Influence of Physical Grain Characteristics on Optimal Rotor Speed During Impact Dehulling of Oats

Central to commercial oat (Avena sativa L.) processing is impact dehulling. During impact dehulling, oats are fed into a spinning rotor that expels the grains against an impact ring. The impact frees the groat from the hulls. To optimize dehulling protocols, we examined the effects of physical grain characteristics and rotor speed on oat dehulling using an impact dehuller. We separated grain of three cultivars (Gem, CDC Dancer, Ronald) according to size by sieve fractionation (separation by width), disk fractionation (separation by length), and by gravity table (separation by density). Grains were characterized by mass, digital image analysis, and bulk density. Samples (50 g) were adjusted to 9% moisture and dehulled at four different rotor speeds. Groat percentage, dehulling efficiency, and groat breakage were measured after dehulling. In general, oats with higher bulk density dehulled more efficiently at slower rotor speeds, regardless of grain mass. Groat breakage increased with rotor speed and with grain mass. Adjusting dehulling conditions according to grain size improved groat yields over optimal dehulling conditions for unfractionated grains for some cultivars. More refined fractionation of grain according to bulk density may provide further improvement of groat yield during impact dehulling.     

Genotypic and Environmental Effects on Oat Milling Characteristics and Groat Hardness

The production of oat bran involves the dehulling of oats, inactivation of their enzymes, and the subsequent grinding and sieving of the clean groats to isolate the larger bran particles. The bran yield from the oat groats may be related to their hardness, as it is in wheat. Groat breakage, which occurs during the dehulling process, reduces milling yield and may also be related to groat hardness. This study sought to investigate genotypic and environmental effects on oat dry milling and oat dehulling characteristics, and attempted to define properties associated with oat groat hardness. Significant genotypic differences in bran yield were largely attributed to groat composition, where higher β‐glucan and oil concentrations in the groat were associated with higher bran yields. Bran composition was largely dependent on a combination of the bran yield and the groat composition. Although groat breakage was correlated with bran yield and with groat β‐glucan concentration, environmental factors appeared to be more influential. Locations that had suffered severe crown rust infestations exhibited higher rates of groat breakage during dehulling. Bran yield was not as strongly affected at the locations infested with crown rust, indicating that bran yield and groat breakage are manifestations of different types of groat hardness and are only partially related.     

Optimizing Conditions for Experimental Oat Dehulling

The determination of groat percentage in experimental oat breeding lines requires the dehulling of oats. Here, we report the results of our efforts to optimize dehulling conditions so that the most accurate and reliable result can be obtained. Hand dehulling was always reliable and accurate, however, it was the most time‐consuming and tedious of the methods studied. Two mechanical methods of oat dehulling, compressed‐air dehulling and impact dehulling, also frequently provided reliable results, however, results were strongly influenced by dehulling conditions. Optimal dehulling conditions represented compromises between unfavorable extremes. Correct aspiration strength was critical to accurate groat percentage determination. We have found that a secondary aspiration is highly desirable after compressed‐air dehulling to remove hulls remaining with the groats after dehulling. Also, increased mechanical stress on oats as exerted either by the number of passes through the impact dehuller, or by the air pressure in the compressed‐air dehuller, resulted in higher dehulling efficiency, but increased groat breakage as well. Dehulling efficiency decreased as moisture increased from 7.5 to 15%, but increased as moisture was further increased to 30%. In contrast, groat breakage with impact dehulling decreased as moisture increased from 7.5 to 30%. A new equation for groat percentage calculation has been introduced where the mass of hulled oats remaining after dehulling is subtracted from the mass of the original oat sample, so that poor dehulling efficiency does not influence the groat percentage.     

Oat Grain/Groat Size Ratios: A Physical Basis for Test Weight

Market value of oat grain is largely determined by test weight or bulk density, yet little is known of the physical basis for test weight in oats. We have hypothesized that a larger sized groat relative to the oat grain (the kernel with the hull) would generate higher test weight oats because the groat is the densest structure in the oat grain. We tested this by measuring oat grain size and oat groat size by digital image analysis for 10 genotypes grown in 10 environments. We also measured other physical characteristics of the oats grains and groats including mean grain and groat mass, test weight, and groat percentage. We found that the groat/ grain size ratio was highly correlated with test weight. Because the oat grain image area was nearly twice that of the groat, we suggest that there are significant amounts of empty space within the oat hull, which detracts from test weight. We also found that oat groat size distributions, like oat grains, fit bimodal distributions better than normal distributions.     

Identification of Sprout Damage in Oats

Sprout damage of oats can occur when mature grain is left in the field after maturity and rainfall induces the germination process in the seeds. Although characteristics of sprout damage have been described thoroughly in many grains including wheat, rye, triticale, and barley, such characteristics have not been identified in oats. During a field study of oat quality, one site exhibited much higher rates of groat breakage during dehulling than other sites. Determination of falling number values on groat flour and a visual inspection of the grain suggested that the higher rates of groat breakage were associated with sprout damage. This hypothesis was confirmed with artificially sprouted oats, where sprouted grain exhibited lower falling numbers and lower stirring number values, suggesting increased α‐amylase activity in the sprouted oats. Sprouted oats exhibited much more breakage during dehulling, decreased β‐glucan concentration, and decreased β‐glucan integrity, as evaluated from the viscosity of steamed groat flour slurries. We suggest that the increased breakage was a result of the weakening of cell walls, as evidenced by the loss of β‐glucan concentration and integrity.     

Theoretical and Empirical Relationships Between Oat Test Weight and Groat Proportion

Test weight and groat proportion are two very important quality characteristics of oat grain. In this study, we pose the hypothesis that these two characteristics are related through characteristics of grain density. Test weight is defined as the product of kernel density and packing proportion. Groat proportion, in theory, is the ratio of the groat mass to the kernel mass. We present two theoretical constructions expressing test weight in terms of groat proportion, packing proportion and kernel density components. To test these, we have applied measurements of test weight, groat proportion, kernel density components, and packing proportion of 18 oat cultivars grown at six environments. Whereas the groat proportion alone accounted for only 34% of the variation in test weight, our theoretical constructions that included groat proportion could account for ≤82% of variation in test weight. Also, we present previously undescribed variation in oat kernel density components across genotypes and environments. Although the kernel density alone could account for most of the variation in test weight across genotypes, packing proportion appeared to be more important in describing variation in test weight of a genotype across different environments. We observed significant variation in both groat and hull density which, together with groat proportion, described most of the variation in kernel density.     

Oat Grain Density Measurement by Sand Displacement and Analysis of Physical Components of Test Weight

Test weight or bulk density of oats (Avena sativa L.) has a major influence on the monetary value of oat grain. We hypothesize that test weight can be attributed to grain density and packing efficiency. We have measured oat grain volume and density by a sand‐displacement method and derived the packing factor for six oat cultivars grown in three environments. Volumes of individual grains were 31–38 mm³ and were highly correlated with grain mass. Grain densities were 0.96–1.03 g/cm³. Packing efficiency, defined as the space proportion occupied by the grains, was 53–55%. Regression analysis suggested that 78% of the variation in test weight could be attributed to grain density. Size fractionation of grain by sieving and analysis by digital image analysis indicated that smaller grains within an oat sample packed more efficiently than larger grains and larger grains in a sample were less dense than the smaller grains. Analysis of oat grain components indicated groat densities were ≈1.29 g/cm³ and hull densities were ≈0.69 g/cm³. The difference in densities between groat and hull provide a physical basis for the recognized relationship between groat percentage and test weight.     

Composition of Oat Bran and Flour Prepared by Three Different Mechanisms of Dry Milling

Three mechanisms of oat milling were tested for laboratory-scale oat bran production. Oat bran consistent with AACC definition and commercially obtained product was generated with either roller-milling or impact-milling of groats, followed by sieving to retain larger particles. These bran preparations were enriched ≈1.7-fold in β-glucan and ash, 1.4-fold in protein, and 1.1-fold in lipid. Bran finishing made further enrichments in protein, β-glucan, and ash. Tempering oat (to 12% moisture for 20 min) improved bran yield from roller-milling nearly two-fold but had little effect on bran composition. Bran yield from the impact-type mill was significantly affected by grinding screen size. Oat bran obtained from a pearling mill was only slightly enriched in β-glucan and protein, but it was more heavily enriched in ash and oil than brans from roller or impact mills. The pearling mill isolated the outer layers of the groat directly, but because of its low β-glucan composition it did not meet the AACC definition of oat bran, indicating a relatively uniform distribution of β-glucan in the groat.     

Factors Affecting Viscosity of Slurries of Oat Groat Flours

Oat grain is routinely kilned and steamed before milling to develop flavor and to inactivate lipid-degrading enzymes. Heat treatments can significantly affect viscous properties, which have functional and nutritional importance. Oat flour slurries (23%, w/w, solids dry basis) made from steamed (for 20 min) or autoclaved (at 121°C, 15 psi, for 10 min) grain developed high viscosities, whereas flour slurries made from raw or kilned (105°C for 90 min) oats did not. Flour slurries made from raw groats, surface-sterilized by 1% hypochlorite, were more viscous than untreated raw groat flour slurries, suggesting that β-glucan hydrolases on the surface of the groat caused the viscosity losses observed in raw or kilned groats. However, because viscosities developed by surface-sterilized groats were not as great as in steamed oat-flour slurries and because some roasting treatments also inactivated enzymes without enhancing viscosity, it appears steaming might also affect the β-glucan polymer, resulting in its greater hydration in solution. Smaller particle size and higher incubation temperature also resulted in increased flour slurry viscosity, presumably because of increased hydration of the β-glucan. Rmoval of lipids from steamed oat flour significantly increased the oat flour slurry viscosity, apparently by increasing the β-glucan concentration in the flour.     

Comparisons of β-Glucan Content of Barley and Oat

The cholesterol-lowering effect of cereal grains has been associated with the soluble fiber component of dietary fiber. β-Glucan is the major soluble fiber component of barley (Hordeum vulgare L.) and oat (Avena sativa L.). Much research has been conducted to determine the β-glucan content of barley and oat genotypes from many different countries. However, genotypes of both crops always were grown in separate experiments, making direct comparisons between the two crops difficult. This study compares in the same experiment the β-glucan content of nine barley and 10 oat genotypes grown at two locations in each of two years (i.e., four environments) in North Dakota. Averaged across genotypes, total β-glucan content of barley and oat groat was similar. Soluble β-glucan content of oat groat was greater than barley, and oat groat had a greater ratio of soluble-to-total β-glucan than barley. The soluble β-glucan content and ratio of soluble to total β-glucan content of the “best” barley genotypes were less than that of oat genotypes with the highest levels of these two traits.     

Mechanical Behavior of Oats: The Groat Effect

To evaluate the mechanical behavior of groats with constant mass and moisture content, individual groats were compressed along the major axis between parallel flat plates. Compression versus tension testing was adapted easily to groats, which are weaker along the major axis. Regression of maximum apparent stress on moisture content suggested that the proportionality constant was a function of an undefined biological difference (UBD) among tested samples and the y‐intercept depended on groat mass and UBD. No brittle‐ductile transition in the range of 9–30% moisture was observed regardless of genotype. Groats in general tended to fail by plastic compression at low moisture content (9–10%). Groats at high moisture content (>12%) generally buckled and burst near the midpoint of the crease, suggesting higher stress around the crease regardless of genotype. Apparent maximum stress should be considered as an average for a genotype. Knowledge of specific groat characteristics (including but likely not limited to mass and moisture) and how they influence apparent mechanical behavior should lead to improvements in oat milling.     

Discriminating Oat and Groat Kernels from Other Grains Using Near‐Infrared Spectroscopy

Oats and groats can be discriminated from other grains such as barley, wheat, rye, and triticale (nonoats) with near‐infrared spectroscopy. The two instruments tested herein were the manual version of the United States Department of Agriculture–Agricultural Research Service single‐kernel near‐infrared (SKNIR) instrument and the automated QualySense QSorter Explorer high‐speed sorter, both used in similar near‐infrared spectral ranges. Three linear discriminate self‐prediction models were developed: 1) oats versus groats + nonoats, 2) oats + groats versus nonoats, and 3) groats versus nonoats. For all three models, the SKNIR instrument showed high correct classification of oats or groats (94.5–100%), which was similar to results of the QSorter Explorer at 95.0–99.4%. The amount of nonoats that were misclassified as oats or groats was low for both instruments at 0–0.2% for the SKNIR instrument and 0.8–3.7% for the QSorter Explorer. Linear discriminate models from independent prediction and validation sets yielded classification accuracies of 91.6–99.3% (SKNIR) and 90.5–97.8% (QSorter Explorer). Small differences in classification accuracy were attributed to processing speeds between the two instruments: 3 kernels/s for the SKNIR instrument and 35 kernels/s for the QSorter Explorer. This indicated that both instruments are useful for quantifying grain sample compositions of oat and groat samples and that both could be useful tools for meeting consumer demand for gluten‐free or low‐gluten products. Discrimination between grains will help producers and manufacturers meet various regulatory requirements. Examples include requirements such as those from the U.S. Food and Drug Administration and the Commission of European Communities, in which gluten‐free oats or other products can only be labeled as nongluten if they contain gluten at less than 20 ppm, the established safe consumption limit for people with celiac disease. The QSorter Explorer is currently being used to meet these requirements.     

Improved bioavailability of dietary phenolic acids in whole grain barley and oat groat following fermentation with probiotic Lactobacillus acidophilus , Lactobacillus johnsonii , and Lactobacillus reuteri

The aim of this study was to improve the bioavailability of the dietary phenolic acids in flours from whole grain barley and oat groat following fermentation with lactic acid bacteria (LAB) exhibiting high feruloyl esterase activity (FAE). The highest increase of free phenolic acids was observed after fermentation with three probiotic strains, Lactobacillus johnsonii LA1, Lactobacillus reuteri SD2112, and Lactobacillus acidophilus LA-5, with maximum increases from 2.55 to 69.91 μg g(-1) DM and from 4.13 to 109.42 μg g(-1) DM in whole grain barley and oat groat, respectively. Interestingly, higher amounts of bound phenolic acids were detected after both water treatment and LAB fermentation in whole grain barley, indicating higher bioaccessibility, whereas some decrease was detected in oat groat. To conclude, cereal fermentation with specific probiotic strains can lead to significant increase of free phenolic acids, thereby improving their bioavailability.     

NUTRIENT DENSITIES,CARBON: NITROGEN RATIOS,AND MIDDAY DIFFERENTIAL CANOPY TEMPERATURE IMPACT GRAIN YIELD OF STRESSED OAT

Nutrient densities, carbon:nitrogen (C:N) ratio, and midday differential canopy temperature (dT), were assessed in oat plants subjected to biotic stresses during two years. Large portions of variation in nutrient densities and C:N ratio of leaves at the boot stage and of kernels and groats at harvest were negatively impacted by the 2- and 3-way interactions of leaves, kernels, and groats with the biotic stress treatments and years. The C:N ratios, but not nutrient densities, were always smaller in groats than in kernels, and during the stress than the no-stress year. Temporal variation accounted for a small variance associated with nutrients in leaves; whereas, stress treatments accounted for the largest variances associated with nutrients in kernels and groats. These indirect relationships among plant architecture components, dT, nutrient densities and C:N ratios, illustrate the complex interactions of biotic and abiotic stresses and their impact on grain yield and its components in oat.     

Optimization of a Laboratory Dehulling Process for Lentil (Lens culinaris)

Red lentil (Lens culinaris) is mainly processed into dehulled and split forms before human consumption and characteristics such as dehulling efficiency (DE), which is the sum of percent dehulled whole seed (PDW) and percent dehulled split seed (PDS), are important to lentil breeders, processors, and exporters. A laboratory Satake dehuller was used to evaluate the dehulling characteristics of red lentil. The effects of dehulling conditions (abrasive wheel speed, dehulling time, and seed moisture content) were investigated using response surface methodology. Increasing dehulling time and seed moisture content decreased DE. Increasing seed moisture content decreased powder and broken fractions but increased the undehulled whole seed fraction. PDW was decreased but PDS was increased as dehulling time was increased. Percent hull removed during dehulling process decreased as seed moisture content was increased but increased as abrasive wheel speed or dehulling time was increased. The optimum dehulling conditions for the laboratory dehuller, based on maximizing DE and percent hull removed while minimizing powder fraction (loss), were established. Good agreement was found between experimental values for dehulling characteristics (DE, PDW, PDS, hull removed, and powder produced) obtained at optimum dehulling conditions and predicted values for those characteristics obtained using the models developed.     

Relationships of Selected Physical,Chemical, and Sensory Parameters in Oat Grain,Rolled Oats,and Cooked Oatmeal—A Three‐Year Study with Eight Cultivars

Eight Scandinavian oat cultivars were studied as grains, groats before and after kiln drying, rolled oats, and oatmeal for 62 physical, chemical, and sensory parameters over three consecutive crop years. The objectives were to study cultivar and year differences, and the relationships between parameters to find out the cultivars most suitable for rolled oats production and to understand how grain parameters are reflected on groat, rolled oat, and oatmeal characteristics. The cultivars and crop years differed (P < 0.05) in most of the parameters. In addition, many groat, rolled oats, and oatmeal variables showed a significant year by crop interaction. Several grain variables were significantly interrelated but only two, moisture and amount of dark and damaged grains, had a strong correlation (r > 0.70) to rolled oat parameters, the former correlating negatively to crude fat and the latter positively with maximum viscosity value. However, some weaker (r < 0.70) negative associations were found such as those between kernel size value and β‐glucan content and water binding capacity of rolled oats. In general, the variables analyzed at different processing stages correlated strongly, indicating good retention of groat properties during processing. Various associations were found between the physicochemical parameters of rolled oats and sensory properties of oatmeal. In principal component analysis, factor 1 mostly represented rolled oat parameters such as crude fat, color values L and a, and oatmeal parameters such as toasted aromatics, coarseness, and size of swollen particles. Factor 2 represented properties that pertain to structure and water and rolled oats interactions. The cultivars were also grouped according to these properties. The results suggest that the rolled oat process deserves optimization on a cultivar basis, particularly when specific product properties such as good water binding capacity are required.     

Effect of Kiln Drying on Falling Number of Oats

The suitability of the falling number method for oats was studied using samples of oat groats, oat bran, oat endosperm flour, and rolled oats. Sample sizes of 4.5–8 g were tested. The results showed that a standard 7‐g sample falling number determination would be suitable for oats, the falling number of samples varying from 328 to 721 sec. Due to high gelatinization temperature of oat starch, falling number values <300 sec showed greater variation than those >300 sec. Oat groat samples from two separate kiln drying processes showed that kiln drying increased the falling number values by 30–89%. The falling number determination is a potential tool for estimating the adequacy of kiln drying of oats. However, further modifications of the method should be developed to better consider the pasting properties of oat starch.     

Association of Avenanthramide Concentration in Oat (Avena sativa L.) Grain with Crown Rust Incidence and Genetic Resistance

Avenanthramides (avn) are antioxidant compounds found in oat tissues, including the grain, that are of interest from a nutritional standpoint. In this study, we have measured avenanthramide concentration in the grain of 18 oat genotypes grown in six environments. These genotypes varied widely in crown rust (Puccinia coronata) resistance. Crown rust infected two of the six environments studied. The grain avenanthramide concentrations in the crown rust environments were significantly higher than those in the uninfected environments. Avenanthramide concentrations in the crown rust infected environments was also significantly correlated with their genetic crown rust resistance, as evaluated in the field. These results suggest that avenanthramide accumulation in the grain is associated with crown rust infection and that, in most of the cultivars evaluated, the extent of their accumulation also correlated with their genetic disease resistance; avenanthramide accumulation in the grain was, with noted exceptions, highest in those cultivars showing the greatest genetic resistant to crown rust.     

Accumulation of Oxygenated Fatty Acids in Oat Lipids During Storage

Oxygenated fatty acids were identified in oat grain by gas chromatography‐mass spectrometry. We hypothesized that most of these were the results of lipoxygenase activity. This hypothesis was tested by measuring concentrations of these compounds after hydrothermal treatments and storage of oat groats or oat flour for 22 weeks at 37°C and 65% relative humidity. Steam treatments inactivated lipases, whereas roasting at 106°C did not. Free fatty acids accumulated quickly in untreated or roasted flour, but not in steamed flour or groats. A total of six hydroxy and epoxy fatty acids were identified. Oxidized fatty acids were found in both esterified lipids and free fatty acids, indicating that lipase action was not necessary for lipid oxidation. More oxidation products were found in flour than in groats, and less were found in the steamed treatments. Lipoperoxygenase appeared to be involved in the formation of oxidation products, although nonenzymatic mechanisms may also operate. Hydroxy‐fatty acids are associated with strongly bitter flavors and are undesirable. Results indicate the importance of enzyme inactivation before storage of processed oat products.     

Effects of Processing,Cultivar, and Environment on the Physicochemical Properties of Oat β‐Glucan

Several food regulatory agencies around the world have approved health claims for oat‐derived β‐glucan for cholesterol lowering and glycemic control. The biological efficacy of β‐glucan appears to depend both on daily intake and on physicochemical properties, such as molecular weight and viscosity. The objective of this study was to determine the effects of oat processing, genotype, and growing location on the physicochemical properties of β‐glucan. Five oat genotypes (HiFi, Leggett, CDC Dancer, Marion, and CDC Morrison) grown in two locations (Saskatoon and Kernen) were dehulled (untreated) and processed in a pilot facility through kilning (kilned, not flaked) and subsequent steaming and flaking (kilned, flaked). Untreated groats gave a relatively low Rapid Visco Analyzer (RVA) apparent viscosity (164 cP) and a low extractable β‐glucan molecular weight (332,440) but exhibited high β‐glucan solubility (90.49%). Compared with untreated groats, the kilned (not flaked) samples had significantly increased RVA apparent viscosity (314 cP) and extractable β‐glucan molecular weight (604,710). Additional processing into kilned and flaked products further increased RVA apparent viscosity (931 cP) and β‐glucan molecular weight (1,221,760), but β‐glucan solubility (63.83%) was significantly reduced. Genotype and growing environment also significantly affected β‐glucan viscosity and molecular weight, but no significant interaction effects between processing, genotype, and environment were found. Results indicate that there is potential for processors to improve the physicochemical and nutritional properties of oat end products through processing of specific oat genotypes from selected growing locations.