ARTICLE IN PRESS

0733-5210/$ - se

doi:10.1016/j.jc

�CorrespondE-mail addr

Journal of Cereal Science 47 (2008) 239–244

www.elsevier.com/locate/jcs

Interaction of heat-moisture conditions and physical properties in oatprocessing: I. Mechanical properties of steamed oat groats

F.K. Gatesa,�, B.J. Dobraszczykb, F.L. Stoddardc, T. Sontag-Strohma, H. Salovaaraa

aDepartment of Food Technology, University of Helsinki, P.O. Box 66, 00014, FinlandbSchool of Food Biosciences, University of Reading, P.O. Box 226, Whiteknights, Reading RG6 6AP, UK

cDepartment of Applied Biology, University of Helsinki, P.O. Box 27, 00014, Finland

Received 25 September 2006; received in revised form 23 March 2007; accepted 5 April 2007

Abstract

Research interest in oats has focussed on their nutritional value, but there have been few studies of their food processing. Heat

treatment is characteristic of oat processing, as it is needed to inactivate lipase and to facilitate flaking. A Texture Analyser was used to

characterise the mechanical properties of unkilned and kilned oat groats after steaming and tempering in an oven for 30, 60 and 90min at

80, 95 and 110 1C. Maximum force, number of peaks before maximum and final force after 5 s hold were used to characterise the

behaviour of the groats during compression. Kilned groats were larger and softer before steaming. After steaming and tempering, the

moisture content of the kilned groats was higher than for unkilned groats. Hot, steamed oats were softer than cold, unsteamed groats,

indicated by a decrease in maximum force from 59 to 55N, and there was no significant difference between kilned and unkilned groats.

However, higher temperatures during tempering increased maximum force. These results suggest that mild steam treatment yields softer

oat groats, whereas cold or over-treated groats tend to be harder.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Heat treatment; Kilning; Strength

1. Introduction

Steaming is routinely used during the food processing ofoats (i) to inactivate lipolytic enzymes and (ii) to facilitateflaking, reducing breakage. Kilning is often used in theindustrial process to inactivate lipase and some millers saythat the process also improves the flavour. Oats, or morecommonly groats (hull-less oats), are kilned by addingsteam to increase the moisture content to about 17% andthe temperature to over 80 1C (Ganssmann and Vorwerck,1995). Enzyme inactivation is closely related to moisturecontent (Hutchinson et al., 1951; Vorwerck, 1988) and at20% results in rapid inactivation in lipase at even 80 1C.

Radiator sections in the kiln further increase thetemperature to about 100 1C. The retention time in thekiln is about 90–120min, at the end of which thetemperature is about 21 1C and the moisture content

e front matter r 2007 Elsevier Ltd. All rights reserved.

s.2007.04.003

ing author. Tel.: +358 9 191 58239.

ess: fred.gates@helsinki.fi (F.K. Gates).

around 9.5% (Ganssmann and Vorwerck, 1995). Aseparate steaming process is then used to temper the groatsbefore flaking, which raises the temperature to about100 1C and the moisture content to about 18%. It is alsopossible to use this steam treatment alone, without kilning.This extended, high temperature treatment results in theproduction of flavour components (Molteberg et al., 1996;Sides et al., 2001).Interest in the food uses of oats has increased and there

has been considerable research into the health benefits ofchemical constituents of oats, especially of the soluble fibre(Malkki and Virtanen, 2001) and antioxidants (Peterson,2001). Studies related to the technological aspects of thefood processing of oats have rarely looked at flaking,although this is a major use of oats. Most of the reportedstudies have concentrated on the quality of the resultingoatmeal (Lapvetelainen and Rannikko, 2000; Rhymer etal., 2005), and sensory properties (Molteberg et al., 1996;Lapvetelainen et al., 2001) or looked at the effect of oatquality on flaking (Lapvetelainen et al., 2001; Ames and

ARTICLE IN PRESS

Fig. 1. Schematic representation of the laboratory oat steamer. The

steaming unit was rotated using an electrical motor at a speed of 36 rpm.

The mixing blades were curved to improve the mixing. The perforations

were at 5 cm intervals on either side of the mixing blade.

F.K. Gates et al. / Journal of Cereal Science 47 (2008) 239–244240

Rhymer, 2003). There is a need to study the flakingprocess, how groat properties are influenced by steamingand how the properties of the groat affect the process.

In an optimised process, the energy consumption shouldbe the minimum needed to ensure that the flakes are ofacceptable quality. Energy is required for steaming and torun the flaking rolls, and the efficiency of the processshould be considered on the basis of the total energyrequired. A previous study showed that heating inducedstiffening in oat groats (Gates and Talja, 2004). Stiffergroats would require more energy to form into flakes.However, this study used dynamic mechanical analysis(DMA), in which a small, sinusoidal deformation(o7.5 mm) was applied, and may not relate to thebehaviour of the groat during flaking.

The aim of this study was to develop a laboratory scalesteaming process and use it to characterise the effect ofsteaming on the mechanical behaviour of unkilned andkilned oat groats. This information could be used tooptimise the process.

2. Materials and methods

2.1. Oat material

Oat groats were provided by Myllyn Paras, Finland inpaper bags (about 20 kg). The samples were taken from thesame batch of oats. The main samples were stored in acool, dry store (10 1C, 40% RH) until required. All testswere carried out within 6 months of dehulling. Themoisture content of both samples was 9.6%. Since millsdo not store cultivars separately, the sample probablyconsisted of a mixture of the main cultivars received at themill. These were mainly Belinda, with smaller amounts ofAslak and Freja. The kilned samples were treated for about90min using a profile typical of the mill.

A laboratory steaming device was developed (Fig. 1) andused to steam 300 g batches for 30 s. The temperature of thesteam in the tube was regulated to 101 1C. The temperatureof the groat bulk during steaming was approximately100 1C. The steamed batches were transferred to a metalcontainer as quickly as possible to minimise cooling andcovered with a loosely fitting lid. Tempering refers to thetime that the groats were kept in an oven at a settemperature and moisture. At the end of tempering, thegroats were thoroughly mixed by hand and the temperatureof the bulk was determined.

A full-factorial experimental design was used with threereplicates and three factors, namely kilning (unkilned,kilned), oven temperature (80, 95, 110 1C) and temperingtime (30, 60, 90min). A 50 g sample of the temperedgroats was sealed in a plastic bag and the moisturecontent was determined within 24 h using ICC method110/1 (ICC, 1976).

The crystallinity of starch granules was determined byobserving thin, hand-cut sections of selected groatsbetween crossed polarisers in a microscope.

2.2. Measurement of mechanical properties

From each replicate of each treatment, 20 temperedgroats were tested with a Texture Analyser TA-XT2iequipped with a Peltier temperature control unit set at65 1C (Stable Microsystems, UK). Individual groats wereplaced crease-down and the 36mm cylindrical probe wasslowly lowered (0.2mm/s) until a 0.05N force wasdetected, which was defined as the surface of the groat(groat height). The groat was then compressed to 50% ofthis height at a speed of 1mm/s. The probe was then heldfor 5 s with data collected at 500 points per second.Dedicated software (Texture Expert Exceed, Stable Micro-systems, UK) was used to determine the maximum force,number of peaks before hold and final force. Thepercentage of cracked groats, that is those with a peakbefore maximum force, was also determined. Means ofthese 20 sub-samples were calculated for further statisticalanalysis. For comparison, unkilned and kilned sampleswere tested at room temperature (21 1C).

ARTICLE IN PRESSF.K. Gates et al. / Journal of Cereal Science 47 (2008) 239–244 241

2.3. Statistical analysis

Statistical analyses were carried out using SPSS 12 (SPSSCorp., Chicago, IL). An independent sample t-test wasused to compare the properties of kilned and unkilnedgroats before steaming. The univariate analysis of varianceprocedure of the general linear model was used to test theeffects of kilning, oven temperature and tempering time ongroat properties. Pearson correlation coefficients werecalculated from the treatment means.

3. Results

In the samples tested, maximum force generally occurredat maximum deformation or at least close to this point. Inmany groats, fracture events occurred before maximumdeformation, as shown in Fig. 2. It was observed that thesize of these peaks related to the size of the crack induced inthe groat.

3.1. Unsteamed groats

Unsteamed, kilned groats were significantly larger(Po0.001), with a mean height of 2.00mm (s.d.0.015mm) compared with 1.92mm (s.d. 0.014mm) forthe unsteamed, unkilned groats. The range of groat heightwas 1.68–2.44mm for the unsteamed groats. The majorityof groats cracked, indicated by one or more peak beforemaximum force. Slightly fewer (77%) raw groats cracked,when compared with kilned groats (87%). There was nosignificant difference in maximum force (mean 59N) ornumber of peaks (mean 1.5) between the unkilned andkilned samples. The force deformation curves were highlyvariable, in particular the fracture behaviour. In some casesthere were a few distinct fracture events before maximum

Fig. 2. Example of a compression curve for an oat groat showing the

parameters measured.

force, whereas in others the peak was extended and anumber of smaller fractures could be detected.

3.2. Steamed and tempered groats

The temperature of the groats after tempering variedbetween 71.7 and 97.3 1C. As expected, oven temperaturehad a significant influence on groat temperature; thegroats tempered at 80 1C had a significantly lowertemperature than those tempered at 95 1C (Po0.05), butthe mean temperature of the groats tempered at 110 1Cwas somewhat lower than those tempered at 95 1C. Meangroat temperatures for each of the treatments is shown inTable 1.Groat moisture varied between 14.0% and 21.2%, and

was significantly higher in the kilned samples (mean18.9%) than in the unkilned (17.1%). Mean groat heightvaried between 1.80 and 2.23mm, with a standarddeviation of 0.077mm. The results of the analysis ofvariance are shown in Table 2. All identifiable starchgranules showed complete Maltese crosses, indicating thatthe starch was not gelatinised.The steamed samples were softer, with a lower maximum

force, than the unsteamed samples, with a mean of 54.2N.The steamed groats were less susceptible to fracture, asindicated by fewer peaks (mean 0.5 peaks) and the splitswere observed to be smaller and the peaks smaller than inthe unsteamed samples. The effect of oven temperature onthe number of cracked grains approached significance(P ¼ 0.055). To further investigate this, the data for thehigher oven temperatures were pooled, and it was shownthat significantly fewer groats cracked at 80 1C than whenthe temperature was greater than 95 1C (P ¼ 0.014).A significant inequality of variance between treatments

was observed for maximum force. The standard deviationwas considerably higher at the lowest tempering time(30min).Both maximum force and final force increased with

increasing oven temperature (Fig. 3). The differencebetween maximum force and final force also increased,but this was not significant. The other parametersmeasured were not significantly affected by tempering.Significant correlations between groat properties were

found from the single grain measurement data (Table 3).Moisture content showed a significant, negative correlationwith groat strength and final force. Groat moisture alsocorrelated weakly with the height of the groat. Groatheight correlated with all the mechanical properties andthis was expected as mechanical properties are dependenton sample geometry.

4. Discussion

4.1. Laboratory steaming

Oat processing generally involves three heat treatments,namely grain drying, kilning and steaming prior to flaking.

ARTICLE IN PRESS

Table 1

Temperature, moisture content and mechanical characteristics of oat groats following factorial combinations of kilning, three temperatures of steam

treatment and three durations of tempering

Kilning Oven temperature

(1C)

Tempering time

(min)

Temperature after tempering

(1C)

Moisture

(%)

Maximum force

(N)

Final force

(N)

Cracked groats

(%)

Unkilned 80 30 85.1 16.8 46.7 23.5 23

80 60 79.1 16.7 52.1 28.6 48

80 90 79.2 16.9 54.9 29.7 50

95 30 84.3 16.7 56.4 33.8 57

95 60 81.2 17.8 57.8 31.1 51

95 90 84.8 18.2 51.7 28.6 55

110 30 79.7 17.6 55.5 29.4 55

110 60 82.8 17.8 59.3 33.9 45

110 90 81.6 15.7 63.2 37.6 50

Kilned 80 30 79.8 18.8 50.6 27.2 32

80 60 74.8 19.8 49.2 25.4 43

80 90 74.1 18.1 51.0 27.4 38

95 30 86.6 18.3 51.9 27.1 48

95 60 85.3 21.2 55.9 31.2 52

95 90 87.4 18.5 51.8 26.4 48

110 30 87.1 18.4 46.6 23.2 42

110 60 87.1 18.6 58.1 31.8 52

110 90 87.5 17.9 62.2 37.7 44

SE

(n ¼ 54)

0.96 0.29 1.01 0.87 2.1

Sample

size

300 g 2� 5 g 20 grains 20 grains 20 grains

Data show means of three replicates.

Table 2

Results of analysis of variance of temperature, moisture content and

mechanical characteristics of oat groats following factorial combinations

of kilning, three temperatures of steam treatment and three durations of

tempering

Source of variation d.f. Mean square

Moisture Temperature Max. force Final force

Kilning (K) 1 39.70** 22.69 67.92 58.80

Oven temperature (O) 2 3.147 210.4* 204.5* 127.6*

Tempering time (T) 2 5.872 20.62 112.1 72.80

K�O 2 0.821 138.2 8.430 7.610

K�T 2 1.907 0.189 2.979 3.199

O�T 4 2.127 30.13 82.39 79.54

K�O�T 4 2.114 3.074 31.47 25.77

Residual 36 4.568 49.48 48.47 35.34

*,**,*** Po0.05, 0.01, 0.001, respectively.

Fig. 3. Effect of oven temperature on groat strength and final force. Error

bars show 71 standard error.

F.K. Gates et al. / Journal of Cereal Science 47 (2008) 239–244242

In this study, a laboratory system was developed forinvestigating the effect of steaming and tempering para-meters on the quality of oat flakes. Kilning was carried outat an industrial mill. The only detectable difference due tokilning in these experiments was to allow the groats to takeup more water.

Industrial steaming processes are continuous, so theconditions are difficult to reproduce in small scaleequipment, which tend to clog. Thus a batch process wasused instead. The small-scale process emulated the

industrial process by injecting high pressure steam intothe bed of groats, instead of adding water and heatseparately as in previous model systems (Ames andRhymer, 2003; Rhymer et al., 2005). The system was open,so that the maximum temperature in the bulk was around100 1C. It was assumed that water condensed on the surface

ARTICLE IN PRESS

Table 3

Correlations between properties of oat groats after steaming and

tempering (n ¼ 54 for moisture and temperature, n ¼ 1080 for mechanical

properties)

Moisture Temperature Height Max. force Breaks

Temperature �0.191

Height �0.274* �0.023

Maximum force �0.271* 0.012 0.290**

No. peaks 0.035 �0.101 0.089** 0.027

Final force �0.257** 0.037 0.174** 0.955** 0.022

*,** Po0.05, 0.01, respectively.

F.K. Gates et al. / Journal of Cereal Science 47 (2008) 239–244 243

of the groats, as well as on the walls of the steamer, socontinuous mixing was applied to ensure that heat andmoisture were evenly distributed. As with the industrialprocess, the experimental treatments did not result inextensive starch gelatinisation, confirmed by the presenceof complete Maltese crosses in the granules observed inpolarised light.

Tempering allowed the redistribution of water withinand between groats (Hsu, 1984). To simulate the conditionsin the industrial process, the groats were not mixed duringtempering and the container was not sealed. The temperingtemperatures were selected as they slightly extended therange used in industrial kilning (Ganssmann and Vor-werck, 1995). Temperature gradients within the bulk wereassumed to depend on oven temperature. These may alsohave set up moisture gradients, as relative humidity isdependent on temperature. Temperature gradients withinthe kernel exist only for a short time and are seldomreported (Wu et al., 2004). Moisture gradients withinkernels have also been demonstrated in similar systems,such as during the cooking of wheat (Stapley et al., 1997)and during the drying of maize (Kovacs and Neemenyi,1999) and rice. The location of the water in the systemcould be expected to have a significant effect on themechanical properties of the groat.

4.2. Mechanical behaviour of oat groats during compression

Uniaxial compression tests, such as the one used in thecurrent study, have been widely used to characterise themechanical behaviour of cereal grains, mainly wheat(Zoerb and Hall, 1960; Shelef and Mohsenin, 1967; Glennet al., 1991; Haddad et al., 1998) but also maize (Zoerb andHall, 1960) and rice (Shitanda et al., 2002). To determinethe fundamental mechanical properties of the material, thecomplex geometry of the groat needs to be eliminated. Thiswas done for wheat by Shelef and Mohsenin (1967) whoapplied Hertz’s method to mathematically account for thegeometry of the grain during compression between two flatplates, whilst Haddad et al. (1998) introduced a simplemethod for machining endosperm pieces with a simplegeometry. Neither of these methods could be used in thepresent study, since the plastic behaviour of oat invalidatedthe assumptions made in the Hertz method and the time

required to prepare test pieces made this approachimpractical for investigating the time-dependent effect ofthe steaming process.Engleson and Fulcher (2002a, b) have measured the

mechanical behaviour of oat along the major axis of thegroat during compression and related this to groat damageimpact hulling. In the current study, however, the groatswere compressed normal to the crease, as this test wascloser to the type of compression that occurs duringflaking. A large deformation (50%) was used in the currentstudy, since this closely approximates industrial flaking.The maximum force was used to characterise the

resistance to flaking, and was used as a measure of theenergy requirement for flaking. Final force was used as ameasure of elasticity of the compressed groat. Thedifference between maximum force and final force providesan indication of the extent of mainly plastic flow. Thedifference between maximum force and final force variedwith treatment. Variations in flake thickness occur evenwhen oats are flaked with a constant roll gap, and these arepart of a continuing investigation.The number of peaks during the compression phase

(i.e. before the maximum force) was found to correspondto cracks in the groat. Cracks are sources of weakness andmay also affect other properties, such as water absorptionand flake integrity, which we are currently investigating.

4.3. Effect of steaming and tempering on the mechanical

properties

Steaming increased the temperature and moisture con-tent of the groats and this caused the groats to becomesofter and more plastic, as shown by decreases in peakforce and the number of fracture events during compres-sion following steaming and tempering. This behaviour istypical of biopolymers, and this softening effect of waterand heat is typically understood in terms of the glasstransition and molecular mobility (Roudaut et al., 2004).However, within the tempering treatments, an increasedoven temperature resulted in an increase in groat strength.This confirms earlier findings from a DMA study of themechanical properties of groats (Gates and Talja, 2004).The results are consistent with findings in other cereal-based foods, where water-induced stiffening has beenobserved and attributed to swelling that reduces structuraldiscontinuities, such as flaws and air cells (Gondek andLewicki, 2006; Harris and Peleg, 1996).In the industrial steaming process, it is not possible to

control the parameters used in this study directly. Themiller can, however, either regulate the mass flow of groatsor regulate groat temperature by adjusting the set-point forthe steam inlet valve. It is also possible to adjust thepressure of the steam, but if this is too low it could lead toproblems with condensation.Our previous study suggested that kilning did not have

much influence on the mechanical properties of oat flakes(Gates et al., 2004). The current study showed that kilning

ARTICLE IN PRESSF.K. Gates et al. / Journal of Cereal Science 47 (2008) 239–244244

did not have a significant influence on the mechanicalproperties of oat groats before or after steaming. Thisstudy also showed that tempering parameters affected thestrength of steamed oat groats, which would be expected torelate to the energy required to flake the oats. Thus,excessive heat treatment may decrease the efficiency of oatflaking, both in terms of the extra steam required and alsobecause more energy would be required to flake strongergroats. The next part of this study looks at the effect ofsteaming on flake quality.

Acknowledgements

Support from Helsinki Mills Ltd. and Myllyn Paras Ltd.through a project jointly funded by the Finnish FundingAgency for Technology and Innovation (Tekes) is grate-fully acknowledged.

References

Ames, N.P., Rhymer, C.R., 2003. Development of a laboratory-scale

flaking machine for oat end product testing. Cereal Chemistry 80,

699–702.

Engleson, J.A., Fulcher, R.G., 2002a. Mechanical behavior of oats: the

groat effect. Cereal Chemistry 79, 787–789.

Engleson, J.A., Fulcher, R.G., 2002b. Mechanical behavior of oats:

specific groat characteristics and relation to groat damage during

impact dehulling. Cereal Chemistry 79, 790–797.

Ganssmann, W., Vorwerck, K., 1995. Oat milling, processing and storage.

In: Welch, R.W. (Ed.), The Oat Crop: Production and Utilization.

Chapman & Hall, London, pp. 369–408.

Gates, F.K., Talja, R.A., 2004. Effect of temperature and moisture on the

mechanical properties of oat. Transactions of the ASAE 47,

1223–1227.

Gates, F.K., Dobraszczyk, B.J., Salovaara, H., 2004. Influence of some

processing and storage conditions on the mechanical properties of oat

flakes. Transactions of the ASAE 47, 223–226.

Glenn, G.M., Younce, F.L., Pitts, M.J., 1991. Fundamental physical

properties characterizing the hardness of wheat endosperm. Journal of

Cereal Science 13, 179–194.

Gondek, E., Lewicki, P.P., 2006. Antiplasicization of cereal based

products by water. Part II: breakfast cereals. Journal of Food

Engineering 77, 644–652.

Haddad, Y., Benet, J.C., Abecassis, J., 1998. A rapid method for

appraising the rheological properties of the starchy endosperm of

cereal grains. Cereal Chemistry 75, 673–676.

Harris, M., Peleg, M., 1996. Patterns of textural change in brittle cellular

cereal foods caused by moisture sorption. Cereal Chemistry 73,

225–231.

Hsu, K.H., 1984. A theoretical approach to the tempering of grains.

Cereal Chemistry 61, 466–470.

Hutchinson, J.B., Martin, H.F., Moran, T., 1951. Location and

destruction of lipase in oats. Nature 167, 758–759.

ICC, 1976. Determination of Moisture Content of Cereals and Cereal

Products (Practical Method). ICC Standard Methods.

Kovacs, A.J., Neemenyi, M., 1999. Moisture gradient vector calculation

as a new method for evaluating NMR images of corn (Zea mays L.)

kernels during drying. Magnetic Resonance Imaging 17,

1077–1082.

Lapvetelainen, A., Rannikko, H., 2000. Quantitative sensory profiling of

cooked oatmeal. Lebensmittel-Wissenschaft und-Technologie 33,

347–379.

Lapvetelainen, A., Alho-Lehto, P., Sinn, L., Laukkanen, T., Lindman, T.,

Kallio, H., Kaitaranta, J., Katajisto, J., 2001. Relationship of selected

physical, chemical, and sensory parameters in oat grain, rolled oats

and cooked oatmeal—a three year study with eight cultivars. Cereal

Chemistry 78, 322–329.

Malkki, Y., Virtanen, E., 2001. Gastrointestinal effects of oat bran and

oat gum: a review. Lebensmittel-Wissenschaft und-Technologie 34,

337–347.

Molteberg, E.L., Solheim, R., Dimberg, L.H., Frølich, W., 1996.

Variation in oat groats due to variety, storage and heat-treatment II:

sensory quality. Journal of Cereal Science 24, 273–282.

Peterson, D.M., 2001. Oat antioxidants. Journal of Cereal Science 33,

115–129.

Rhymer, C., Ames, N., Malcolmson, L., Brown, D., Duguid, S., 2005.

Effects of genotype and environment on the starch properties and end-

product quality of oats. Cereal Chemistry 82, 197–203.

Roudaut, G., Simatos, D., Champion, D., Contreras-Lopez, E., Le Meste,

M., 2004. Mobility around the glass transition temperature: a mini-

review. Innovative Food Science & Emerging Technologies 5,

127–134.

Shelef, L., Mohsenin, N.N., 1967. Evaluation of the elastic modulus of

wheat grain. Cereal Chemistry 44, 392–402.

Shitanda, D., Nishiyama, Y., Koide, S., 2002. Compressive strength

properties of rough rice considering variation in contact area. Journal

of Food Engineering 53, 53–58.

Sides, A., Robards, K., Helliwell, S., An, M., 2001. Changes in the volatile

profile of oats induced by processing. Journal of Agricultural and

Food Chemistry 49, 2125–2130.

Stapley, A.G.F., Hyde, T.M., Gladden, L.F., Fryer, P.J., 1997. NMR

imaging of the wheat grain cooking process. International Journal of

Food Science and Technology 32, 355–375.

Vorwerck, K., 1988. Hydrothermische Behandlung von Hafer. Getreide

Mehl Brot 42, 199–202.

Wu, B., Yang, W., Jia, C., 2004. A three-dimensional numerical

simulation of transient heat and mass transfer inside a single

kernel during the drying process. Biosystems Engineering 87,

191–200.

Zoerb, G.C., Hall, C.W., 1960. Some mechanical and rheological

properties of grains. Journal of Agricultural Engineering Research 5,

83–93.