Review Cite This: J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Key Aroma Compounds in Oats and Oat Cereals Robert J. McGorrin*
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Department of Food Science and Technology, Oregon State University, 100 Wiegand Hall, Corvallis, Oregon 97331-6602, United States ABSTRACT: Oats possess a unique flavor, comprising grain and nut-like sensory characteristics. The first comprehensive study of oat flavor by Heydanek and McGorrin [Heydanek, M. G.; McGorrin, R. J. Gas chromatography−mass spectroscopy investigations on the flavor chemistry of oat groats. J. Agric. Food Chem. 1981, 29 (5), 950−954, 10.1021/jf00107a016] identified 110 volatile components in oat groats, including C8−C9 unsaturated aldehydes and ketones contributing raw oat grain, hay-feedy, and grassy aromas. A following study on heat-processed oats and cooked oatmeal by Heydanek and McGorrin (Heydanek, M. G.; McGorrin, R. J. Oat flavor chemistry: Principles and prospects. In Oats: Chemistry and Technology, 1st ed.; Webster, F. H., Ed.; AACC International: St. Paul, MN, 1986; pp 335−369) identified a series of Maillard-derived compounds, including furanones, thiazoles, and 2-methyl-, 2,5-dimethyl-, C3-, and C4-substituted pyrazines. In the subsequent 38 years since these initial research findings, additional identifications of aroma compounds in oat flakes and flours have been reported. This review addresses significant recent developments of the current understanding of oat flavor chemistry and the key aroma compounds that contribute to the unique flavor of oat cereals. KEYWORDS: flavor characterization techniques, oat flavor, oat cereal, oat aroma volatiles, key aroma compounds, analysis
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INTRODUCTION Interest in the flavor chemistry of oats (Avena sativa L.) derives from consumer awareness of their health benefits. Oats are widely consumed as ingredients in porridges, ready-to-eat (RTE) cereals, breads, cookies, and snack bars and are enjoyed for their pleasant nutty, browned, and cereal grain flavor.1−3 Concurrently, oats are associated with positive health images related to their abundance of β-glucans as soluble fiber and their role in serum cholesterol reduction.4,5 The “heart healthy” and “cholesterol reduction” health claims were approved by the U.S. Food and Drug Administration (FDA) in 1997,6 and the FDA allows oat cereal manufacturers to make the claim for daily consumption of 3 g of soluble oat fiber in a diet low in unsaturated fat and cholesterol.7 Oats contain considerably higher amounts of protein (17%) relative to other widely consumed cereal grains (wheat, 13%; barley, 12%; and rice, 8%) with a high quality.8−10 Oats have the highest fat levels compared to other cereal grains, e.g., 5 times higher than wheat,11 and have a favorable ratio of saturated and mono- and polyunsaturated fatty acids.12 Natural phenolic antioxidants are evenly distributed among the lipid in the endosperm fraction, which makes oats unique for their oxidative stability.13 Oats represent a highly complex matrix of proteins, complex carbohydrates, fats, and volatile compounds. The identification of the complex flavor constituents in oats and oat cereals remains a challenge and has been the subject of numerous investigations. Hrdlic̆ka and Janic̆ek14,15 first studied the nut-like flavor of toasted oat flakes and identified carbonyls and amines by paper chromatography. The first comprehensive gas chromatography−mass spectroscopy (GC−MS) studies by Heydanek and McGorrin1 identified more than 110 compounds in the volatile profile of oats. These aroma volatile studies were conducted on oat groats (dehulled oats) prior to enzyme inactivation, rancid oat groats,2,16 toasted oat groats,2 oat flakes,2 and oatmeal porridge2 (cooked oat flakes). © XXXX American Chemical Society
Subsequent oat aroma investigations focused on extruded oat flour,17−20 oatmeal porridge,21,22 oat groats,23−26 and oat flakes.21,23,24,27,28 Furthermore, the volatile composition in oils extracted from crude oats and unroasted and roasted milled oats has been investigated.29 The flavor contributions of volatile compounds in oat cereals was the subject of previous reviews.30,31 This review examines the various methods/techniques used to isolate and identify key volatile compounds in oats and oat products since the first reported GC−MS studies.1,2 It provides a contextual overview of various sensory-directed methods that were applied to identify oat compounds at various stages of processing. This report summarizes the current understandings of oat flavor chemistry and the significant aroma compounds that contribute to the distinctive flavor of oats.
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OAT PROCESSING Oatmeal is a broad term for whole oats (oat groats) that have been ground into a meal. Commercially processed oatmeal can be in the form of steel-cut oats, rolled oats or oat flakes, quick oats, instant oats, oat bran, and oat flour. Cooked oatmeal or oat porridge is consumed as a hot cereal. Definitions of various forms of raw and processed oats are shown in Table 1. Harvested oats lack flavor, and the development of the desirable nutty, grain-like oat aroma characteristics requires a heat treatment.1 In the absence of a thermal process, oats retain a raw green aroma and slightly bitter taste.1,2 The flavor quality of raw oats and oat groats will decline over extended Special Issue: 2nd International Flavor Fragrance Shanghai Received: February 11, 2019 Revised: April 11, 2019 Accepted: April 12, 2019
A
DOI: 10.1021/acs.jafc.9b00994 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Review
Journal of Agricultural and Food Chemistry
generation pathways involved. A critical consideration is to avoid the generation of artifacts during the isolation process, particularly because oats are prone to oxidative flavor formation if improperly processed.16 A particular challenge for oat flavor volatile studies is the relatively low quantity of flavor-active volatiles, which contribute to the characteristic oat flavor and, consequently, the lack of required sensitivity needed for identification posed by instrumental limits of detection. Isolation technique temperatures needed to be controlled to minimize unwanted flavor artifacts and to maintain the fidelity between the volatile isolates and the flavor of the oat starting material. Early recognition of this problem influenced which methods were selected for pre-concentration of oat volatiles to deliver the required sensitivity for subsequent GC−MS analysis. Volatile Isolation Techniques. The flavor isolation techniques employed by various researchers provide insights of their continued advancements during the past 4 decades. It is evident from oat flavor studies that the chosen isolation procedure influences the profile of extracted volatile compounds. Results obtained from different methods should be used complementarily to obtain representative data for the characterization of oat flavor to facilitate quantitative analysis and aroma assessment by gas chromatography−olfactometry (GC−O). A summary of oat aroma chemistry studies is listed in Table 2, representing the volatile isolation technique used, quantity of sample analyzed, and number of volatile compounds identified. The sample quantity is dependent upon the concentration of trace key aroma compounds in the sample and the relative sensitivity of the analytical method. In the initial volatile studies reported on oat groats, oat flakes, and cooked oatmeal, both vacuum steam distillation and dynamic headspace sampling were used to concentrate oat aroma components.1,2,16 The vacuum distillation method was initially selected to provide the required sensitivity and to avoid thermally generated artifacts. Unlike the Likens− Nickerson simultaneous distillation−extraction technique at atmospheric pressure,35 the vacuum distillation apparatus is operated at reduced pressure to lower the sample boiling point. Vacuum distillation used 8 kg of oat groat samples, which were heated for 4 h at 55 °C, either dry or with 8 L of water at a 0.02−20 Torr vacuum, and distillates were condensed in dry ice/2-propanol traps.1,2 The frozen distillates were thawed, evaluated by taste for sensory fidelity, extracted with dichloromethane, and concentrated for subsequent GC−MS analysis. The amount of volatile recovery and composition of volatiles were significantly influenced by whether the distillation of oat groats was conducted in the dry or hydrated state.1 Vacuum distillation was applied for analysis of volatiles in toasted oat groats,2 rancid oat groats,2,16 oat flakes,2 cooked oatmeal,2 and volatiles in oil obtained from crude and roasted oats.29 Dichloromethane extraction of a dry oat extrudate, followed by preparative high-performance liquid chromatography (HPLC) and vacuum distillation, was used to fractionate and pre-concentrate the volatiles in extruded oat flour for GC− MS and aroma extract dilution analysis.17 Headspace sampling was applied to the initial studies of oat volatiles1,2 and, historically, has been one of the most popular methods for flavor isolation as a result of its simplicity, convenience, and absence of solvent extraction. The dynamic headspace procedure (purge and trap) involves passing an inert gas through the sample, collecting the stripped volatile constituents onto a Tenax (polyphenylene oxide) trap, and
Table 1. Processed Forms of Oats and Oat Cereal Products oat form raw oats whole oats (oat groats) naked oats steel-cut oats rolled oats (oat flakes) quick oats instant oats oat bran oat flour porridge
processing step oats as harvested, which contain an outer hull or husk whole oat kernels, with the outer hull removed newly developed hull-less oat variety (Avena nuda) whole oats/oat groats, cut to one-third size with steel blades whole oats, steamed and then milled between two steel rollers; resulting flake is thick or thin, depending upon the roller gap whole oats cut into pieces, then steamed, and rolled into flakes precooked and dried thin oat flakes, for rapid cooking/rehydration whole oat without the germ (endosperm), ground into a meal steel-cut oats, steamed and ground into a flour cooked oatmeal prepared from thick or thin oat flakes
storage time as a result of the degradation of oat lipids by enzymatic hydrolysis, followed by oxidation.18,32 The prevention of oat rancidity and off-flavor development is typically achieved by heat treatment combined with elevated moisture levels to inactivate oat lipase enzymes, which could release free fatty acids susceptible to oxidation and contribute to rancidity.33 Commercial oat processing of dehulled oat groats begins with saturated (wet) steam treatment to deactivate enzymes, followed by kiln drying (up to 100 min at 88−98 °C) to develop the characteristic oat flavor.34 The maximum process temperature is controlled to minimize oxidative rancidity, whereas underprocessing could leave residual enzyme activity. Processed oat groats are dried to ∼7.5% moisture. In subsequent processing, oat groats are steamed, pressed into oat flakes with steel rollers, and then dried to a target 11% moisture content. The steps and conditions for processing raw oats into various oat products and RTE oat cereals are outlined in Figure 1.
Figure 1. Oat milling steps and conditions for processing raw oats into assorted oat products.
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ANALYTICAL METHODS FOR OAT FLAVOR IDENTIFICATION The focus of oat flavor chemistry studies is directed toward identification of key volatile compounds and their correlation with sensory attributes. The essential aspect is determining which volatiles are key and most significant to the flavor. The identification of certain classes of compounds also provides insights into their mechanism of formation and various B
DOI: 10.1021/acs.jafc.9b00994 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Review
Journal of Agricultural and Food Chemistry Table 2. Oat Aroma Chemistry Studies oat form oat groats
toasted oat groats rancid oat groats oat oil from groats oat flakes (uncooked)
cooked oatmeal (porridge)
oat flour (extruded)
investigators
isolation method
sample size
volatiles ID
Heydanek and McGorrin1 (1981) Heydanek and McGorrin2 (1986) Sides et al.23 (2001) Klensporf and Jelen24 (2005) Heydanek and McGorrin2 (1986) Heydanek and McGorrin2,16 (1981 and 1986) Heinio et al.26 (2002) Fors and Schlich29 (1989) Heydanek and McGorrin2 (1986) Morello21 (1998) Sides et al.23 (2001) Schuh and Schieberle27 (2005) Klensporf and Jelen24 (2005) Klensporf and Jelen28 (2008)
vacuum distillation; dynamic headspace vacuum distillation; dynamic headspace SPME SPME vacuum distillation; dynamic headspace vacuum distillation static headspace vacuum distillation vacuum distillation supercritical CO2 SPME SAFE SPME SPME SAFE vacuum distillation supercritical CO2 SPME extraction/HPLC/vacuum distillation static headspace dynamic headspace dynamic headspace
8 kg 8 kg 2g 5 ga 2 kg 2 kg 0.5 g 5g 2 kg 6 ga 2g 50 g 5 ga 5 ga 20 g 2 kg 10 ga 20 g 50 g 3g 2.5 g 10 g
110 110 16 25 >220 45 15 >100 19 >15 16 9 30 25 18 35 >15 20 25 11 27 >120
Heydanek and McGorrin2 (1986) Morello21 (1998) Zhou et al.22 (2000) Guth and Grosch17 (1993) Molteberg et al.18 (1996) Sjovall et al.19 (1997) Parker et al.20 (2000)
a
Personal communication.
requirements for increased sensitivity. A disadvantage of SPME is the limited trapping capacity of the thin coating on the fiber, causing competition among analytes for absorption sites and volatile displacement, especially with large dynamic ranges of analytes.38 The first reported application of SPME to oat analysis by Sides et al.23 was a comprehensive comparison of volatiles produced at intermediate stages of processing from raw oats and groats to flaked oats. The divinylbenzene/ carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fiber was selected for oat volatile analysis because of its highest adsorption capacity across a range of volatile polarities. Samples (2 g) in sealed vials were equilibrated with the fiber for 1 h at 60 °C before desorption and GC−MS analysis. A second HS-SPME processing comparison of raw oats to oat flakes was conducted by Klensporf and Jelen28 with quantitation of individual volatiles. A third application of HS-SPME to analyze the volatiles in oat flakes provided the following optimum conditions: 30 min, 50 °C, and 10% water.24 Authors noted that the addition of water to oat samples at 40−60% markedly reduced the extraction efficiency. Oat flake samples were analyzed as both whole flakes and a milled powder. Better extraction efficiency of volatile lipid oxidation products was observed when whole flake samples were analyzed, because lipid oxidation occurs mostly on the oat surface.24 After milling, volatile components are likely bound-up with the flour and make extraction more difficult. Aroma volatiles in oatmeal porridge (20 g of oat flakes and 100 mL of water) were adsorbed on a PDMS SPME fiber for 20 min at 100 °C during the cooking process.22 Solvent-assisted flavor evaporation (SAFE) provides the advantages of simultaneous distillation−extraction and vacuum distillation, without the risk of unwanted artifacts formed during the volatile isolation process.39 SAFE in combination with aroma extract dilution analysis (AEDA) was applied in two studies to isolate and determine the key aroma compounds in oat flakes. In the first study by Schuh and Schieberle,27 50 g
then desorbing and cryofocusing the volatile concentrate directly onto a GC capillary column for GC−MS analysis. An alternative desorption approach is solvent elution of volatiles from the Tenax trap, concentration, and injection onto the GC column. Initial dynamic headspace studies applied to oats used 300 g samples of oat groats with 300 mL of water at 30 °C, swept for 1 h with 170 mL/min of helium (10.2 L of headspace trapped).1,2 Subsequent studies on extruded oat flour used dynamic headspace as the isolation technique, with sample sizes ranging from 2.5 to 10 g.19,20 The static headspace sampling technique equilibrates the sample in a sealed glass vial, and the headspace sample is withdrawn by a gas syringe via a septum. Oat sample sizes are typically 0.5−3 g; however, limited parts per million (ppm) to mid-parts per billion (ppb) volatile sensitivity is achieved. Static headspace has been applied to recover volatiles from extruded flaked oatmeal,36 lipid oxidation products from oat groats,18 and volatiles in native and crushed oat groats.26 Supercritical carbon dioxide extraction was used to isolate aroma compounds in cooked oatmeal porridge and oat flakes.21 Supercritical carbon dioxide offers mild thermal conditions at 50 °C and 200 atm and reasonable extraction times. Cooked oatmeal was combined with Hydromatrix (diatomaceous earth) in a 1:1 ratio to create a free-flowing material for 120 min of extraction. Freshly ground oat flakes were combined with water and Hydromatrix in a 3:1:1 ratio of a free-flowing material for a 100 min extraction. Volatiles were trapped in 20 mL of dichloromethane containing 1% methanol and concentrated to 0.1 mL prior to GC−MS analysis. Solid-phase microextraction (SPME) is a routinely used technique for volatile isolation with complementary sensitivity to dynamic headspace.37 Headspace (HS)-SPME uses a 1 cm adsorbent-coated fused silica fiber, which is suspended in the headspace above oat samples. Its rapid, low-cost, solvent-free aspects have provided advantages, however, with limited application to oat cereals, presumably as a result of the C
DOI: 10.1021/acs.jafc.9b00994 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Review
Journal of Agricultural and Food Chemistry Table 3. Sensory Assessments of Oat Flavor Isolatesa separated vacuum steam isolate source groats
oatmeal (oat flakes)
cooked oatmeal
initial flavor
vacuum steam distillate flavor
raw oat weedy-hay grassy mild oat hay-weedy browned oat-nutty browned-burnt weak weedy
raw grain hay-feedy grassy mild oat raw-green brown-nutty oaty, slightly burnt nutty-browned weedy-grain
acidic/neutral fraction
basic fraction
green-weedy mashy grain
weak/none
green-weedy mashy grain
brown-nutty pleasant
mashy grain malty caramelized slightly oat
nutty-pecan raw potato harsh-chalky
a
This table was reproduced with permission from ref 2. Copyright 1986 AACC International.
Figure 2. Comparison of volatile profiles obtained by headspace trapping and vacuum distillation of dry and hydrated oat groats, with SE-30 capillary gas chromatography−flame ionization detector responses of oat groat total volatile isolates as a function of Kovats retention indices. Peak identifications correspond to compounds listed in Table 4. This figure was adapted and reproduced with permission from ref 1.
of dry oat flakes were powdered in liquid nitrogen and extracted 2 h with 300 mL of diethyl ether. A 100 mL ether concentrate was distilled at 40 °C by the SAFE technique. In a second investigation by Klensporf and Jelen,28 20 g of oat flakes was ground and mixed with 150 mL of water and distilled by SAFE at 40 °C, with subsequent extraction of the condensate with 50 mL of 1:1 pentane/diethyl ether. Chemical Class Separation Techniques. Following isolation of a volatile concentrate, the extract can be fractionated by acid−base separation into basic and acidic/ neutral fractions. This was accomplished by acidification of the oat volatile aqueous distillate with 0.1 M hydrochloric acid, extraction with methylene chloride (acidic/neutral fraction), treatment of the aqueous layer with 0.1 M sodium carbonate, and subsequent extraction with dichloromethane (basic fraction).2 Identification: High-Resolution GC−MS. Volatiles recovered from oat flavor isolates were analyzed by GC−MS and GC−O. For the majority of oat volatile studies in this review, the capillary GC column phase selected was nonpolar DB-5 (5% diphenyl-dimethylpolysiloxane), followed by SE-30 (dimethylpolysiloxane) and SE-54 (5% phenyl-1% vinyldimethylpolysiloxane). In three instances, DB-1701 (14%
cyanopropylphenyl-dimethylpolysiloxane), DB-WAX (polyethylene glycol), and FFAP (nitroterephthalic acid-modified polyethylene glycol) phases were selected to achieve separations by polarity. Sensory-Directed Aroma Analytical Techniques. GC− O was used in the majority of oat volatile studies to assign specific odors to peaks and to focus the identifications on those peaks with sensory relevance. AEDA in combination with highresolution capillary gas chromatography was used to assign relative intensities and flavor dilution (FD) factors after serial dilutions and re-analysis of volatile extracts.40
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AROMA VOLATILES IDENTIFIED IN OATS AND OAT CEREALS The earliest studies by Hrdlic̆ka and Janic̆ek14,15 on toasted oat flakes identified 15 C2−C8 aldehydes and ketones and unusual amines using paper chromatography and chemical derivatization techniques. However, the authors acknowledged that these compounds could not be considered as the sole source of toasted oat flavor. Heydanek and McGorrin1,2,16 reported the first systematic investigations of oat flavor volatiles to attempt to understand the basis of oat flavor chemistry. This review summarizes the current understandings of oat aroma D
DOI: 10.1021/acs.jafc.9b00994 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Review
Journal of Agricultural and Food Chemistry Table 4. Principal Volatiles in Oat Groatsa compound
RI (SE-30)
pentanal 3-methyl-1-butanol 1-pentanol hexanal 2-methylpyrazine 1-hexanol 2-heptanone heptanal 2,5-dimethylpyrazine benzaldehyde 1-octen-3-ol 2-octanone 2-pentylfuran 3-octen-2-one limonene (Z,E)-3,5-octadien-2-one 3-ethyl-2,5-dimethylpyrazine 2-ethyl-3,5-dimethylpyrazine (E,E)-3,5-octadien-2-one nonanal 2-decanone (E,E)-2,4-decadienal γ-nonalactone
669 743 754 774 798 858 867 878 883 931 971 972 978 1018 1021 1046 1055 1062 1069 1084 1174 1293 1324
peak IDb
A B
fresh groats
× ×
C × × × D
E F
× × × × ×
G H
× ×
×
hydrated groats
rancid groats
odor description
× × × × × × × × × × ×
× × × ×
× × × × × × × × × × ×
× ×
green malty fusel alcohol green, cut grass nutty, brown, musty cut grass blue cheese fatty nutty, roasted, baked potato almond mushroom fatty, green, blue cheese fruity/earthy, rum earthy, herbal, sweet hay citrus, orange grassy, straw-like earthy, roasty earthy, roasty grassy, straw-like fatty, citrus orange, floral, fatty old chicken fat coconut, creamy
×
× × ×
a
Data were adapted from ref 1. bLetter code correlates with peaks labeled in Figure 2.
hydrated with 300 mL of water) purged for 1 h at 30 °C. Figure 2 compares the volatile profiles obtained by headspace trapping and vacuum distillation of dry and hydrated oat groats, and Table 4 lists the components identified by GC−MS analysis of these volatiles. Most notable in the hydrated sample is an observed increase in C5 and C6 alcohols and aldehydes. When the volatile isolation was conducted in the presence of water, it was concluded that lipoxygenase and aldehyde oxidoreductase activity was still present in dried oat groats, and additional volatiles were generated as a result of this activity. The presence of latent enzyme activity at high moisture levels is important to understand because of the potential for flavor or off-flavor development during the volatile isolation steps. The detection of small quantities of 2methyl-, 2,5-dimethyl-, and C4-substituted pyrazines in oat groats suggests that a small amount of browning occurs during kiln drying in commercial oat processing. Rancid Oat Groats. Flavor deterioration of oats after hightemperature processing and development of oxidative rancidity was reported in two studies by Heydanek and McGorrin.2,16 Heated oat groats were prepared by boiling for 30 min in distilled water, followed by freeze drying. When analyzed at 3 weeks of storage, the oats had a pronounced “old oil, rancid, old chicken fat” aroma. Vacuum steam distillation and GC− MS volatile analysis identified a total of 45 components, including 24 aldehydes, ketones, and alcohols. Table 4 lists the major aroma volatiles identified in rancid oat groats compared to unoxidized oat groats. The indicated volatiles were present in both samples but at significantly higher levels in oxidized oat groats. Hexanal at an estimated 10−15 μg g−1 concentration was the most abundant volatile in rancid oat groats, in addition to pentanal, 1-pentanol, and 3,5-octadien-2-one.16 Hexanal was present well above its odor threshold (4.5 ng g−1)41 and is expected to provide a major impact. However, the sensory characterization of the rancid odor as “old chicken fat”
compounds for each type of oat product, beginning with this initial work and subsequent studies by others. Oat Groats. Heydanek and McGorrin1,2 initially reported 111 volatiles in oat groats, the precursors of commercial oat food products. Low concentrations of C10 terpenes, alkyl benzenes, alcohols, and ketones were identified at
