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CROP QUALITY & UTILIZATION

Factors Affecting Groat Percentage in Oat

^a USDA/ARS Wheat Quality Lab., Harris Hall, North Dakota State Univ., Fargo, ND 58105 USA
^b Dep. of Plant Sciences, North Dakota State Univ., Fargo, ND 58105 USA

doehlert{at}plains.nodak.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
The groat percentage represents an important quality characteristic of oat (Avena sativa L.). Our objectives were to characterize mechanical factors of the oat dehulling process and the physical characteristics of the oat grain that affect groat percentage. Mechanical factors were determined with a compressed air dehuller. Physical characteristics of oat grain were evaluated from 10 genotypes grown at three locations with digital image analysis. Groat percentage as determined by hand dehulling was compared with mechanical dehulling. The strength and duration of mechanical stress required to separate the hull from the groat and the strength of the aspiration required to remove free hulls from the groats had significant effects on groat percentage results. Insufficient mechanical stress resulted in ineffective dehulling, but excessive stress resulted in groat breakage. Excessive aspiration removed groats as well as hulls, but insufficient aspiration left excessive hulls with groats. Groat percentage values obtained by hand dehulling or by mechanical means correlated well. Hand sorting of mechanically dehulled groats to remove hulls remaining after dehulling improved their correlation, indicating the importance of hand-sorting mechanically dehulled oats. Test weight and oat size uniformity were highly correlated with groat percentage. Negative correlation between hulls remaining after dehulling with groat percentage suggested that heavier hulls, associated with lower groat percentage, were more difficult to remove by aspiration. Positive correlations between groat breakage during dehulling and groat percentage suggest that thin hulls provide less protection to the groat during dehulling, resulting in higher levels of breakage.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
THE OAT GROAT OR CARYOPSIS

is typically covered with a lemma and palea, or hull after harvest. This hull must be removed before the groat can be processed for food purposes. The proportion of the groat (the oat without the hull) to the whole oat is known as the groat percentage. The groat percentage is important because it represents the economic yield an oat miller can derive from a given lot of oat grain. It also provides an estimate of the digestible portion of the grain, if being fed to animals.

Groat percentage, also referred to as groat proportion, caryopsis percentage, or reported as hull percentage, has long been recognized as an important indicator of oat quality (Stoa et al., 1936; Love et al., 1925; Atkins, 1943; Bartley and Weiss, 1951). Peek and Poehlman (1949) considered test weight to be a more valuable oat quality evaluation tool because hand dehulling of oats was considered too tedious. Stoa et al. (1936) suggested that early maturing oats were superior in groat percentage, and rust susceptible lines were generally high in percent hull. These conclusions were also supported by the findings of Bunch and Forsberg (1989). The studies of Bartley and Weiss (1951) indicated strong environmental effects on groat percentage and demonstrated correlations between groat percentage and yield, test weight, and kernel weight. Youngs and Shands (1974) demonstrated that tertiary kernels had a higher groat percentage than primary and secondary kernels. However, Palagyi (1983) found that genotypes with higher levels of tertiary kernels had lower groat percentage, and suggested that tertiary kernels compete with primary and secondary kernels for assimilate, preventing them from filling properly. Groat percentage is a quantitatively inherited trait with a broad sense heritability of 36 to 92% (Wesenberg and Shands, 1973; Ronald et al., 1999). Stuthman and Granger (1977) found a narrow sense heritability of 34 to 72%.

Any of three methods are commonly used to evaluate groat percentage on an experimental basis. The most basic of these is hand dehulling, where oat grains are stripped of their hulls by hand. Two mechanical methods of oat dehulling include the impact dehuller (Ganssmann and Vorwerck, 1995), and the compressed air dehuller (Kittlitz and Vetterer, 1972). The impact dehuller feeds grain into a spinning rotor. The impact of the grain with wall as it is expelled from the rotor knocks the hull from the groat. The hulls are then removed by aspiration. The compressed air dehuller uses a stream of pressurized air to apply mechanical shock to a sample of oat grain, which knocks the hull off the groat. Hulls are removed by aspiration. Compressed air dehullers have only recently been marketed commercially and are just starting to become widely used among oat research laboratories. Hand dehulling is thought to provide a ceiling value for groat percentage, because no groats are lost in mechanical treatments. However, mechanical dehullers allow for the dehulling of larger samples, which is impractical by hand dehulling. Impact dehullers are commonly used by commercial millers to dehull oat, although an additional method, known as stone hulling, may also be used by some commercial milling operations (Ganssmann and Vorwerck, 1995).

Factors influencing groat percentage are not well understood. This study was initiated to understand better the mechanical and biological factors affecting groat percentage. Various mechanical manipulations were made with a compressed air dehuller to determine how these physical treatments affected groat percentage and related dehulling characteristics. Ten oat cultivars grown at three locations were dehulled by the three methods of laboratory dehulling. Digital image analysis was used to characterize the architecture of the oat kernels to determine relationships between the physical structure of the oat grain and groat percentage.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
Plant Material
The oat cultivar Dumont was used to characterize the compressed air dehulling system. The grain was grown at Fargo, ND, in the 1996 growing season. Ten oat cultivars (AC Assiniboia, Bay, Belle, Brawn, Gem, Hytest, Jerry, Jud, AC Medallion, and Youngs) were used to compare different dehulling methods. These were grown at three locations in southeastern North Dakota (Edgeley, Fargo, and Fullerton). The soil type at Fargo is Fargo silty clay (fine montmorillonitic Typic Endoquert). Soils at Edgeley are Barnes (fine-loamy, mixed Udic Haploboroll) and Cresbard (fine, Montmorillonitic Glossic Udic Natriboroll) loam complex. Soils at Fullerton are Svea (fine-loamy, mixed Pachic Udic Haplobroll) and Cresbard loam. The previous crop at Fargo was soybean [Glycine max (L.) Merr.], at Edgeley , summer fallow, and at Fullerton, corn (Zea mays L.). The experiments were planted at Edgeley and Fullerton on 13 May 1996 and were harvested at Fullerton on 14 Aug. 1996 and at Edgeley on 21 Aug. 1996. The Fargo experiment was planted 29 April 1996 and harvested 7 Aug. 1996. A seeding rate of 2.47 x 10⁶ ha^-1 was used for all experiments. Herbicide treatments consisted of pre-emergence application of 3.93 kg ha^-1 propchlor (2-chloro-N-isopropylacetanilide) and post-emergence application at the 3-leaf stage with a tank mix of 0.14 kg ha^-1 thifensulfuron {methyl 3 [[[[(4-methoxy-6-methyl-1,3,5-triazin-2yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylate, 0.07 kg ha^-1 tribenuron {methyl 2-[[[[N-(methoxy-6-methyl-1,3,5-triazin-2yl) methylamino] carbonyl] amino] sulfonyl] benzoate} and 0.14 kg ha^-1 clopyralid (3,6-dichloro-2-pyridinecarboxylic acid, monoethonolamine salt). Experimental units consisted of four rows spaced 0.3 m apart and 2.4 m long. The two center rows were harvested with a two-row binder and threshed with a plot thresher. Seed was cleaned with an air screen cleaner to remove chaff. Test weight of grain was measured from the mass of a standard volume, multiplied by a constant.

Dehulling
Two-gram samples were dehulled by hand. The mass of samples were measured before and after dehulling. The ratio of the groat mass to the oat mass times 100 was the groat percentage.

Prior to dehulling by mechanical means, 50-g samples were incubated in dessicators over saturated potassium carbonate solutions (Wolf et al., 1984) to equilibrate grain moisture to a uniform level of about 95 g kg^-1 water. Previous results (Kittlitz and Vetterer, 1972) indicated that grain moisture strongly affected oat-dehulling characteristics.

For the determination of the effects of mechanical conditions during dehulling on groat percentage and dehulling characteristics, 40-g samples of Dumont oat were dehulled with the compressed-air dehuller. All experiments on dehulling conditions were repeated using the cultivar Whitestone, but because results were similar, only Dumont results were presented. The compressed-air dehuller was the Codema¹ Laboratory Oat Huller (Eden Prairie, MN). Compressed air pressure was 0.414, 0.483, 0.522, 0.621, and 0.69 MPa. The dehulling times were 30, 45, 60, 75, and 90 s. The vent opening (which controlled aspiration strength) was 0, 2, 4, 6, 8, and 12 mm. Larger vent openings resulted in weaker aspiration.

For the comparison of cultivars, the compressed air dehuller was operated at 0.522 MPa for 60 s, with a vent opening of 7 mm. Crude groat percentage was defined as the ratio of the mass of material recovered in the groat recovery container to the mass of whole oats fed into the dehuller multiplied by 100. The crude groat preparation was sorted by hand into hulls remaining after dehulling, broken groats, and whole groats. The hulls remaining fraction contained both free hulls and undehulled oats. Any groat with more than 10% of its mass broken off was considered broken for sorting purposes. Corrected groat percentage was the ratio of the mass of the crude groat preparation minus the mass of the hulls remaining, to the mass of the whole oats fed into the dehuller, multiplied by 100. The milling yield was defined as the mass of whole oats necessary to obtain 100 kg of clean whole groats. Milling yield was calculated by subtracting the mass of hulls remaining and broken groats from the mass of the crude groat, dividing this by the mass of the oat starting material, and multiplying the inverse of this value by 100. Percentage hulls remaining is the ratio of the mass of hulls or undehulled oats recovered from the crude groat preparation to the mass of the crude groat preparation, multiplied by 100. Percentage broken groats is defined as the mass ratio of broken groats recovered after dehulling to the mass of the crude groat preparation multiplied by 100.

The impact dehuller was manufactured by local contract according to plans provided by the Quaker Oat Company (Barrington, IL). Its design is similar to those shown by Ganssmann and Vorwerck (1995) or by Deane and Commers (1986). Fifty-gram oat samples were passed through the dehuller, then hulls were removed by aspiration on a "clipper" dockage tester with constant fan opening settings. After aspiration, samples dehulled by the impact dehuller were sorted into whole groats, broken groats, and hulls remaining, as were the compressed air dehuller samples, and data obtained were treated as described for the compressed air dehuller.

Digital Image Analysis
Oat and groat architecture was described by digital image analysis. Samples of about 10-g were spread on a lightbox so that a photograph could be taken including every kernel in the sample. Kernels were arranged so that none were touching each other. A photograph was taken of the kernels, including a transparent ruler, for distance calibration, and a sample number label, with a 35-mm SLR optical camera. Prints of these photographs (10 x 15 cm) were scanned into a personal computer, at 236 dots per cm resolution, to create digital images. The computer package, SigmaScan (Jandel Corporation, San Rafael, CA), was used to calculate the length, width, and image area of each kernel in each photograph. Data were downloaded into a spreadsheet program, where the mean kernel lengths, widths, and image areas were calculated, along with the coefficients of variation (CV) for length, width, and image areas. The number of kernels in each image (also provided by the computer image analysis package) was divided by the mass of the sample photographed to get the mean kernel mass. The mean kernel mass was divided by the mean image area to obtain the oat mass: area ratio. Photographs of both whole oat kernels and cleaned groats were analyzed. Additional values derived from the oat and groat data included the oat length: groat length difference, oat mass: groat mass difference, and the groat mass: oat mass ratio.

Experimental Design
Effects of compressed air dehuller conditions on dehulling characteristics of Dumont oat were determined by one-way ANOVA by the Statistix computer package (Analytical Software, Tallahassee, FL) with three replicates, where dehuller conditions were considered fixed. Differences among means were compared by Tukey's Honestly Significant Difference (HSD), as described by Steel et al. (1997)(p. 191–193). For the evaluation of cultivars, oats were grown in the field at three locations in a randomized complete-block design with three replicates. Effects of cultivar, location and dehulling method on groat percentage, hulls remaining after dehulling, broken groats, and milling yield were determined by a three-way analysis (ANOVA) by the Statistix computer package, where cultivar and dehulling method were considered fixed and location was considered random. Physical characteristics of oat kernels determined by digital image analysis were analyzed by two-way ANOVA, where cultivar was considered fixed and location was considered random. Differences among means were compared by HSD also calculated by Statistix computer package. Phenotypic correlations of dehulling characteristics among cultivars were initially calculated for each location by the Statistix computer package, then r values from the separate locations were pooled according to Steel et al. (1997)(p. 295–296). A chi-square test (Steel et al., 1997, p. 296–297) indicated the homogeneity of all pooled r values.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
Influence of Dehuller Conditions on Oat Dehulling
Increased air pressure during dehulling resulted in a decrease in the crude groat percentage, a decrease in the hulls remaining after dehulling, an increase in broken groats, and an increase in the corrected groat percentage, without affecting the mill yield (Table 1) . The decrease in the crude groat percentage appeared largely to be due to the decrease in the hulls remaining after dehulling. The increased air pressure appeared to allow for a greater dehulling efficiency, and improved the removal of hulls from the groats. However, the increased mechanical stress associated with the increased air pressure also caused more groat breakage. The improved dehulling efficiency at the higher air pressures appeared to be responsible for the increased corrected groat percentages. But the improved dehulling efficiency at higher air pressures was offset by the increased groat breakage, resulting in no net change in milling yield (Table 1).

Cultivar Effects on Oat Dehulling Characteristics
ANOVA (not shown) indicated significant location x genotype interaction, and significant location and genotype main effects (P < 0.01) for groat percentage as determined by three methods of oat dehulling. Significant interaction is attributed to differences in ranking among genotypes at different locations and not to differences in magnitude. Method of dehulling had a significant effect (P < 0.01) and the method x genotype and method x location interactions were also significant (P < 0.01). Genotypic means of hand dehulled groat percentages, crude and corrected groat percentage derived from the impact dehuller, and the compressed air dehuller are presented in Table 4 . Large differences between crude and corrected groat percentages are indicative of errors which can be introduced into groat percentage estimations if hulls are not removed by hand sorting from crude groat preparations. Larger differences between crude and corrected groat percentage with the impact dehuller than the compressed air dehuller indicated that aspiration was stronger with the compressed air dehuller. Higher values of groat percentage were derived from hand dehulling for all cultivars. Groat percentages derived from the impact dehuller were higher than those derived from the air pressure dehuller. Larger corrected groat percentage values from the impact dehuller suggest that the impact dehuller applied more physical stress on the oat kernels and achieved a greater dehulling efficiency over the air pressure dehuller. Larger ranges in groat percentage were found with mechanical dehulling than with hand dehulling (Table 4).

Certain cultivars ranked higher in the crude groat percentage than in the corrected groat percentage, as determined by the impact dehuller. These included Jud, AC Medallion, and Youngs. The hulls of these cultivars appeared to be more difficult to separate from the groats. Because aspiration removes material according to mass, the hulls of these cultivars may have been heavier than the others. Rankings of crude and corrected groat percentage were more similar by the compressed air dehuller than by the impact dehuller. Cultivars that ranked lower with the correction for groat percentage by the air pressure dehuller included AC Assiniboia, Youngs, Brawn, and Jud. Cultivars that ranked higher included Belle, AC Medallion, Jerry, and Gem. These changes in ranking were due to the relative amounts of hulls remaining in the crude groat preparation. Those cultivars with small amounts of hulls remaining advanced in the rankings and those with large amounts of hulls remaining declined in ranking.

Analysis of Hulls Remaining and Groat Breakage after Mechanical Dehulling
ANOVA (not shown) indicated significant genotype main effect and genotype x location interactions for hulls remaining after dehulling (P < 0.01). The location effect was not significant (P > 0.05). The significant cultivar x location interaction appeared to be due to differences in ranking of cultivars at different locations, as well as differences in magnitude. Four cultivars: Gem, Assiniboia, Brawn, and Youngs, had higher hulls remaining at the Fullerton location than at the other locations. These were also the cultivars with the largest kernels. Failure of the larger groats to fill completely at this location may have contributed to the retention of hulls. ANOVA (not shown) also indicated that method of dehulling had a significant effect (P < 0.01) on hulls remaining after dehulling. More hulls remained with the groats after impact dehulling than after compressed air dehulling (Table 5) . This indicated that the aspiration used after the impact dehulling was not as strong as that used by the compressed air dehuller. More hulls remained with the cultivars Brawn, Jud, and Youngs by either method. Belle and Hytest had the lowest amount of hulls remaining by either method. Because mass of hulls is likely to have influenced the efficiency of their removal by aspiration, it appears likely that those cultivars with greater hulls remaining also had heavier hulls.

Milling Yield
By definition, values of milling yield are inverse to groat percentage. A high groat percentage will lead to a low mill yield. ANOVA (not shown) indicated a significant genotype effect, a significant genotype x location interaction (P < 0.01), but not a significant location effect (P > 0.05) on mill yield. The genotype x location interaction appeared to be due primarily to a difference in ranking of cultivars among the locations. Although the dehulling method main effect was not significant (P > 0.05), the method x genotype interaction was significant (P < 0.01). Mill yield of AC Medallion was much higher by the impact dehuller than by the compressed air dehuller, because of increased groat breakage with the impact dehuller. Hytest, Belle, and AC Assiniboia were low ranking by both methods, which reflects their high groat percentage and low breakage. Brawn was high ranking in mill yield by both methods. Jerry, Gem, and Youngs were in the midrange for both methods. Jud was high in milling yield by impact dehulling, but in the midrange by the compressed air dehuller. Bay was high in milling yield by the compressed air dehuller, but in the midrange by the impact dehuller (Table 5). The higher corrected groat percentage of Bay by the impact dehuller, attributed to the lower aspiration of the compressed air dehuller, is likely to be responsible for the difference.

Image Analysis
ANOVA (not shown) for whole oat size characteristics as determined by image analysis indicated significant location and genotype main effects (P < 0.01) and significant genotype x location interaction (P < 0.01) for all characteristics tested, including kernel mass, kernel length, kernel width, image area, mass: area ratio, and area CV. Genotype x location interactions appeared to be due to differences in ranking as well as magnitude of genotypes among locations. Genotypic means of whole oat and groat size characteristics are shown in Tables 6 and 7 .

Whole oat size uniformity, as estimated by the whole oat image area CV was also highly correlated with groat percentage (Table 9). The relationship between oat size uniformity and groat percentage is well known to many oat millers, who will sort oats according to size and dehull the more uniformly sized oat preparations separately to maximize milling yield (Ganssmann and Vorwerck 1995). Different size oats dehull optimally at different rotor speeds in an impact dehuller. Although the size distributions of oat kernels is typically multimodal, because of size differences among primary, secondary and tertiary kernels (Symons and Fulcher, 1988), very little work has been done to characterize differences is size distributions among cultivars and environments. In light of the importance of this characteristic to groat percentage and oat milling yield, this subject may deserve further attention.

The groat weight: oat weight ratio, calculated from oats and groats dehulled with the compressed air dehuller was significantly correlated with groat percentage (Table 9). The observation that it was not more highly correlated with experimentally obtained groat percentage values (which are also groat weight: oat weight ratios) indicates that significant amounts of groats were lost in the air pressure dehulling process, either by breakage or by aspiration. The groat area: oat area ratio is also an estimate of groat percentage, based on image areas, and was highly correlated with groat percentage (Table 9).

Additional correlation analyses (Table 10) indicated a strong negative correlation between the hulls remaining after dehulling with groat percentage (by either of the mechanical dehulling methods used). If oats with heavier hulls have lower groat percentage, then these heavier hulls may be more difficult to remove by aspiration. Thus, oats with lower groat percentages might be expected to have more hulls remaining after dehulling. This observation emphasizes the importance of hand-sorting crude groat preparations from mechanical oat dehullers, because hulls remaining with the groats will increase the crude groat percentage. If the crude groat percentage were the only selection factor for groat percentage, one could inadvertently select for a low groat percentage cultivar because its groat preparation contained more hulls, when one intended to select for a high groat percentage cultivar. Oat length was also correlated with hulls remaining, suggesting that longer hulls are heavier hulls (Table 10).

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
Mechanical dehulling affected values for groat percentage, as did cultivar and environmental factors. The results suggest that excess mechanical stress from either higher air pressure or increased dehulling duration resulted in increased dehulling efficiency, but also resulted in increased groat breakage. Increased aspiration removed greater amounts of hull, but also removed small groats and broken groats, resulting in decreased groat percentages.

Groat percentage determined by hand dehulling is considered to be a ceiling value. Although crude groat percentages were often higher than the hand dehulled value, this was because samples were often heavily contaminated with hulls. Corrected groat percentages were highly correlated with hand dehulled groat percentage values, indicating the value of hand sorting mechanically dehulled samples to obtain reliable groat percentage values. Cultivars with lower groat percentage were more likely to provide erroneously high crude groat percentage values because more hulls remained with the groats after dehulling by mechanical means. Interestingly, these cultivars were also less likely to have broken groats after dehulling. Thicker hulls may provide more protection to groats during dehulling, resulting in less breakage.

Test weight and size uniformity were whole oat characteristics that were most highly correlated with groat percentage. More detailed analyses of oat size distributions may provide improved strategies for identifying oat size uniformity.

	NOTES

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
¹ Mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned.

Received for publication November 6, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 

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