AGRICULTURAL AND FOOD SCIENCE Vol. 13 (2004): 198–211. Dehulling capacity and storability of naked oat Anna-Maija Kirkkari Work Efficiency Institute, Department of Agriculture, PO Box 13, FIN-05201 Rajamäki, Finland, e-mail: [email protected] Pirjo Peltonen-Sainio MTT Agrifood Research Finland, Plant Production Research, FIN-31600 Jokioinen, Finland Pekka Lehtinen Helsinki University of Technology, Laboratory of Biochemistry and Microbiology, PO Box 6100, FIN-02015 TKK, Finland Harvested naked oat is not completely hulless. Hull content of various cultivars ranged between one and six percent. Genotype and environment control expression of nakedness. Using different threshing settings at different grain moisture contents, it was investigated whether grain moisture at threshing and combine harvester settings affected hull content and its relationship to germina- tion capacity. Naked groats were stored at room temperature and analysed for protein content and fatty acid composition to determine storability. Grain moisture content at threshing had contrary effects on hull content and degree of hull retention in different years. Small grains tended to retain hulls more tightly during threshing. Grain filling capacity appears to be the dominant factor deter- mining degree of nakedness rather than stage of maturity. The postulated protective nature of hulls was confirmed only for cultivar Lisbeth. Highly viable samples of grain of cv. Lisbeth, threshed at normal settings, contained a higher percentage of hulls than those with low germination capacity, while for cv. Bullion, a protective effect of the hulls was not evident. Grain moisture content at threshing did not affect protein content of naked cultivars, but some differences in fatty acid com- position were recorded. Changes in lipid composition and volatile oxidation products during stor- age of groats were relatively moderate, indicating no major problems related to storage when na- ked oat was dried well. Key words: Avena sativa L. nuda, germination, grain, grain moisture, groat, combine harvesters, hulls, naked oat, storage, threshing © Agricultural and Food Science Manuscript received June 2003 198 AGRICULTURAL AND FOOD SCIENCE Vol. 13 (2004): 198–211. The spikelet of naked oat is multiflorous, often Introduction bearing three or more florets per spikelet. Con- ventional oat usually has two fertile florets. The Oat ( Avena sativa L.) florets are protected by two earlier the floret is set in the panicle, the more bracts or scales, the lemma on the outside and physiologically mature it is at threshing and the the palea on the inside (Bonnett 1961). Unlike more probably it is also dehulled. Grains retain- conventional oat, the lemma of naked oat is thin ing hulls are often from the least mature spike- and papery, containing only a little lignin, and lets. from which the naked grain threshes free simi- In addition to genotype, environmental con- larly to wheat and rye (Valentine 1995, Ougham ditions markedly control the expression of na- et al. 1996). However, dehulling of naked oat is kedness (Boland and Lawes 1973). Lawes and not complete. Experiments carried out with na- Boland (1974) found that naked oat grown in the ked oat in Finland showed that the hull content greenhouse regularly produced more naked of different cultivars generally ranged between groats than that grown in the field. They surmised one and six percent, although some cultivars had that temperature has a major effect: at 25˚C, the hull contents as high as 13 percent (Kangas et expression of nakedness was complete in all cul- al. 2001). Variation in hull retention was report- tivars studied, while at 20˚C some cultivars de- ed to be substantial (Lawes 1971, Machan 1998). hulled incompletely and at 15˚C only one culti- It is possible to increase the expression of na- var expressed complete nakedness. Jenkins kedness through plant breeding. Modern culti- (1973) also reported that cool conditions prior vars can have less than one percent of hulled to heading resulted in more hulled groats when grains. Kangas et al. (2001) reported that the lat- compared with warm conditions. According to est breeding lines of naked oat had a lower pro- Lawes and Boland (1974), the degree of naked- portion of hulls in grains. This study aimed to ness of early-sown naked oat was higher than compare dehulling capacity of different cultivars that of late sown. One possible reason for this under Finnish growing conditions. was that the late sown oat developed under con- The single gene N-1 controls the attachment ditions more prone to higher temperature. More- of the hull to the groat. Three additional alleles over, the longer the light period, the higher the ( N-2, N-3 and N-4) interact with N-1, and their degree of nakedness. Furthermore, there are combined effect contributes to the degree of na- some indications that dry growing conditions kedness of naked oat (Kibite 2002). There are increase hull retention (Cuddeford 1995). Thus, four principal phenotypes of naked oat: com- the aim of this study was to evaluate whether pletely naked, partially naked, partially hulled grain moisture at threshing and combine harvest- and completely hulled. Lawes and Boland (1974) er settings affect hull content. As hulls are evi- reported that ten of 89 diverse naked genotypes dently important for successful germination and grown in a field produced completely naked seedling establishment through protecting the grains. The mosaic phenotype is a mixture of groat from mechanical damage (Fulcher 1986, naked and covered kernels, in various propor- Peltonen-Sainio et al. 2001), the connection be- tions, depending on the alleles at the N-2 and N- tween the hull content and germination capacity 3 loci (Kibite 2002). was also investigated. The attachment of the hull is associated with When the groat is damaged, lipase becomes factors including lignin content of the lemma, active and breaks down fatty acids to produce a the multiflorous character and the length of the rancid flavour. Because naked oat is far more rachilla. In cultivars with mosaic expression of susceptible to mechanical damage than conven- nakedness, the lignin content of the lemma var- tional oat (Valentine 1995, Thorton 1986, Kirk- ies greatly. Some groats retain the lemma while kari et al. 2001, Peltonen-Sainio et al. 2001), it others thresh free from them (Valentine 1995). has been assumed to be also more susceptible to 199 AGRICULTURAL AND FOOD SCIENCE Kirkkari, A.-M. et al. Dehulling capacity and storability of naked oat developing a distinctive rancid flavour associat- tents. The threshing was carried out using stand- ed with hydrolysis and oxidation of oat lipids. ard farm machinery at three different threshing Our additional aim was to gauge the storability settings. The normal setting was used in 1998 of naked oat. with the cylinder (diameter 450 mm) speed at 1200 rpm (28 m s -1) (recommended in the user manual for threshing conventional oat) and with the front concave clearance set at 18 mm and the rear concave clearance at 7 mm. In 1999, two Material and methods cylinder speeds were used, 1200 rpm (28 m s -1) and 900 rpm (21 m s -1) and normal concave clear- Experiment 1 ance set. In 2000, the concave clearance was re- duced, with the front concave clearance setting The Finnish naked oat cultivar Lisbeth and the at 15 mm and the rear at 5 mm and only one British cultivar Bullion were grown in an exper- cylinder speed, 1200 rpm (28 m s -1), was used. iment carried out at the research farm of the Work (Table 1). Efficiency Institute in Rajamäki, southern Fin- After each threshing round, a sample of ap- land, in 1998–2000. The experiment was sown proximately 3 kg of grain was taken from the at three different dates using standard farm ma- hopper of the combine harvester. To analyse the chinery in spring to obtain grain samples from grain moisture, 10 g of whole grain was dried at plants grown under different growing conditions 130˚C for 19 hours and weighed. After harvest- and at different threshing moisture contents. The ing, the samples were dried with cold air and naked oat cultivars tested differed in their groat the germination analyses were carried out ap- properties. A hectolitre of the seed were used proximately four months after harvesting. Ger- each year in test weighed of cv. Bullion 68.6 kg mination analyses (4 × 100 seeds on blotting and cv. Lisbeth 62.7 kg. Correspondingly, one paper) were made at the Plant Production Inspec- thousand groats of cv. Bullion weighed 29.5 g tion Centre. Germination was classified from and of cv. Lisbeth 27.8 g. four replicates as being normal, abnormal or non- The naked oat cultivars were sown in 250 m 2 viable. Only normally germinating grains with field plots, samples from which were threshed undamaged radicles and hypocotyls were includ- in three to seven different rounds at different ed in calculation of the germination percentage. grain moisture contents. The autumn weather The samples from 1998–2000 were analysed conditions determined the number of threshing for the proportion of hulls remaining attached rounds each autumn to get as many samples as to the groats as a percentage of the weight of possible with different threshing moisture con- three sub-samples of approximately 5 grams Table 1. Threshing settings and moisture content at harvest. Year Threshing settings Cylinder speed, rpmConcave clearance, mm Moisture content, % 1998 1200 18/7 20–44 1999 900 18/7 15–37 1200 18/7 15–36 2000 1200 18/7 17–28 1200 15/5 17–26 200 AGRICULTURAL AND FOOD SCIENCE Vol. 13 (2004): 198–211. each. The hulled groats were separated from the 23.0, 27.4 and 45.0% in 1998 and, 13.7, 19.9, sample, dehulled by hand and the proportion of 23.6 and 42.5% in 1999.