- Email: [email protected]
Interpreting Crush Response Profiles from the Single Kernel Characterization System
Journal of Cereal Science 57 (2013) 222e229
Contents lists available at SciVerse ScienceDirect
Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs
Interpreting Crush Response Profiles from the Single Kernel Characterization System Nikiforos Misailidis, Grant M. Campbell* Satake Centre for Grain Process Engineering, School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2012 Received in revised form 12 October 2012 Accepted 13 November 2012
Modern versions of the Single Kernel Characterization System allow force profiles recorded for individual kernels during crushing to be retrieved. The current understanding of the meaning of the averaged Crush Response Profile (aCRP) is challenged in the current work. When bran layers are removed, the features of the aCRP that have been interpreted as the bran layers’ elastic response and subsequent collapse are retained. By contrast, when the crease is removed, these features are largely eliminated. The crease depth correlates with the first peak force, providing further evidence that wheat kernels tend initially to break across the crease. The initial slope of the force-deformation curve depends on the inherent strength of the endosperm material opposite the crease, while the magnitude of the initial peak force depends on the strength and thickness of this material, and hence on the crease depth. It is concluded that through the averaging process, aCRPs give a misleading picture of wheat kernel breakage in the SKCS. The Hardness Index currently reported by the SKCS, which is based on identifying breakage events as indicated by the jaggedness of the force profile, would appear to give a more fundamentally sound basis for quantifying wheat properties relevant to breakage. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Wheat hardness Breakage Crease Bran
1. Introduction Hardness is, along with gluten strength, the most important characteristic of wheat (Pomeranz and Williams, 1990); indeed it could be argued that the twin themes of hardness and gluten quality underpin the majority of wheat research, from fundamental genetics to practical processing. The introduction of the Perten Single Kernel Characterization System (SKCS) provided the facility to report the hardness of individual wheat kernels in a sample, along with mass, moisture content and diameter (Martin et al., 1991, 1993). This has had several applications including identifying mixtures of wheat classes, understanding the origins of wheat hardness and understanding the effects of distributions of hardness on processing, as reviewed by Osborne and Anderssen (2003), who also discuss the model by which the SKCS Hardness Index is calculated.
Abbreviations: SKCS, Single Kernel Characterization System; HI, Hardness Index; aCRP, averaged Crush Response Profile; iCRP, individual Crush Response Profile; aNCRP, averaged Crush Response Profile constructed from normalised iCRPs. * Corresponding author. E-mail address: [email protected] (G.M. Campbell). 0733-5210/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jcs.2012.11.004
In more recent versions of the SKCS, greater computing power has allowed the individual force profiles recorded during crushing to be stored and retrieved. These individual Crush Response Profiles (iCRPs) are jagged and highly variable; in fact, the jaggedness is the basis for the calculation of the Hardness Index. However, averaging the iCRPs of a sufficiently large sample yields an averaged Crush Response Profile (aCRP) that is smooth and reproducible. The aCRP generally exhibits an initial small peak followed by a trough and a second, larger peak. Osborne et al. (2001) proposed an interpretation of this general aCRP curve, as shown in Fig. 1 (adapted from Edwards et al., 2007; Osborne et al., 2001, 2004; Osborne and Anderssen, 2003). They interpret the first peak as indicating the response to deformation of the “shell” of the wheat kernel, and the second peak as the response of the endosperm. The “shell” is interpreted as “the aleurone layer” (Osborne et al., 2001; Osborne and Anderssen, 2003) or as “the bran layer” (Anderssen and Haraszi, 2009), or for similar work on barley, as “the structure of the outer layers in barley grain” (Osborne et al., 2007b) or “the husk, pericarp and aleurone layers of the grain” (Iwami et al., 2005). Anderssen and Haraszi (2009) summarise the view, presented in all these papers, that information “can be recovered about the mechanical strengths of the internal botanical layers from the morphology of an aCRP”. They give a systematic description of their understanding of “the deformation and fracturing of a wheat kernel
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
Fig. 1. The generic structure of the averaged Crush Response Profile (aCRP) and its previous interpretation (adapted from Edwards et al., 2007; Osborne et al., 2001, 2004; Osborne and Anderssen, 2003).
during its crushing” as indicated by the aCRP, in a section entitled “The rheology of aCRPs”, structured into “The elastic response of the bran layer”, “Bran layer collapse into the structure of the endosperm”, “The equilibrium response”, “The viscoelastic compressive response of the endosperm” and “Collapse of the endosperm cellular structure”. They conclude “an aCRP compartmentalizes the rheology of wheat hardness into the two distinct and independent aspects of aleurone strength and elasticity and endosperm viscoelasticity”. The objective of the current work is to present evidence for an alternative interpretation of Crush Response Profiles. As the previous interpretation emphasised the response of the bran layers in interpreting the initial part of the aCRP, it seems natural to consider the response when the bran layers are removed; this is considered first. The crease, which is a distinctive feature of the wheat kernel, gives a natural point of weakness that seems likely to affect breakage; the effect of the crease on the aCRP is considered second. Thirdly, the strain at which wheat endosperm fractures is known from independent studies; this is compared with the strains corresponding to the “endosperm response” portion of the aCRP, to demonstrate that the wheat kernels must have fractured well before this point, such that they cannot be exhibiting a viscoelastic response of the endosperm prior to fracture. Finally, the nature of the averaging process is scrutinised, to demonstrate that the aCRP gives a misleading picture of the nature of wheat breakage during crushing in the SKCS. The utility of the SKCS Hardness Index is reevaluated in the light of these considerations. 2. Materials and methods The following studies employed the Perten Single Kernel Characterization System, Model 4100 (Perten Instruments, Sweden). 2.1. Effect of debranning on Crush Response Profiles For this study, two representative wheats, Malacca (hard, SKCS Hardness Index ¼ 68.4) and Consort (soft, SKCS Hardness Index ¼ 33.4) at 14.5% moisture content were debranned in a Satake TM05 laboratory debranner to remove 4, 8, 12 and 16% of the original material. 300 kernels of the debranned samples were tested in the SKCS, the individual CRPs for each kernel retrieved, and the aCRPs constructed by averaging the individual data using an Excel spreadsheet. 2.2. Effect of crease characteristics on Crush Response Profiles This study comprised three parts. In the first part, 100 kernels of each of a hard and a soft wheat (Malacca and Consort described
223
above) were broken or cut along the crease, to create 200 halfkernels from which the crease had been eliminated. Another 100 kernels of each wheat were cut across the crease, to create 200 halfkernels in which the crease had been retained. Each set of halfkernels was processed through the SKCS, along with 300 unbroken kernels of each wheat. The second part of this study measured the crease morphology and SKCS breakage patterns of a set of 128 wheats from another project. These wheats comprised 30 varieties harvested from three different sites and grown over four seasons in the UK, representing a wide range of commercial wheats and of contrasting endosperm texture, as described elsewhere (Sylvester-Bradley et al., 2010). These samples gave a convenient set of data that sheds light on the issue of aCRP interpretation. For each wheat, four kernels were manually sliced into two halves across the thickest part of the kernel. The germ end of each half was scanned using a commercial scanner with 6400 dots per inch (dpi) resolution (Epson V500, Epson, Japan), and the crease depth and total kernel depth measured with the aid of commercial image processing software (Adobe Photoshop Elements 4.0, Adobe systems Inc., USA). 2.3. aCRPs from different grains In the third part of this study, a range of grains, some with a crease, some without, were tested in the SKCS and their aCRPs constructed, to see whether the presence of a crease gave a pattern in the aCRP. 3. Results and discussion 3.1. Effect of debranning on Crush Response Profiles Fig. 2 shows the aCRPs following debranning of a hard and a soft wheat to 0, 4, 8, 12 and 16% removal. Clearly the removal of material gives a lower area under the curve e each kernel comprises less material and therefore less energy is required to break it. However, removal of the bran layers, even up to 16% of the original kernel weight, fails to eliminate the initial peak in the aCRP. In fact, the effect of bran removal appears to be predominantly in the second half of the curve (particularly for the hard wheat), in the portion previously conceived as “Endosperm response”. This is in agreement with the results of Osborne et al. (2004), who found that debranning wheat kernels similarly had little effect on the first peak and substantially affected the second peak. Those and the current results provide the first indicative evidence that the initial peak in the aCRP is not indicative of the rheological response of the “shell” layers. 3.2. Effect of crease characteristics on Crush Response Profiles In fracture mechanics terms, cuts and notches provide natural points of weakness that concentrate stress and initiate breakage (Bourne, 2002, p. 102). The wheat kernel is distinctive in featuring a crease. This provides a natural point of weakness (Dobraszczyk, 1994; Evers and Millar, 2002), and it seems reasonable to suppose that this point of weakness might affect breakage in the SKCS. Fig. 3 shows the aCRPs resulting from intact wheat kernels and from half-kernels created by separating either along or across the crease. Clearly the area under the curve is lower for the halfkernels, as is to be expected. For both the hard and the soft wheat, the half-kernels created by cutting across the crease retain the initial peak in the aCRP and appear to be, for all intents and purposes, smaller versions of the corresponding whole-kernel aCRPs. By contrast, the half-kernels created by breaking or cutting
224
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
Fig. 2. Averaged Crush Response Profiles following debranning of (a) a hard wheat and (b) a soft wheat.
along the crease, so that the crease was eliminated, look quite different. For the soft wheat, the first peak disappeared, and for the hard wheat it was considerably less pronounced. (The aCRPs are also considerably shorter than those for the whole kernels or the “cut across” half-kernels, suggesting breakage finished somewhat sooner for these long, thin half-kernels compared with their short, fat counterparts.) The results from Fig. 2 provided evidence that the first peak in the aCRP does not relate to the bran; the results in Fig. 3 provide initial evidence that the first peak relates to the crease. This is strengthened by considering Fig. 4, which shows the initial peak force versus the ratio of crease depth:kernel depth. There is a significant negative correlation of 0.468 (p < 0.001) which indicates that kernels with a relatively deeper crease are, not surprisingly, easier to break. The correlation between the initial peak force and the ratio crease cavity width:kernel width was even stronger at 0.608. The general message is that the crease characteristics affect the first peak of the aCRP. This is most easily conceptualised in terms of crease depth, as a relatively deeper crease implies less material in the area opposite the crease, and it is this material that breaks. (Note that the crease depth is typically 60e70% of the total thickness of the kernel; there is relatively little material holding the two halves of the kernel together.) These results provide further evidence that the initial breakage depends on the crease characteristics. The correlations are evident despite the fact that the wheats are of inherently different hardnesses; even though the material that is breaking is inherently variable in its breakage strength, and even though only a few kernels of each wheat were examined, the effect of crease geometry
Fig. 3. Averaged Crush Response Profiles resulting from (a) intact wheat kernels and from half-kernels created by separating either (b) across or (c) along the crease, for a hard wheat (Malacca) and a soft wheat (Consort).
is sufficiently pronounced to be visible. In terms of a mechanism, a deeper crease implies less material (which is predominantly endosperm) in the region opposite the crease (i.e. between the crease cavity and the dorsal side of the kernel). It seems reasonable that less material in this region would give a kernel that is easier to break. Thus the evidence from the correlations is consistent with a reasonable mechanical explanation, and consistent with the earlier evidence from manipulating the crease geometry. Additional evidence comes from the observation of Iwami et al. (2005) that there was a strong negative correlation between the initial slope of the aCRP and the percentage of broken kernels following pearling of barley. If this initial slope indicates the strength of the endosperm material opposite the crease, then it makes physical
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
Fig. 4. Effects of crease morphology on the initial peak force of the averaged Crush Response Profile (aCRP).
sense that weaker material breaks more readily to give broken kernels following pearling. A physical mechanism that interprets the initial slope as indicating the strength of the outer layers makes less physical sense, given that the outer layers are removed early on in the pearling process and would have little relevance to broken kernels produced after pearling to 35% of the initial whole grain weight. 3.3. aCRPs from different grains Fig. 5 shows aCRPs from a range of grains and other seeds, separated into those that feature a significant crease (barley, rye
Fig. 5. Averaged Crush Response Profiles from (a) seeds that feature a crease; and (b) seeds that do not feature a crease.
225
and triticale) and those that do not have a crease (rice, quinoa, mung bean and white pepper). As for wheat, the initial peak is clearly evident for those other grains that have a crease, but is absent, minimal or very different for seeds that do not. Osborne and Anderssen (2003) report aCRPs for durum, barley, soft wheat, sorghum, oats and rice; their results point to a similar pattern. Taken together, the results from processing debranned wheat, from half-kernels, from relating crease characteristics to the first peak, and from examining the aCRPs from seeds with and without a crease, seem to point strongly to a conclusion that the first peak in the aCRP is not related to breakage of the “shell” layers, but depends principally on the material opposite the crease e on its thickness (which depends on the crease depth) and on its mechanical properties (which depend principally on the strength of the endosperm). Thus a picture emerges of kernels that tend to break initially within the SKCS mechanism by breaking into two halves along the natural point of weakness of the crease. This is consistent with images reported by Fang and Campbell (2002) showing a kernel breaking along the crease during roller milling, and with the observation reported by Evers and Millar (2002) that wheat kernels with a closed crease tended to require a greater force to fracture than those with a more open crease. Kernels enter the SKCS rotor-crescent mechanism in somewhat random orientations, so undoubtedly some kernels do not break initially along the crease, but the tendency to break preferentially in this way seems to be evident in the data from Figs. 3e5. A further consideration regarding the interpretation of the aCRP is the known elastic and fracture responses of wheat endosperm. The endosperm of wheat kernels is brittle and able to accept deformation strains of the order of 3e10% before fracturing (Delwiche, 2000; Haddad et al., 1999). It can be calculated that the time intervals of the predominant upward slope of the aCRP correspond to strains of 20e60%, well after the initial breakage of the kernel. Thus it is not possible that this slope, which we call the “post-failure upslope”, indicates the elastic response of the unbroken endosperm. However, the first portion of the aCRP, which we call the “prefailure upslope”, does appear to give an indication of the inherent strength of the endosperm material opposite the crease. Prior to breakage, the kernel does undergo elastic deformation, such that this first portion of the curve can indeed be considered in some sense a stressestrain curve. (We say “in some sense”, because an under-acknowledged feature of the force profile generated by the SKCS is that the direction of the force changes during crushing; as the kernel passes along the arc between the rotor and the crescent, the load cell only measures the component of the force in one direction. Thus the magnitude of the force recorded in the early part of the curve is not directly comparable with that recorded in the later part of the curve.) Acknowledging the consequence of the geometrical arrangement of the SKCS crescent and load cell, nevertheless, the initial slope of this pseudo-stressestrain curve can be interpreted as a measure of strength or stiffness in a conventional fracture mechanics sense. Coate (2011), reanalysing the data for the set of 45 wheat samples later reported in Campbell et al. (2012), found a strong positive correlation (0.726, p < 0.001) between the average of the initial slopes from the iCRPs and the average SKCS hardness, although the correlation between the initial slope of the aCRP and the average SKCS hardness was weaker (0.340, p w0.01). Iwami et al. (2005) similarly reported a high correlation (0.527, p < 0.001) between this initial aCRP slope and SKCS hardness for barley samples. This provides evidence that this initial slope can be interpreted in a fracture mechanics sense to indicate the inherent stiffness (modulus of elasticity) of the endosperm material. It provides further evidence that the initial portion of the curve is influenced by endosperm properties rather than
226
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
bran properties. It also hints at the problem of calculating the aCRP as a simple average of the iCRPs. Experimental evidence has been presented that the kernel breaks preferentially along the crease and that the “pre-failure slope” that occurs prior to the first peak relates to endosperm stiffness. Evidence based on a consideration of the strains occurring in the SKCS and on the known fracture properties of wheat endosperm support this view that fracture occurs early on during crushing in the SKCS. This is further supported by examining the individual Crush Response Profiles from which the aCRP is generated. A key question is: What is the relationship between the aCRP and the iCRPs from which it is calculated. This is best illustrated by asking initially: What is the relationship between the maximum force of the aCRP and the average of the maximum forces of the iCRPs? Fig. 6(a) illustrates the iCRPs of six kernels of a wheat of HI ¼ 47.8, and the aCRP resulting from the averaging of just these six iCRPs. The curves are labelled using the maximum force recorded for each curve. An interesting observation is that the average of these peak forces is 1781 N, but the peak force of the average CRP is much lower at 1562 N (a 12% reduction). The reason is that the maximum forces for the iCRPs occur at different times, because the kernels are different sizes. Another way of looking at this is to ask: How much smaller is the maximum force of the aCRP (aCRPmax) than the average of the iCRPmax values (iCRPmaxave)? This depends on how variable the
iCRPs are. If they were all identical, aCRPmax and iCRPmaxave would be the same. They are not identical because the kernels are different sizes; the more variable the kernel sizes, the greater the variation. This is demonstrated in Fig. 6(b). Twelve samples, each of 300 kernels, were tested in the SKCS, and aCRPmax and iCRPmaxave compared for each sample. Clearly those samples that had more widely varying kernel diameters (as indicated by the reported standard deviation of diameters) gave greatly reduced values of aCRPmax compared with iCRPmaxave, while samples with more uniformly sized kernels gave a smaller discrepancy. The standard deviation of the kernel diameters accounted for 93% of the observed variation, indicating that the relationship between aCRPmax and iCRPmaxave is dictated overwhelmingly by kernel diameter variation. Another noteworthy observation is that as the broken kernels exit the SKCS crushing mechanism, the iCRP force drops rapidly to zero. The aCRP curve shows however a gradual reduction in the force. Anderssen and Haraszi (2009) describe this portion of the aCRP as “Collapse of the endosperm structure”, saying that it contains “information about the rheology of the wheat at this stage” in terms of “viscoelastic rigidity.contained in the conglomerate”. In fact, this slope is an artefact resulting from kernels of different size exiting the crushing mechanism at different times. More generally, many of the features of the aCRPs are likely to be determined by the variation in kernel size. Thus, two wheat samples of identical endosperm and bran mechanical
Fig. 6. Relationships between individual Crush Response Profiles and averaged Crush Response Profiles: (a) iCRPs of six kernels, and their corresponding aCRP; (b) The relationship between the percentage reduction of the peak force of the aCRP compared with the average peak force of the iCRPs, and the standard deviation of the kernel diameter; (c) And (d) the early portion of example iCRPs; (e) The iCRPs of (a) normalised against kernel diameter on the x-axis and against mass on the y-axis; (f) the average of 250 normalised iCRPs (aNCRP) and the original aCRP normalised against the average kernel diameter.
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
properties and the same average kernel size, but varying only in kernel size distribution, would yield different measures derived from their aCRPs. Some of the distinctions reported in the literature based on parameters derived from aCRP curves are probably no more than artefacts of variations in kernel size distribution. To take yet another, even more revealing, aspect, many of the iCRPs return to a zero force at points within the profile. Fig. 6(c) and (d) zoom in on the early portion of some of the curves in order to illustrate this. The curve labelled “1850” in Fig. 6(c) shows a sharp initial peak followed by a sharp drop to zero, followed by a smoother but similar sized peak and a second drop to zero, after which the force increases jaggedly as the collection of particles break together. The curve labelled “1845” shows a similar initial peak followed by essentially a single return to zero. In Fig. 6(d), the curve labelled “1957” returns to essentially zero for the entire period from about 12 to 50 ms, while the curve labelled “1842” has an initial small peak, returns to zero for the period from 5 to 17 ms, has a second peak, returns to zero at around 40 ms, then increases thereafter. Anderssen and Haraszi (2009) similarly show example iCRPs that drop to zero force after the initial peak or peaks. The aCRP, by contrast, never returns to zero midway through crushing, because the zero force events in the iCRPs occur at different times. The aCRP thus gives no hint regarding these patterns of breakage. These “returns to zero” indicate the production of particles that are, for a time, too small to traverse the gap between the rotor and the crescent and hence to transmit forces. This observation is a clue as to the nature of the breakage mechanisms occurring during crushing in the SKCS. Combining this with the above information about the crease, it appears as a general pattern that kernels tend to break into two halves along the crease, these two smaller particles are then too small to traverse the gap until they move further along the decreasing arc between the rotor and the crescent, at which point they begin to transmit force, deform and break further. The number of “return to zero” events point to the number of these discrete breakage events before the collection of particles fills the area between the rotor and crescent and begins to break as a conglomerate. Extending these observations to the entire profile, it becomes clear that the aCRP does not indicate the average of the forces involved in distinct, identifiable crushing events, since it is influenced by the variation in the time at which those events occur for different kernels. The aCRP does not give a meaningful representation of the breakage of an “average” kernel. In fact, no kernel breaks in a way suggested by the aCRP. By not taking account of the effects of kernel size on the timing of events in the iCRPs, the aCRP obscures important features and gives “average” measurements that are less meaningful than the average of the individual measurements. Interpreting the aCRP as if it indicated the breakage pattern of a typical, or indeed any, kernel is fundamentally misconceived. Osborne et al. (2007a) asserted that the SKCS “has previously been shown to provide in situ measurements of the rheological properties of the bran and endosperm layers of wheat, otherwise only possible following their isolation by dissection or machining”, in which the aCRP can “be interpreted as a pseudo stress/strain characterization of the crushing as it progresses from the outer grain layers into the endosperm”. The interpretation presented in Fig. 1 was proposed in 2001 but never explicitly tested or demonstrated and cannot be considered as established fact, as that paper and others imply. The evidence presented in the current paper demonstrates that this interpretation is incorrect. We acknowledge the possibility of observing correlations between aCRP features and other measures of wheat properties or performance (Anderssen and Haraszi, 2009; Edwards et al., 2008; Iwami et al., 2005; Osborne et al., 2007a). This underlines that there is a difference
227
between a correlation and a mechanistic link, and that correlations, in the absence of mechanisms, do not provide reliable guides to the underlying physics. We therefore dispute the mechanistic interpretations in these and other papers that have tried to relate kernel characteristics to aCRP features, on the basis that the interpretation of aCRP features as relating to bran or endosperm is flawed in all of those papers. We acknowledge that there may still be useful information in the aCRP, even if the current interpretation is incorrect. We propose, however, that the interpretation of the aCRP would be both more meaningful (as a physical representation of wheat kernel breakage) and more powerful (in terms of yielding quantitative measurements related to wheat breakage and other aspects of wheat genetics or usage) if the aCRP were constructed from iCRPs in a way that takes account of the time variation. It would also be appropriate to account for the effect of kernel mass. Osborne et al. (2004) divided the aCRP force by the average kernel mass, in order to take account of mass removal following debranning of wheat. We propose the construction of aCRPs from iCRPs for which the force is normalised against each kernel’s mass, and the time is normalised against kernel diameter and the speed of the rotor, in order to obtain a dimensionless time. This creates a problem for averaging, that the intermediate times are now different for each kernel. In order to average, it is necessary to interpolate the intermediate forces for a standardised set of normalised times. Fig. 6(e) shows the interpolated iCRPs of Fig. 6(a) normalised against kernel diameter on the x-axis and against mass on the yaxis. The crushing rotor in the SKCS is 127 mm in diameter and it rotates at 120 rpm, giving a tip speed of 800 mm s1. A typical crushing time for a 3 mm kernel is 125 ms, corresponding to a movement of the rotor edge of 100 mm Thus, a typical 3 mm kernel crushed over a 125 ms timescale would yield a dimensionless time of 0.125 800/3 ¼ 33.33, i.e. ignoring slippage, crushing takes place over a distance approximately 30 times the diameter of the kernel. The x-axis of Fig. 6(e) can thus be interpreted as “Equivalent kernel diameters” (acknowledging that the decreasing gap between the rotor and crescent obscures precise interpretation). Interpolation was performed in an Excel spreadsheet using the following function (Anonymous, 2012): ¼FORECAST(NewX,OFFSET(KnownY,MATCH(NewX,KnownX,1)1,0,2), OFFSET(KnownX,MATCH(NewX,KnownX,1)-1,0,2)) There is a tantalising suggestion in Fig. 6(e) that normalisation has tended to make equivalent breakage events coincide, such that in the aCRP of these six kernels, several troughs become evident at around 2, 7.5, 12 and 14 ms that correspond to relatively consistent breakage patterns in the different kernels. However, as Fig. 6(f) shows, by the time the normalisation and averaging is applied to 250 kernels (dictated by the limitations of the size of the spreadsheet that Excel can handle), the curve once again becomes smooth. Fig. 6(f) compares the average of the normalised iCRPs (aNCRP) with the original aCRP (normalised on the x-axis against the average kernel diameter and on the y-axis against average kernel mass). The aNCRP has a similar overall shape, with a similar prefailure upslope and trough, longer post-failure upslope and sharper final drop. The initial pre-failure slope is similar for the two curves, for this particular example, while the final sharp drop in the aNCRP force profile is closer to the pattern of real kernels; however, the rest of the aNCRP continues to give a representation that does not meaningfully indicate the breakage of actual kernels. We suspect that, by avoiding the confounding effects and artefacts arising from variations in kernel size distribution, summary information extracted from the aNCRP would give better correlations with other measures of wheat processing and end-use quality than from the aCRP (but at a cost of greater complexity in constructing the
228
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
aNCRP). To take one measure, the longer post-failure upslope in the aNCRP is likely to allow a measure of this slope that is more accurate than from the (non-normalised) aCRP, and that therefore correlates better with the currently reported SKCS Hardness Index and with other factors affecting or affected by hardness. We acknowledge that the aCRP or aNCRP may contain useful measures of wheat properties that might be used to inform studies of wheat genetics, processing and end use performance. However, with the exception of the initial pre-failure slope, which indicates endosperm stiffness, and the initial peak, which relates to both endosperm stiffness and the thickness of endosperm material opposite the crease, we caution against trying to identify specific features of the aCRP or aNCRP as descriptive of specific aspects of wheat kernel morphology or breakage. In our view, the Hardness Index currently reported by the SKCS is fundamentally more soundly conceived than measures based on averaged profiles. The Hardness Index is calculated based on the jaggedness of the iCRP, measured as the total number of positive crush force slope values, and the number with a value greater than 8 N (corresponding over a 25 ms interval to a rate of increase greater than 320 N s1). These measures are used to calculate the Hardness Index using an unpublished model. As the relevance of wheat hardness is in relation to how the wheat kernel breaks, and as the jagged features of the iCRP indicate breakage events, it seems that an interpretation of hardness that utilises these jagged features is fundamentally more meaningful than an interpretation that averages these jagged features out. Similarly, information about the behaviour of specific botanical layers in the kernel is more likely to reside in the jagged profile than in the smooth averaged profiles. Meanwhile, the most directly relevant consequence of wheat kernel hardness, breakage during roller milling, has proved to relate well to the currently reported Hardness Index (Campbell et al., 2007, 2012). We acknowledge that there may be scope to improve the model currently used to calculate hardness, and that there may be complementary information in averaged Crush Response Profiles. Nevertheless, despite the attractions of the smooth aCRP or aNCRP curves, we would encourage continued use of the iCRPs as a firmer basis for extracting meaningful information about wheat breakage. 4. Conclusions The current work demonstrates that when the bran layers are removed, the features of the aCRP that have been interpreted as the bran layers’ elastic response and subsequent collapse are retained. By contrast, when the crease is removed, these features are largely eliminated. Meanwhile, the crease depth correlates with the first peak force, providing further evidence that the initial breakage is in some respect a function of the crease. A picture thus emerges of breakage of wheat kernels in the SKCS in which kernels tend initially to break along the crease. The initial slope of the forcedeformation curve depends on the inherent strength of the endosperm material opposite the crease, while the magnitude of the initial peak force depends on the strength of this material as well as its thickness, and hence on the crease depth. Averaged Crush Response Profiles give a misleading picture of the nature of wheat kernel breakage in the SKCS, in part because the timings of breakage events are different for kernels of different size. Constructing averaged Crush Response Profiles from iCRPs that have been normalised against each kernel’s diameter may give a stronger basis for extracting information about wheat quality and its relation to genetics, processing performance and end-use, although this entails greater computational complexity. However, the current SKCS Hardness Index, which is based on breakage events indicated by the jaggedness of the force profile, appears to
give a more meaningful basis for quantifying wheat properties related to breakage. Acknowledgements NM acknowledges the Home-Grown Cereals Authority (HGCA) for funding for PhD studies, and the GREEN Grain consortium for provision of wheat samples (funding provided through the UK Sustainable Arable LINK programme, sponsored by the Department for Environment Food and Rural Affairs and the Scottish Government). The Satake Corporation of Japan is gratefully acknowledged for its help in establishing the Satake Centre for Grain Process Engineering. References Anderssen, R.S., Haraszi, R., 2009. Characterizing and exploiting the rheology of wheat hardness. European Food Resources Technology 229, 159e174. Anonymous, 2012. http://www.blueleafsoftware.com/Products/Dagra/LinearInter polationExcel.php, (accessed 19.07.12.). Bourne, M., 2002. Food Texture and Viscosity; Concept and Measurement, second ed. Academic Press, London, UK. Campbell, G.M., Fang, C.Y., Muhamad, I.I., 2007. On predicting roller milling performance VI. Effect of kernel hardness and shape on the particle size distribution from First Break milling of wheat. Transactions of the Institution of Chemical Engineers Part C: Food and Bioproducts Processing 85, 7e23. Campbell, G.M., Sharp, C., Wall, K., Mateos-Salvador, F., Gubatz, S., Huttly, A., Shewry, P., 2012. Modelling wheat breakage during roller milling using the Double Normalised Kumaraswamy Breakage Function: effects of kernel shape and hardness. Journal of Cereal Science 55, 415e425. Coate J., 2011. Effects of Wheat Kernel Shape and Hardness on the Normalised Kumaraswamy Breakage Function. MEng. Dissertation, University of Manchester. Delwiche, S.R., 2000. Wheat endosperm compressive strength properties as affected by moisture. Transactions of the American Society of Agricultural Engineers 43, 365e373. Dobraszczyk, B.J., 1994. Fracture mechanics of vitreous and mealy wheat endosperm. Journal of Cereal Science 19, 273e282. Edwards, M.A., Osborne, B.G., Henry, R.J., 2007. Investigation of the effect of conditioning on the fracture of hard and soft wheat grain by the single-kernel characterization system: a comparison with roller milling. Journal of Cereal Science 46, 64e74. Edwards, M.A., Osborne, B.G., Henry, R.J., 2008. Effect of endosperm starch granule size distribution on milling yield in hard wheat. Journal of Cereal Science 48, 180e192. Evers, T., Millar, S., 2002. Cereal grain structure and development: some implications for quality. Journal of Cereal Science 36, 261e284. Fang, C., Campbell, G.M., 2002. Stress-strain analysis and visual observation of wheat kernel breakage during roller milling using fluted rolls. Cereal Chemistry 794, 511e517. Haddad, Y., Mabille, F., Mermet, A., Abecassis, J., Benet, J.C., 1999. Rheological properties of wheat endosperm with a view on grinding behaviour. Powder Technology 105, 89e94. Iwami, A., Osborne, B.G., Huynh, H.-N., Anderssen, R.S., Wesley, I.J., Kajiwara, Y., Takashita, H., Omori, T., 2005. The measurement of structural characteristics of barley for Shochu using single-kernel characterization system 4100 crushresponse profiles. Journal of the Institute of Brewing 111, 181e189. Martin, C.R., Rousser, R., Brabec, D.L., 1991. Rapid Single Kernel Grain Characterization System, US Patent No 5005774. Martin, C.R., Rousser, R., Brabec, D.L., 1993. Development of a single kernel wheat characterisation system. Transactions of the American Society of Agricultural Engineers 36, 1399e1404. Osborne, B.G., Anderssen, R.S., 2003. Single-kernel characterization principles and applications. Cereal Chemistry 80, 613e622. Osborne, B.G., Turnbull, K.M., Anderssen, R.S., Rahman, S., Sharp, P.J., Appels, R., 2001. The hardness locus in Australian wheat lines. Australian Journal of Agricultural Resources 52, 1275e1286. Osborne, B.G., Anderssen, R.S., Huynh, H.-N., 2004. In situ measurement of the rheological properties of wheat and barley grain using the SKCS 4100. pp. 207e 211. In: Cauvain, S.P., Salmon, S.S., Young, L.S. (Eds.), Using Cereal Science and Technology for the Benefit of Consumers: Proceedings of the 12th ICC Cereal and Bread Congress, Harrogate, 23rde26th May 2004, Woodhead Publishing Ltd., Cambridge, UK. Osborne, B.G., Henry, R.J., Southan, M.D., 2007a. Assessment of commercial milling potential of hard wheat by measurement of the rheological properties of whole grain. Journal of Cereal Science 45, 122e127. Osborne, B.G., Fox, G.P., Kelly, A.M., Henry, R.J., 2007b. Measurement of barley grain rheology for the quality selection of breeding material. Journal of the Institute of Brewing 113, 135e141. Pomeranz, Y., Williams, P.C., 1990. Wheat hardness: its genetic, structural and biochemical background, measurement and significance. In: Pomeranz, Y. (Ed.),
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229 Advances in Cereal Science and Technology, vol. 10. American Association of Cereal Chemists, St. Paul, Minnesota, USA, pp. 471e544. Sylvester-Bradley, R., Kindred, D.R., Weightman, R.M., Thomas, W., Swanston, J.S., Thompson, D., Feuerhelm, D., Creasy, T., Argillier, O., Melichar, J., Brosnan, J., Agu, R., Bringhurst, T., Foulkes, J.F., Pask, A., Cowe, I.,
229
Hemongway, D., Robinson, D., Wilcox, S., 2010. Genetic Reduction of Energy Use and Emissions of Nitrogen through Cereal Production: GREEN Grain. HGCA Project Report No. 468. Home Grown Cereals Authority, Stoneleigh Park, Kenilworth, Warwickshire, CV8 2TL, UK, June 2010.
Contents lists available at SciVerse ScienceDirect
Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs
Interpreting Crush Response Profiles from the Single Kernel Characterization System Nikiforos Misailidis, Grant M. Campbell* Satake Centre for Grain Process Engineering, School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2012 Received in revised form 12 October 2012 Accepted 13 November 2012
Modern versions of the Single Kernel Characterization System allow force profiles recorded for individual kernels during crushing to be retrieved. The current understanding of the meaning of the averaged Crush Response Profile (aCRP) is challenged in the current work. When bran layers are removed, the features of the aCRP that have been interpreted as the bran layers’ elastic response and subsequent collapse are retained. By contrast, when the crease is removed, these features are largely eliminated. The crease depth correlates with the first peak force, providing further evidence that wheat kernels tend initially to break across the crease. The initial slope of the force-deformation curve depends on the inherent strength of the endosperm material opposite the crease, while the magnitude of the initial peak force depends on the strength and thickness of this material, and hence on the crease depth. It is concluded that through the averaging process, aCRPs give a misleading picture of wheat kernel breakage in the SKCS. The Hardness Index currently reported by the SKCS, which is based on identifying breakage events as indicated by the jaggedness of the force profile, would appear to give a more fundamentally sound basis for quantifying wheat properties relevant to breakage. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Wheat hardness Breakage Crease Bran
1. Introduction Hardness is, along with gluten strength, the most important characteristic of wheat (Pomeranz and Williams, 1990); indeed it could be argued that the twin themes of hardness and gluten quality underpin the majority of wheat research, from fundamental genetics to practical processing. The introduction of the Perten Single Kernel Characterization System (SKCS) provided the facility to report the hardness of individual wheat kernels in a sample, along with mass, moisture content and diameter (Martin et al., 1991, 1993). This has had several applications including identifying mixtures of wheat classes, understanding the origins of wheat hardness and understanding the effects of distributions of hardness on processing, as reviewed by Osborne and Anderssen (2003), who also discuss the model by which the SKCS Hardness Index is calculated.
Abbreviations: SKCS, Single Kernel Characterization System; HI, Hardness Index; aCRP, averaged Crush Response Profile; iCRP, individual Crush Response Profile; aNCRP, averaged Crush Response Profile constructed from normalised iCRPs. * Corresponding author. E-mail address: [email protected] (G.M. Campbell). 0733-5210/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jcs.2012.11.004
In more recent versions of the SKCS, greater computing power has allowed the individual force profiles recorded during crushing to be stored and retrieved. These individual Crush Response Profiles (iCRPs) are jagged and highly variable; in fact, the jaggedness is the basis for the calculation of the Hardness Index. However, averaging the iCRPs of a sufficiently large sample yields an averaged Crush Response Profile (aCRP) that is smooth and reproducible. The aCRP generally exhibits an initial small peak followed by a trough and a second, larger peak. Osborne et al. (2001) proposed an interpretation of this general aCRP curve, as shown in Fig. 1 (adapted from Edwards et al., 2007; Osborne et al., 2001, 2004; Osborne and Anderssen, 2003). They interpret the first peak as indicating the response to deformation of the “shell” of the wheat kernel, and the second peak as the response of the endosperm. The “shell” is interpreted as “the aleurone layer” (Osborne et al., 2001; Osborne and Anderssen, 2003) or as “the bran layer” (Anderssen and Haraszi, 2009), or for similar work on barley, as “the structure of the outer layers in barley grain” (Osborne et al., 2007b) or “the husk, pericarp and aleurone layers of the grain” (Iwami et al., 2005). Anderssen and Haraszi (2009) summarise the view, presented in all these papers, that information “can be recovered about the mechanical strengths of the internal botanical layers from the morphology of an aCRP”. They give a systematic description of their understanding of “the deformation and fracturing of a wheat kernel
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
Fig. 1. The generic structure of the averaged Crush Response Profile (aCRP) and its previous interpretation (adapted from Edwards et al., 2007; Osborne et al., 2001, 2004; Osborne and Anderssen, 2003).
during its crushing” as indicated by the aCRP, in a section entitled “The rheology of aCRPs”, structured into “The elastic response of the bran layer”, “Bran layer collapse into the structure of the endosperm”, “The equilibrium response”, “The viscoelastic compressive response of the endosperm” and “Collapse of the endosperm cellular structure”. They conclude “an aCRP compartmentalizes the rheology of wheat hardness into the two distinct and independent aspects of aleurone strength and elasticity and endosperm viscoelasticity”. The objective of the current work is to present evidence for an alternative interpretation of Crush Response Profiles. As the previous interpretation emphasised the response of the bran layers in interpreting the initial part of the aCRP, it seems natural to consider the response when the bran layers are removed; this is considered first. The crease, which is a distinctive feature of the wheat kernel, gives a natural point of weakness that seems likely to affect breakage; the effect of the crease on the aCRP is considered second. Thirdly, the strain at which wheat endosperm fractures is known from independent studies; this is compared with the strains corresponding to the “endosperm response” portion of the aCRP, to demonstrate that the wheat kernels must have fractured well before this point, such that they cannot be exhibiting a viscoelastic response of the endosperm prior to fracture. Finally, the nature of the averaging process is scrutinised, to demonstrate that the aCRP gives a misleading picture of the nature of wheat breakage during crushing in the SKCS. The utility of the SKCS Hardness Index is reevaluated in the light of these considerations. 2. Materials and methods The following studies employed the Perten Single Kernel Characterization System, Model 4100 (Perten Instruments, Sweden). 2.1. Effect of debranning on Crush Response Profiles For this study, two representative wheats, Malacca (hard, SKCS Hardness Index ¼ 68.4) and Consort (soft, SKCS Hardness Index ¼ 33.4) at 14.5% moisture content were debranned in a Satake TM05 laboratory debranner to remove 4, 8, 12 and 16% of the original material. 300 kernels of the debranned samples were tested in the SKCS, the individual CRPs for each kernel retrieved, and the aCRPs constructed by averaging the individual data using an Excel spreadsheet. 2.2. Effect of crease characteristics on Crush Response Profiles This study comprised three parts. In the first part, 100 kernels of each of a hard and a soft wheat (Malacca and Consort described
223
above) were broken or cut along the crease, to create 200 halfkernels from which the crease had been eliminated. Another 100 kernels of each wheat were cut across the crease, to create 200 halfkernels in which the crease had been retained. Each set of halfkernels was processed through the SKCS, along with 300 unbroken kernels of each wheat. The second part of this study measured the crease morphology and SKCS breakage patterns of a set of 128 wheats from another project. These wheats comprised 30 varieties harvested from three different sites and grown over four seasons in the UK, representing a wide range of commercial wheats and of contrasting endosperm texture, as described elsewhere (Sylvester-Bradley et al., 2010). These samples gave a convenient set of data that sheds light on the issue of aCRP interpretation. For each wheat, four kernels were manually sliced into two halves across the thickest part of the kernel. The germ end of each half was scanned using a commercial scanner with 6400 dots per inch (dpi) resolution (Epson V500, Epson, Japan), and the crease depth and total kernel depth measured with the aid of commercial image processing software (Adobe Photoshop Elements 4.0, Adobe systems Inc., USA). 2.3. aCRPs from different grains In the third part of this study, a range of grains, some with a crease, some without, were tested in the SKCS and their aCRPs constructed, to see whether the presence of a crease gave a pattern in the aCRP. 3. Results and discussion 3.1. Effect of debranning on Crush Response Profiles Fig. 2 shows the aCRPs following debranning of a hard and a soft wheat to 0, 4, 8, 12 and 16% removal. Clearly the removal of material gives a lower area under the curve e each kernel comprises less material and therefore less energy is required to break it. However, removal of the bran layers, even up to 16% of the original kernel weight, fails to eliminate the initial peak in the aCRP. In fact, the effect of bran removal appears to be predominantly in the second half of the curve (particularly for the hard wheat), in the portion previously conceived as “Endosperm response”. This is in agreement with the results of Osborne et al. (2004), who found that debranning wheat kernels similarly had little effect on the first peak and substantially affected the second peak. Those and the current results provide the first indicative evidence that the initial peak in the aCRP is not indicative of the rheological response of the “shell” layers. 3.2. Effect of crease characteristics on Crush Response Profiles In fracture mechanics terms, cuts and notches provide natural points of weakness that concentrate stress and initiate breakage (Bourne, 2002, p. 102). The wheat kernel is distinctive in featuring a crease. This provides a natural point of weakness (Dobraszczyk, 1994; Evers and Millar, 2002), and it seems reasonable to suppose that this point of weakness might affect breakage in the SKCS. Fig. 3 shows the aCRPs resulting from intact wheat kernels and from half-kernels created by separating either along or across the crease. Clearly the area under the curve is lower for the halfkernels, as is to be expected. For both the hard and the soft wheat, the half-kernels created by cutting across the crease retain the initial peak in the aCRP and appear to be, for all intents and purposes, smaller versions of the corresponding whole-kernel aCRPs. By contrast, the half-kernels created by breaking or cutting
224
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
Fig. 2. Averaged Crush Response Profiles following debranning of (a) a hard wheat and (b) a soft wheat.
along the crease, so that the crease was eliminated, look quite different. For the soft wheat, the first peak disappeared, and for the hard wheat it was considerably less pronounced. (The aCRPs are also considerably shorter than those for the whole kernels or the “cut across” half-kernels, suggesting breakage finished somewhat sooner for these long, thin half-kernels compared with their short, fat counterparts.) The results from Fig. 2 provided evidence that the first peak in the aCRP does not relate to the bran; the results in Fig. 3 provide initial evidence that the first peak relates to the crease. This is strengthened by considering Fig. 4, which shows the initial peak force versus the ratio of crease depth:kernel depth. There is a significant negative correlation of 0.468 (p < 0.001) which indicates that kernels with a relatively deeper crease are, not surprisingly, easier to break. The correlation between the initial peak force and the ratio crease cavity width:kernel width was even stronger at 0.608. The general message is that the crease characteristics affect the first peak of the aCRP. This is most easily conceptualised in terms of crease depth, as a relatively deeper crease implies less material in the area opposite the crease, and it is this material that breaks. (Note that the crease depth is typically 60e70% of the total thickness of the kernel; there is relatively little material holding the two halves of the kernel together.) These results provide further evidence that the initial breakage depends on the crease characteristics. The correlations are evident despite the fact that the wheats are of inherently different hardnesses; even though the material that is breaking is inherently variable in its breakage strength, and even though only a few kernels of each wheat were examined, the effect of crease geometry
Fig. 3. Averaged Crush Response Profiles resulting from (a) intact wheat kernels and from half-kernels created by separating either (b) across or (c) along the crease, for a hard wheat (Malacca) and a soft wheat (Consort).
is sufficiently pronounced to be visible. In terms of a mechanism, a deeper crease implies less material (which is predominantly endosperm) in the region opposite the crease (i.e. between the crease cavity and the dorsal side of the kernel). It seems reasonable that less material in this region would give a kernel that is easier to break. Thus the evidence from the correlations is consistent with a reasonable mechanical explanation, and consistent with the earlier evidence from manipulating the crease geometry. Additional evidence comes from the observation of Iwami et al. (2005) that there was a strong negative correlation between the initial slope of the aCRP and the percentage of broken kernels following pearling of barley. If this initial slope indicates the strength of the endosperm material opposite the crease, then it makes physical
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
Fig. 4. Effects of crease morphology on the initial peak force of the averaged Crush Response Profile (aCRP).
sense that weaker material breaks more readily to give broken kernels following pearling. A physical mechanism that interprets the initial slope as indicating the strength of the outer layers makes less physical sense, given that the outer layers are removed early on in the pearling process and would have little relevance to broken kernels produced after pearling to 35% of the initial whole grain weight. 3.3. aCRPs from different grains Fig. 5 shows aCRPs from a range of grains and other seeds, separated into those that feature a significant crease (barley, rye
Fig. 5. Averaged Crush Response Profiles from (a) seeds that feature a crease; and (b) seeds that do not feature a crease.
225
and triticale) and those that do not have a crease (rice, quinoa, mung bean and white pepper). As for wheat, the initial peak is clearly evident for those other grains that have a crease, but is absent, minimal or very different for seeds that do not. Osborne and Anderssen (2003) report aCRPs for durum, barley, soft wheat, sorghum, oats and rice; their results point to a similar pattern. Taken together, the results from processing debranned wheat, from half-kernels, from relating crease characteristics to the first peak, and from examining the aCRPs from seeds with and without a crease, seem to point strongly to a conclusion that the first peak in the aCRP is not related to breakage of the “shell” layers, but depends principally on the material opposite the crease e on its thickness (which depends on the crease depth) and on its mechanical properties (which depend principally on the strength of the endosperm). Thus a picture emerges of kernels that tend to break initially within the SKCS mechanism by breaking into two halves along the natural point of weakness of the crease. This is consistent with images reported by Fang and Campbell (2002) showing a kernel breaking along the crease during roller milling, and with the observation reported by Evers and Millar (2002) that wheat kernels with a closed crease tended to require a greater force to fracture than those with a more open crease. Kernels enter the SKCS rotor-crescent mechanism in somewhat random orientations, so undoubtedly some kernels do not break initially along the crease, but the tendency to break preferentially in this way seems to be evident in the data from Figs. 3e5. A further consideration regarding the interpretation of the aCRP is the known elastic and fracture responses of wheat endosperm. The endosperm of wheat kernels is brittle and able to accept deformation strains of the order of 3e10% before fracturing (Delwiche, 2000; Haddad et al., 1999). It can be calculated that the time intervals of the predominant upward slope of the aCRP correspond to strains of 20e60%, well after the initial breakage of the kernel. Thus it is not possible that this slope, which we call the “post-failure upslope”, indicates the elastic response of the unbroken endosperm. However, the first portion of the aCRP, which we call the “prefailure upslope”, does appear to give an indication of the inherent strength of the endosperm material opposite the crease. Prior to breakage, the kernel does undergo elastic deformation, such that this first portion of the curve can indeed be considered in some sense a stressestrain curve. (We say “in some sense”, because an under-acknowledged feature of the force profile generated by the SKCS is that the direction of the force changes during crushing; as the kernel passes along the arc between the rotor and the crescent, the load cell only measures the component of the force in one direction. Thus the magnitude of the force recorded in the early part of the curve is not directly comparable with that recorded in the later part of the curve.) Acknowledging the consequence of the geometrical arrangement of the SKCS crescent and load cell, nevertheless, the initial slope of this pseudo-stressestrain curve can be interpreted as a measure of strength or stiffness in a conventional fracture mechanics sense. Coate (2011), reanalysing the data for the set of 45 wheat samples later reported in Campbell et al. (2012), found a strong positive correlation (0.726, p < 0.001) between the average of the initial slopes from the iCRPs and the average SKCS hardness, although the correlation between the initial slope of the aCRP and the average SKCS hardness was weaker (0.340, p w0.01). Iwami et al. (2005) similarly reported a high correlation (0.527, p < 0.001) between this initial aCRP slope and SKCS hardness for barley samples. This provides evidence that this initial slope can be interpreted in a fracture mechanics sense to indicate the inherent stiffness (modulus of elasticity) of the endosperm material. It provides further evidence that the initial portion of the curve is influenced by endosperm properties rather than
226
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
bran properties. It also hints at the problem of calculating the aCRP as a simple average of the iCRPs. Experimental evidence has been presented that the kernel breaks preferentially along the crease and that the “pre-failure slope” that occurs prior to the first peak relates to endosperm stiffness. Evidence based on a consideration of the strains occurring in the SKCS and on the known fracture properties of wheat endosperm support this view that fracture occurs early on during crushing in the SKCS. This is further supported by examining the individual Crush Response Profiles from which the aCRP is generated. A key question is: What is the relationship between the aCRP and the iCRPs from which it is calculated. This is best illustrated by asking initially: What is the relationship between the maximum force of the aCRP and the average of the maximum forces of the iCRPs? Fig. 6(a) illustrates the iCRPs of six kernels of a wheat of HI ¼ 47.8, and the aCRP resulting from the averaging of just these six iCRPs. The curves are labelled using the maximum force recorded for each curve. An interesting observation is that the average of these peak forces is 1781 N, but the peak force of the average CRP is much lower at 1562 N (a 12% reduction). The reason is that the maximum forces for the iCRPs occur at different times, because the kernels are different sizes. Another way of looking at this is to ask: How much smaller is the maximum force of the aCRP (aCRPmax) than the average of the iCRPmax values (iCRPmaxave)? This depends on how variable the
iCRPs are. If they were all identical, aCRPmax and iCRPmaxave would be the same. They are not identical because the kernels are different sizes; the more variable the kernel sizes, the greater the variation. This is demonstrated in Fig. 6(b). Twelve samples, each of 300 kernels, were tested in the SKCS, and aCRPmax and iCRPmaxave compared for each sample. Clearly those samples that had more widely varying kernel diameters (as indicated by the reported standard deviation of diameters) gave greatly reduced values of aCRPmax compared with iCRPmaxave, while samples with more uniformly sized kernels gave a smaller discrepancy. The standard deviation of the kernel diameters accounted for 93% of the observed variation, indicating that the relationship between aCRPmax and iCRPmaxave is dictated overwhelmingly by kernel diameter variation. Another noteworthy observation is that as the broken kernels exit the SKCS crushing mechanism, the iCRP force drops rapidly to zero. The aCRP curve shows however a gradual reduction in the force. Anderssen and Haraszi (2009) describe this portion of the aCRP as “Collapse of the endosperm structure”, saying that it contains “information about the rheology of the wheat at this stage” in terms of “viscoelastic rigidity.contained in the conglomerate”. In fact, this slope is an artefact resulting from kernels of different size exiting the crushing mechanism at different times. More generally, many of the features of the aCRPs are likely to be determined by the variation in kernel size. Thus, two wheat samples of identical endosperm and bran mechanical
Fig. 6. Relationships between individual Crush Response Profiles and averaged Crush Response Profiles: (a) iCRPs of six kernels, and their corresponding aCRP; (b) The relationship between the percentage reduction of the peak force of the aCRP compared with the average peak force of the iCRPs, and the standard deviation of the kernel diameter; (c) And (d) the early portion of example iCRPs; (e) The iCRPs of (a) normalised against kernel diameter on the x-axis and against mass on the y-axis; (f) the average of 250 normalised iCRPs (aNCRP) and the original aCRP normalised against the average kernel diameter.
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
properties and the same average kernel size, but varying only in kernel size distribution, would yield different measures derived from their aCRPs. Some of the distinctions reported in the literature based on parameters derived from aCRP curves are probably no more than artefacts of variations in kernel size distribution. To take yet another, even more revealing, aspect, many of the iCRPs return to a zero force at points within the profile. Fig. 6(c) and (d) zoom in on the early portion of some of the curves in order to illustrate this. The curve labelled “1850” in Fig. 6(c) shows a sharp initial peak followed by a sharp drop to zero, followed by a smoother but similar sized peak and a second drop to zero, after which the force increases jaggedly as the collection of particles break together. The curve labelled “1845” shows a similar initial peak followed by essentially a single return to zero. In Fig. 6(d), the curve labelled “1957” returns to essentially zero for the entire period from about 12 to 50 ms, while the curve labelled “1842” has an initial small peak, returns to zero for the period from 5 to 17 ms, has a second peak, returns to zero at around 40 ms, then increases thereafter. Anderssen and Haraszi (2009) similarly show example iCRPs that drop to zero force after the initial peak or peaks. The aCRP, by contrast, never returns to zero midway through crushing, because the zero force events in the iCRPs occur at different times. The aCRP thus gives no hint regarding these patterns of breakage. These “returns to zero” indicate the production of particles that are, for a time, too small to traverse the gap between the rotor and the crescent and hence to transmit forces. This observation is a clue as to the nature of the breakage mechanisms occurring during crushing in the SKCS. Combining this with the above information about the crease, it appears as a general pattern that kernels tend to break into two halves along the crease, these two smaller particles are then too small to traverse the gap until they move further along the decreasing arc between the rotor and the crescent, at which point they begin to transmit force, deform and break further. The number of “return to zero” events point to the number of these discrete breakage events before the collection of particles fills the area between the rotor and crescent and begins to break as a conglomerate. Extending these observations to the entire profile, it becomes clear that the aCRP does not indicate the average of the forces involved in distinct, identifiable crushing events, since it is influenced by the variation in the time at which those events occur for different kernels. The aCRP does not give a meaningful representation of the breakage of an “average” kernel. In fact, no kernel breaks in a way suggested by the aCRP. By not taking account of the effects of kernel size on the timing of events in the iCRPs, the aCRP obscures important features and gives “average” measurements that are less meaningful than the average of the individual measurements. Interpreting the aCRP as if it indicated the breakage pattern of a typical, or indeed any, kernel is fundamentally misconceived. Osborne et al. (2007a) asserted that the SKCS “has previously been shown to provide in situ measurements of the rheological properties of the bran and endosperm layers of wheat, otherwise only possible following their isolation by dissection or machining”, in which the aCRP can “be interpreted as a pseudo stress/strain characterization of the crushing as it progresses from the outer grain layers into the endosperm”. The interpretation presented in Fig. 1 was proposed in 2001 but never explicitly tested or demonstrated and cannot be considered as established fact, as that paper and others imply. The evidence presented in the current paper demonstrates that this interpretation is incorrect. We acknowledge the possibility of observing correlations between aCRP features and other measures of wheat properties or performance (Anderssen and Haraszi, 2009; Edwards et al., 2008; Iwami et al., 2005; Osborne et al., 2007a). This underlines that there is a difference
227
between a correlation and a mechanistic link, and that correlations, in the absence of mechanisms, do not provide reliable guides to the underlying physics. We therefore dispute the mechanistic interpretations in these and other papers that have tried to relate kernel characteristics to aCRP features, on the basis that the interpretation of aCRP features as relating to bran or endosperm is flawed in all of those papers. We acknowledge that there may still be useful information in the aCRP, even if the current interpretation is incorrect. We propose, however, that the interpretation of the aCRP would be both more meaningful (as a physical representation of wheat kernel breakage) and more powerful (in terms of yielding quantitative measurements related to wheat breakage and other aspects of wheat genetics or usage) if the aCRP were constructed from iCRPs in a way that takes account of the time variation. It would also be appropriate to account for the effect of kernel mass. Osborne et al. (2004) divided the aCRP force by the average kernel mass, in order to take account of mass removal following debranning of wheat. We propose the construction of aCRPs from iCRPs for which the force is normalised against each kernel’s mass, and the time is normalised against kernel diameter and the speed of the rotor, in order to obtain a dimensionless time. This creates a problem for averaging, that the intermediate times are now different for each kernel. In order to average, it is necessary to interpolate the intermediate forces for a standardised set of normalised times. Fig. 6(e) shows the interpolated iCRPs of Fig. 6(a) normalised against kernel diameter on the x-axis and against mass on the yaxis. The crushing rotor in the SKCS is 127 mm in diameter and it rotates at 120 rpm, giving a tip speed of 800 mm s1. A typical crushing time for a 3 mm kernel is 125 ms, corresponding to a movement of the rotor edge of 100 mm Thus, a typical 3 mm kernel crushed over a 125 ms timescale would yield a dimensionless time of 0.125 800/3 ¼ 33.33, i.e. ignoring slippage, crushing takes place over a distance approximately 30 times the diameter of the kernel. The x-axis of Fig. 6(e) can thus be interpreted as “Equivalent kernel diameters” (acknowledging that the decreasing gap between the rotor and crescent obscures precise interpretation). Interpolation was performed in an Excel spreadsheet using the following function (Anonymous, 2012): ¼FORECAST(NewX,OFFSET(KnownY,MATCH(NewX,KnownX,1)1,0,2), OFFSET(KnownX,MATCH(NewX,KnownX,1)-1,0,2)) There is a tantalising suggestion in Fig. 6(e) that normalisation has tended to make equivalent breakage events coincide, such that in the aCRP of these six kernels, several troughs become evident at around 2, 7.5, 12 and 14 ms that correspond to relatively consistent breakage patterns in the different kernels. However, as Fig. 6(f) shows, by the time the normalisation and averaging is applied to 250 kernels (dictated by the limitations of the size of the spreadsheet that Excel can handle), the curve once again becomes smooth. Fig. 6(f) compares the average of the normalised iCRPs (aNCRP) with the original aCRP (normalised on the x-axis against the average kernel diameter and on the y-axis against average kernel mass). The aNCRP has a similar overall shape, with a similar prefailure upslope and trough, longer post-failure upslope and sharper final drop. The initial pre-failure slope is similar for the two curves, for this particular example, while the final sharp drop in the aNCRP force profile is closer to the pattern of real kernels; however, the rest of the aNCRP continues to give a representation that does not meaningfully indicate the breakage of actual kernels. We suspect that, by avoiding the confounding effects and artefacts arising from variations in kernel size distribution, summary information extracted from the aNCRP would give better correlations with other measures of wheat processing and end-use quality than from the aCRP (but at a cost of greater complexity in constructing the
228
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229
aNCRP). To take one measure, the longer post-failure upslope in the aNCRP is likely to allow a measure of this slope that is more accurate than from the (non-normalised) aCRP, and that therefore correlates better with the currently reported SKCS Hardness Index and with other factors affecting or affected by hardness. We acknowledge that the aCRP or aNCRP may contain useful measures of wheat properties that might be used to inform studies of wheat genetics, processing and end use performance. However, with the exception of the initial pre-failure slope, which indicates endosperm stiffness, and the initial peak, which relates to both endosperm stiffness and the thickness of endosperm material opposite the crease, we caution against trying to identify specific features of the aCRP or aNCRP as descriptive of specific aspects of wheat kernel morphology or breakage. In our view, the Hardness Index currently reported by the SKCS is fundamentally more soundly conceived than measures based on averaged profiles. The Hardness Index is calculated based on the jaggedness of the iCRP, measured as the total number of positive crush force slope values, and the number with a value greater than 8 N (corresponding over a 25 ms interval to a rate of increase greater than 320 N s1). These measures are used to calculate the Hardness Index using an unpublished model. As the relevance of wheat hardness is in relation to how the wheat kernel breaks, and as the jagged features of the iCRP indicate breakage events, it seems that an interpretation of hardness that utilises these jagged features is fundamentally more meaningful than an interpretation that averages these jagged features out. Similarly, information about the behaviour of specific botanical layers in the kernel is more likely to reside in the jagged profile than in the smooth averaged profiles. Meanwhile, the most directly relevant consequence of wheat kernel hardness, breakage during roller milling, has proved to relate well to the currently reported Hardness Index (Campbell et al., 2007, 2012). We acknowledge that there may be scope to improve the model currently used to calculate hardness, and that there may be complementary information in averaged Crush Response Profiles. Nevertheless, despite the attractions of the smooth aCRP or aNCRP curves, we would encourage continued use of the iCRPs as a firmer basis for extracting meaningful information about wheat breakage. 4. Conclusions The current work demonstrates that when the bran layers are removed, the features of the aCRP that have been interpreted as the bran layers’ elastic response and subsequent collapse are retained. By contrast, when the crease is removed, these features are largely eliminated. Meanwhile, the crease depth correlates with the first peak force, providing further evidence that the initial breakage is in some respect a function of the crease. A picture thus emerges of breakage of wheat kernels in the SKCS in which kernels tend initially to break along the crease. The initial slope of the forcedeformation curve depends on the inherent strength of the endosperm material opposite the crease, while the magnitude of the initial peak force depends on the strength of this material as well as its thickness, and hence on the crease depth. Averaged Crush Response Profiles give a misleading picture of the nature of wheat kernel breakage in the SKCS, in part because the timings of breakage events are different for kernels of different size. Constructing averaged Crush Response Profiles from iCRPs that have been normalised against each kernel’s diameter may give a stronger basis for extracting information about wheat quality and its relation to genetics, processing performance and end-use, although this entails greater computational complexity. However, the current SKCS Hardness Index, which is based on breakage events indicated by the jaggedness of the force profile, appears to
give a more meaningful basis for quantifying wheat properties related to breakage. Acknowledgements NM acknowledges the Home-Grown Cereals Authority (HGCA) for funding for PhD studies, and the GREEN Grain consortium for provision of wheat samples (funding provided through the UK Sustainable Arable LINK programme, sponsored by the Department for Environment Food and Rural Affairs and the Scottish Government). The Satake Corporation of Japan is gratefully acknowledged for its help in establishing the Satake Centre for Grain Process Engineering. References Anderssen, R.S., Haraszi, R., 2009. Characterizing and exploiting the rheology of wheat hardness. European Food Resources Technology 229, 159e174. Anonymous, 2012. http://www.blueleafsoftware.com/Products/Dagra/LinearInter polationExcel.php, (accessed 19.07.12.). Bourne, M., 2002. Food Texture and Viscosity; Concept and Measurement, second ed. Academic Press, London, UK. Campbell, G.M., Fang, C.Y., Muhamad, I.I., 2007. On predicting roller milling performance VI. Effect of kernel hardness and shape on the particle size distribution from First Break milling of wheat. Transactions of the Institution of Chemical Engineers Part C: Food and Bioproducts Processing 85, 7e23. Campbell, G.M., Sharp, C., Wall, K., Mateos-Salvador, F., Gubatz, S., Huttly, A., Shewry, P., 2012. Modelling wheat breakage during roller milling using the Double Normalised Kumaraswamy Breakage Function: effects of kernel shape and hardness. Journal of Cereal Science 55, 415e425. Coate J., 2011. Effects of Wheat Kernel Shape and Hardness on the Normalised Kumaraswamy Breakage Function. MEng. Dissertation, University of Manchester. Delwiche, S.R., 2000. Wheat endosperm compressive strength properties as affected by moisture. Transactions of the American Society of Agricultural Engineers 43, 365e373. Dobraszczyk, B.J., 1994. Fracture mechanics of vitreous and mealy wheat endosperm. Journal of Cereal Science 19, 273e282. Edwards, M.A., Osborne, B.G., Henry, R.J., 2007. Investigation of the effect of conditioning on the fracture of hard and soft wheat grain by the single-kernel characterization system: a comparison with roller milling. Journal of Cereal Science 46, 64e74. Edwards, M.A., Osborne, B.G., Henry, R.J., 2008. Effect of endosperm starch granule size distribution on milling yield in hard wheat. Journal of Cereal Science 48, 180e192. Evers, T., Millar, S., 2002. Cereal grain structure and development: some implications for quality. Journal of Cereal Science 36, 261e284. Fang, C., Campbell, G.M., 2002. Stress-strain analysis and visual observation of wheat kernel breakage during roller milling using fluted rolls. Cereal Chemistry 794, 511e517. Haddad, Y., Mabille, F., Mermet, A., Abecassis, J., Benet, J.C., 1999. Rheological properties of wheat endosperm with a view on grinding behaviour. Powder Technology 105, 89e94. Iwami, A., Osborne, B.G., Huynh, H.-N., Anderssen, R.S., Wesley, I.J., Kajiwara, Y., Takashita, H., Omori, T., 2005. The measurement of structural characteristics of barley for Shochu using single-kernel characterization system 4100 crushresponse profiles. Journal of the Institute of Brewing 111, 181e189. Martin, C.R., Rousser, R., Brabec, D.L., 1991. Rapid Single Kernel Grain Characterization System, US Patent No 5005774. Martin, C.R., Rousser, R., Brabec, D.L., 1993. Development of a single kernel wheat characterisation system. Transactions of the American Society of Agricultural Engineers 36, 1399e1404. Osborne, B.G., Anderssen, R.S., 2003. Single-kernel characterization principles and applications. Cereal Chemistry 80, 613e622. Osborne, B.G., Turnbull, K.M., Anderssen, R.S., Rahman, S., Sharp, P.J., Appels, R., 2001. The hardness locus in Australian wheat lines. Australian Journal of Agricultural Resources 52, 1275e1286. Osborne, B.G., Anderssen, R.S., Huynh, H.-N., 2004. In situ measurement of the rheological properties of wheat and barley grain using the SKCS 4100. pp. 207e 211. In: Cauvain, S.P., Salmon, S.S., Young, L.S. (Eds.), Using Cereal Science and Technology for the Benefit of Consumers: Proceedings of the 12th ICC Cereal and Bread Congress, Harrogate, 23rde26th May 2004, Woodhead Publishing Ltd., Cambridge, UK. Osborne, B.G., Henry, R.J., Southan, M.D., 2007a. Assessment of commercial milling potential of hard wheat by measurement of the rheological properties of whole grain. Journal of Cereal Science 45, 122e127. Osborne, B.G., Fox, G.P., Kelly, A.M., Henry, R.J., 2007b. Measurement of barley grain rheology for the quality selection of breeding material. Journal of the Institute of Brewing 113, 135e141. Pomeranz, Y., Williams, P.C., 1990. Wheat hardness: its genetic, structural and biochemical background, measurement and significance. In: Pomeranz, Y. (Ed.),
N. Misailidis, G.M. Campbell / Journal of Cereal Science 57 (2013) 222e229 Advances in Cereal Science and Technology, vol. 10. American Association of Cereal Chemists, St. Paul, Minnesota, USA, pp. 471e544. Sylvester-Bradley, R., Kindred, D.R., Weightman, R.M., Thomas, W., Swanston, J.S., Thompson, D., Feuerhelm, D., Creasy, T., Argillier, O., Melichar, J., Brosnan, J., Agu, R., Bringhurst, T., Foulkes, J.F., Pask, A., Cowe, I.,
229
Hemongway, D., Robinson, D., Wilcox, S., 2010. Genetic Reduction of Energy Use and Emissions of Nitrogen through Cereal Production: GREEN Grain. HGCA Project Report No. 468. Home Grown Cereals Authority, Stoneleigh Park, Kenilworth, Warwickshire, CV8 2TL, UK, June 2010.