5 >2·7 >2·0 2·5 2·3 0·8 0·7 27 40 33 114

1·0 1·3 1·3 2·0 2·0 0·8 0·9 28 39 36 119

>1·3 >3·1 >1·7 >2·5 >2·4 >0·9 0·8 30 45 36 114

Processing unprocessed with hulls without hulls 1·2 >3·1 0·8 <2·0 1·7 0·3 0·2 >33 >48 >50 133

>1·7 1·6 >2·8 2·5 >3·3 >2·0 >2·0 27 39 28 122

<0·8 1·8 0·7 2·3 1·7 0·3 0·2 26 38 31 <94

§ See legends to Figure 1. ∗ Values are given in mg/kg oat grain. † Values are given in peak area units. > Significantly higher value for each compound compared with values within the groups variety, relative humidity, storage time and processing, respectively. < Significantly lower value compared with values within the same groups as above (Tukey’s Studentised Range Test, a=0·05).

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950

Change (% of unprocessed samples)

750

550

350

150

–50 VA

CA

pCA

FA

V

pHBA

CAL

AV1

AV3

AV4

U

Figure 2 Changes in the content of phenolic compounds in oats due to heat treatment of grains with (Ε) or without (Φ) hulls. The values given are the percentage change compared with unprocessed samples.

enanthramides and the unidentified compound were generally lower in processed samples. No notable differences between the two heat processes were found among these five substances (Table I). Increased levels of the phenolic acids and aldehydes were found when the samples were stored (Table I). This was particularly the case for CA, which increased by almost 150% from 3·5 months of storage to 15·5 months. The levels of CAL, U and the avenanthramides, were unchanged, however. Storage at a rh of 80% resulted in significantly higher levels of VA, pCA and, especially, CA, while CAL had the highest levels when stored at 30% rh (Table I). All other compounds were unchanged due to storage humidity. Results from PCA showed that the humidity during storage had only a minor effect on the total variation in phenolic compounds. Thus, by averaging the samples from the various humidity levels and obtaining 18 samples instead of 54, a simplified picture of the main variations in the data set could be achieved, without influencing the PCA plots substantially (results not shown). Therefore, the PCAs presented are mean values of all three storage conditions. The first four factors of the PCA model explained

87% of the variation within the 18 samples. The contribution of phenolic compounds to differences among samples is shown by comparing the score plots with the loading plots for each factor [Fig. 3(a and b); Fig. 4(a and b)]. For Factor 1 all compounds, except U, were separated into two groups. The phenolic acids (except CA), pHBA, V and CAL, all had positive loadings and were thereby related to samples heat-treated with hulls, particularly to the variety Kapp [Fig. 3(a and b)]. The avenanthramides and CA, on the other hand, had negative loadings for Factor 1 and were linked to unprocessed samples of Mustang. The high amount of the avenanthramides in Mustang was also clearly expressed by Factor 2. Ferulic acid was separated from the other phenolic compounds for Factor 1 by a positive loading for Factor 2. This was related to a higher level of FA also in samples of Kapp and Svea, heat-treated without hulls. Factor 3 showed mainly that U was connected to Svea [Fig. 4(a and b)]. The positive loadings for CA, FA and VA for Factor 4 may be associated with the higher levels found after a storage period of 15 months [Fig. 4(a and b)]. PCA provided a readily interpretable graphical representation of the relationships between the

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(a) 4 Heat-treated without hulls K

S k

PCA scores, factor 2 (15%)

2 s S

K

S

k 0

K M

M

s

s

k m

M

Heat-treated with hulls –2 m m

Unprocessed –4 –4

–2

0 2 PCA scores, factor 1 (46%)

4

6

(b) 0.2

FA U

PCA loadings, factor 2 (15%)

0.0

CAL –0.2

V pHBA pCA

VA

CA

–0.4

AV4

AV1 AV3 –0.6 –0.4

–0.2

0.0 0.2 PCA loadings, factor 1 (46%)

0.4

0.6

Figure 3 Principal Component Analysis (PCA) of the phenolic compounds in oats. (a) Scores for Factor 1 and 2, explaining 46 and 15%, respectively, of the variation among samples. (b) Loadings for Factor 1 and 2. The three varieties stored for 3·5 months are M=Mustang, K=Kapp and S=Svea. Small letters are used for 15·5 months of storage. Χ=unprocessed samples, Ε=samples processed with hulls, Μ=samples processed without hulls, pHBA=p-hydroxybenzaldehyde, VA=vanillic acid, CA=caffeic acid, V=vanillin, CAL=coniferyl alcohol, pCA=p-coumaric acid, FA=ferulic acid, AV 1, 3, 4=avenanthramide 1, 3 and 4, U=unidentified.

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(a) 2 Stored 15.5 months k k s PCA scores, factor 4 (11%)

1 m

s k m

m

0

K

s

K M

K S

–1

S M

M

S

Stored 3.5 months –2 –2

–1

0 1 PCA scores, factor 3 (15%)

2

3

(b) 0.8 CA

PCA loadings, factor 4 (11%)

0.6 FA 0.4

VA 0.2

V 0.0 AV1 –0.2

AV3

pCA AV4

pHBA CAL

–0.4 –0.4

–0.2

0.0

0.2 0.4 PCA loadings, factor 3 (15%)

U 0.6

0.8

1.0

Figure 4 Principal Component Analysis (PCA) of the phenolic compounds in oats. (a) Scores for Factor 3 and 4, explaining 15 and 11%, respectively, of the variation among samples. (b) Loadings for Factor 3 and 4. The symbols are the same as described in Figure 3.

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individual samples and the phenolic compounds. Generally, the results from PCA and ANOVA/ Tukey agreed well.

DISCUSSION Low molecular weight phenolic compounds, i.e. aromatic acids and aldehydes, are mostly esterified to cell wall polysaccharides in graminaceous plants20–22. In grains, their highest concentrations are found in the outer grain fractions11,23. The increased content of free phenolic compounds in oat grains heat-treated with hulls may, thus, be explained by thermal liberation of cell wall bound compounds from both hulls and groats. Phenolic compounds may also be conjugated to lignin24, a complex polymer of phenylpropanoid units25, which encrust and penetrate the cellulose cell walls. As Klason lignin amounts to about 22% in oat hulls but only to 1·4% in oat groats26, the lignin monomers p-coumaric acid, p-hydroxybenzaldehyde and coniferyl alcohol, were probably almost exclusively released from the hulls and transported into the groats during the process. Accordingly, they did not add to the phenolic content in grains when the hulls were removed prior to processing27. Also when parboiling rice, the amount of free phenolic acids increased considerably in rough rice compared with dehulled rice23. It is suggested that the acids are released from the cell walls in the chaff and bran and diffuse into the inner layer of the rice grain during parboiling23. Liberation of phenolic compounds may be highly dependent on moisture content, time and temperature during processing. Water and wet processing of various plant materials commonly causes release of phenolic compounds. Soaking of oat meal in water resulted in a ten-fold increase in vanillin28, while pressure cooking of corn caused a substantial increase in the amounts of free ferulic acid, p-coumaric acid and vanillin29. Hot water treatment promoted the liberation of coniferyl alcohol from lignin in alfalfa25 and the amount liberated was dependent upon the extraction time25. Hence, in our experiments, the longer time used during processing and the higher moisture contents obtained in the samples heat-treated with hulls17, may have promoted an increased release of phenolic compounds in those samples compared with the samples processed without hulls. Processing may also have changed the ratio between

various compounds due to thermal degradation. Vanillin and vanillic acid, for instance, can be produced through thermal decomposition of ferulic acid30–32, while p-hydroxybenzaldehyde can be formed from p-coumaric acid32. Caffeic acid was the only phenolic acid that was reduced during both heat processes. Caffeic acid has been found to be heat-sensitive but ferulic- and p-coumaric acids are also susceptible to thermal breakdown13,29–32. Even if no net decrease was observed in our study for any of the phenolic acids, except caffeic acid, degradation may be masked by liberation of bound acids. Some of the minor chromatographic peaks might be products (i.e. vinylphenols) from thermal degradation of p-coumaric or ferulic acids29–33. A relatively small reduction (20%) in the levels of avenanthramides AV1 and AV3 due to heat treatment is in agreement with earlier findings7 as AV1 has been found to be a rather heat-stable antioxidant in oats7. That AV4 was more labile is difficult to explain. Oat varietal differences in phenolic compounds have been reported previously7. Among 10 varieties, free caffeic acid was found to vary in the range of 3–5 mg/kg oat grain and AV1 in the range of 40–132 mg/kg7. The increased content of phenolic acids due to storage is, however, in contrast to results reported previously34. Those results showed that, in wheat flour, the content of free phenolic acids due to oxidation decreases considerably during 6 months of storage. In our study, oxidation of the phenolic acids may have been prevented due to storage of the oats as whole grains. Even though most oat flavour components are volatile8,9, and, as a rule, require a heat process to develop8, non-volatile components can also contribute to the flavour. Phenolic acids are perceived as sour, bitter and astringent flavours at 30–90 mg/ kg12,13 and taste thresholds for combinations of phenolic acids are even lower than those for individual acids12. Also, aromatic aldehydes like vanillin and p-hydroxybenzaldehyde, are considered to take part in the formation of flavour and taste35. As antioxidant activities and flavours of low molecular weight phenolic compounds probably affect the sensory properties of oat products, choice of variety, storage conditions and heat treatment procedures are important for the quality of these food items. Oat phenols and their possible relationships to sensory properties of oats are the subject of the follow-up article18.

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Acknowledgements Professor Olof Theander is acknowledged for initiating this project. Ms Johanne Margrethe Bjo¨rge and Mr Sweneric Andersson are thanked for their skilful technical assistance and Mr Rolf Andersson for running the NMR spectra. Financial support for this project was provided by grants from the Swedish Council for Forestry and Agricultural Research, Cerealia’s Foundation, The Foundation of Va¨stsvenska Lantma¨n, Swedish Farmers’ Foundation for Agricultural Research and Semper AB (to LHD), from The Research Council of Norway (to ELM) and from Volvo Research Foundation and Volvo Educational Foundation (to WF).

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