Influence of growth conditions on barley starch properties

International Journal of Biological Macromolecules 21 (1997) 37 – 45

Influence of growth conditions on barley starch properties Richard F. Tester * Glasgow Caledonian Uni6ersity, Department of Biological Sciences (Food Science), Southbrae Campus, Southbrae Dri6e, Glasgow, G13 1PP, Scotland, UK Received 19 July 1996; received in revised form 2 December 1996; accepted 19 February 1997

Abstract Air equilibrated barley starch comprises amylopectin, amylose, lipid and water. The structure of amylose and amylopectin, and the proportion of amylose in granules is under genetic control and is therefore subject to genotypic variation. The amount of lipid (which is essentially all lysophospholipid) is similarly under genetic control. Environment and especially environmental temperature do, however, have a regulatory effect on the size of starch granules, the amylose to amylopectin ratio and the amount of lipid (which is essentially all complexed with amylose) within barley starch. High growth temperatures probably facilitate amylopectin crystallisation and increase gelatinisation temperatures, (and to some extent the enthalpy of gelatinisation), but delay the onset and depress the extent of swelling of granules when heated in water. © 1997 Elsevier Science B.V. Keywords: Starch; Gelatinisation; Crystallisation

1. General properties Barley starch is composed of a bimodal distribution of starch granules, the large ones (A-type) having a mean diameter of about 10 – 15 mm, with the small ones (B-type) ranging from about 2 – 4 mm [1]. The composition of barley starch granules is subject to quite a large genotypic variation [1 – 4] which is presented in Table 1. Unlike other members of the triticeae, barley starches incorporate a broad range of amylose and lipid contents * Tel.: + 44 141 3374934; fax: + 44 141 3374600.

with the waxy mutants containing in some cases less than 2% amylose and 0.2% lipid, normal cultivars containing circa 28–33% amylose and 0.7–1% lipid, with high amylose mutants containing more than 54% amylose and 1.3% lipid. In common with other cereal starches, part of the amylose fraction exists as lipid complexed amylose (LAM) although a proportion is lipid free (FAM). Within LAM, the lipid accounts for 12.5% of the complex [5]. The ‘total’ amylose fraction of cereal starches determined after defatting with ethanol [6] includes LAM and FAM, whilst the ‘apparent’ amylose fraction includes

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Table 1 Composition of waxy, normal and high amylose barley genotypes Genotypes

Total amylose (%)

Phosphorus (mg/100 g)

Lipid (mg/100 g)

Waxy Normal High amylose

1.7–8.3 27.5–32.7 40.0–54.7

9.5 – 37.2 47.0 – 65.8 74.5 – 106

174 – 630 674 – 1032 828 – 1299

Adapted from [1 – 4].

only FAM. The difference between total and apparent amylose (total minus apparent) or D-amylose is proportional to the LAM content. Most of the lipid in barley starch is in the form of lysophospholipid (LPL) which can be quantified directly by gas liquid chromatography (GLC) of the constituent fatty acid methyl esters (FAME) or it can be calculated indirectly from the starch phosphorus content [1]. In common with other cereal starches, there is a strong positive correlation between amylose and lipid in barley starches [5]. In comparison to some other cereal and non-cereal starches, there is not a great deal of information concerning the size and structure of barley amylose and amylopectin. However, according to DeHass and Goering [7] the amylose fraction of barley starch comprises a molecular weight (Mn) of 190–260 000 while the corresponding weight of amylopectin is of the order 3.6 – 4.1 million. It is recognised that both the amylose and amylopectin fractions of all starches are branched, although amylose branching is quite light [8]. If barley amylopectin is debranched with iso-amylase and the unit chains are fractionated by for example gel permeation chromatography (GPC) or high performance liquid chromatography (HPLC), three major populations can be identified corresponding to approximately DP 10 – 12, 18 – 20 and 45 – 50 [2,9]. The DP 10–12 and 18 – 20 fractions represent A- and small B-chains [8] which are able to form double helices and, if in register, crystalline domains. There is little difference in this distribution for different barley genotypes [1]. When starch granules are heated in excess water, double helices (created from the exterior chains of amylopectin) progressively uncoil and the granules gelatinise and swell. The gelatinisa-

tion onset (To), peak (Tp) and conclusion (Tc) temperatures plus enthalpy (DH) of gelatinisation (usually determined by differential scanning calorimetry, DSC) of starches are in part heritable characteristics, which exhibit some genotypic variability [5]. There is also some variation in the temperature and enthalpy of the higher temperature amylose-lipid dissociation endotherm. Typical amylopectin gelatinisation and amylose lipid dissociation parameters for field grown (Dundee, Scotland) waxy and normal barley starches are presented in Table 2. It is difficult to precisely define the peak gelatinisation temperature for high amylose starches because they exhibit a very flat endotherm. In common with other cereal starches, barley starches have characteristic temperature dependent swelling properties and profiles when heated in excess water [1,5,10,11]. Swelling factors (SF) can be easily determined using blue dextran dye exclusion [10]. There is an onset swelling temperature at circa 40°C which corresponds to the onset of gelatinisation by DSC, an essentially linear swelling region from 40 to 60°C and a plateau region beginning at temperatures above 60°C. At temperatures above 85°C barley starch granules begin to disintegrate and this complicates understanding of temperature dependent swelling properties. The regression equation describing the volume expansion or SF of waxy (containing at least 1.7% total amylose and 0.12% lipid) and normal barley starch heated in excess water at 80°C in terms of the weight fraction of amylopectin (AP) and both free and complexed amylose has been calculated by Morrison et al. [5], where: SF= 41.1(AP+ 0.8× FAM)(1− 2.7 LAM0.485).

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Table 2 Gelatinisation characteristics of waxy and normal barley starches grown in field plots (Dundee, Scotland) Genotype

Waxy Normal

Starch gelatinisation endotherm

LAM dissociation

To (°C)

Tp (°C)

Tc (°C)

DH (J/g)

To (°C)

DH (J/g)

40.7 – 50.7 42.2 – 45.8

55.6–60.1 53.1–57.3

74.0–78.0 69.3–73.2

11.7 – 14.0 7.8 – 10.6

— 94.4 – 98.5

— 1.0 – 3.0

Adapted from [5]. To, Tp and Tc are onset, peak and conclusion temperatures of amylopectin gelatinisation or LAM dissociation and DH represents the transition enthalpy.

This equation does not necessarily take into account the role of leached a-glucan (as FAM) on the barley starch swelling characteristics, but does however account for most of the variation in swelling properties of normal barley starches. In addition, according to this equation all amylose and lipid free waxy starches should have a SF of 41.1 at 80°C and that additional factors like starch damage (which facilitates swelling) can be ignored—which is unlikely to be true. These additional factors are currently under investigation by this author.

2. Influence of growth conditions on barley starch composition The biochemistry of barley starch synthesis has been investigated in relation to the activity of key enzymes involved in the biosynthesis and the consequence of inactivation of particular enzymes on the number and size of granules and hence grain yields. In one study [12], barley plants were grown in a glass house and transferred to growth chambers just prior to anthesis and enzymes involved in starch biosynthesis were assayed. This work indicated that at high temperatures UDP sucrose synthase activity was depressed (although in comparison to wheat [13] one would expect soluble starch synthase activity to also be depressed and perhaps the metabolism and transport of hexose phosphates [14] and that the effects were initiated by a short period of thermal stress close to anthesis. In a related piece of work [15], the same authors used particle sizing to determine what effect temperature induced stress had on both the

size and distribution of barley starch granules. These authors concluded that decreases in dry matter accumulation at high temperatures were due to reductions in both the volume available for starch accumulation and the numbers (of both Aand B-type granules), rather than sizes of starch granules deposited. More recent work on waxy, normal and high amylose barley genotypes grown in constant environment chambers with constant illumination post anthesis [2], indicated that temperature induced stress reduces both the size of A- and B-type granules and the number of B-type granules. However, this work highlighted difficulties with interpretation of results in that different genotypes respond differently to environmental temperature and that adaptive features of individual genotypes are superimposed upon the temperature induced biochemical control and effects. Furthermore, different lighting protocols and sources of light cause additional variation. Work on wheat grown at a range of temperatures in a field in polythene tunnels (which, unlike the barley were subject to diurnal variation) has also shown that higher temperatures cause a decrease in the size of granules and the number of amyloplasts per endosperm [16]. It is obvious that although there are clear variations attributed to temperature stress in both these systems, it is difficult to make direct comparisons between the barley and wheat experiments because of the different species and growing environments employed. However, it does appear that temperature can influence cereal (and in particular barley) yields dramatically and is at least as important a variable as genotype.

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In maize and rice, higher growth temperatures tend to cause a reduction in amylose contents [17 – 21], in potato there is little effect of growth temperature [22] whilst in wheat amylose tends to increase with temperature [16,23]. In soybean seedling hypocotyls elevated growth temperatures produce a decrease in amylose content whilst in seedling cotyledons there is tattle effect [22]. Environmental effects on the amylose content of sweet potato starch appear to be variable [24]. The composition of barley starch has also been reported to be affected to some extent by environment. Goering et al. [25] determined the amylose content of barley cultivars grown under a number of ‘environmental and cultural practices’ and concluded that ‘‘since environmental influence and cultural practices were found to be insignificant, it was possible to examine samples from any available source with some assurance that any marked differences noted would be genetic differences due to variety’’. They claimed that they found amylose contents ranging from 19 – 23% (which they considered to be small) for Compana grown under different conditions whilst (bigger) differences of 13 – 26% were found within different barley varieties. Their data was, however, compromised by the fact that they did not take into account the partitioning of amylose into FAM and LAM and consequently underestimated amylose. Tester et al. [2] reported that in barley there was little effect of growth temperature on the amylose content of normal or waxy cultivars of this cereal. However, they showed that when a high amylose cultivar (Glacier Pentlandfield) was grown at 15 rather than 10°C, there was a 27% decrease in the total amylose content of the starch although there was little change between 15 and 20°C. Although the amylose content of barley starches (for most cultivars) is not dramatically altered by growth temperature, the lipid content tends to increase in waxy, normal and high amylose genotypes as a function of temperature. Tester et al. [2] showed that cultivars grown at 20 rather than 10°C contained about 50% more lipid (as lysophospholipid). This was associated with an increase in the LAM to FAM ratio. Similar results have been reported for wheat [23] although there is some seasonal variability [16]. The reason

for this increase is not clear, nor indeed is any role of the lipid in the biosynthesis of starch granules in cereal grains. Whilst it is tempting to speculate that the lipid controls and regulates the biosynthesis of amylose and maintains the amylose to amylopectin ratio, this ratio is largely independent of temperature in for example potato starches which contain no lipid. There appears to be little effect of growth temperature on the structure of barley starch a-glucans. Using gel permeation chromatography (GPC) to separate solubilised native barley starches grown at 10, 15 or 20°C, this author and colleagues have never been able to identify any differences in the size or polydispersity of the amylose or amylopectin fractions. Furthermore, chromatograms of ’debranched’ (selectively treated with isoamylase to hydrolyse a-(1–6) bonds only) starches extracted from plants grown at 10, 15 or 20°C are essentially identical [2] and contain the three major unit chain populations (as previously described) at DP 10–12, 18–20 and 45–55. In other cereals, however, it has been reported that environmental variation during starch biosynthesis does effect the branching pattern of the constituent a-glucans. In rice, elevated environmental temperature leads to a reduction in the size of amylose molecules and increased amylopectin chain lengths [18–21]. In wheat, elevated growth temperatures reportedly increase the proportion of amylopectin unit chains with a DP of 10–16 but reduce the proportions of unit chains with a DP of 17–21 [23].

3. Influence of growth conditions on barley starch physical properties The characteristic gelatinisation temperatures (To, Tp and Tc by DSC) of waxy, normal and high amylose barley starches [2], in common with wheat [16,23], are all increased as growth temperature is elevated (Table 3). This is associated with an increase in the enthalpy of gelatinisation. Similar effects have been reported for maize starch where planting dates, environmental temperature and day length have been studied as variables [26]. Pasting temperatures (which are comparable

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Table 3 Effect of growth temperature on the gelatinisation characteristics of waxy, normal and high amylose barley starches Genotype

Growth

Starch gelatinisation endotherm

Temperature (°C)

To (°C)

Tp (°C)

Tc (°C)

DH (J/g)

Waxy Waxy Waxy

10 15 20

43.1–44.0 45.0–51.5 48.5–50.3

52.1 – 54.1 57.5 – 61.6 62.7 – 63.1

70.0 – 70.5 75.9 – 76.0 73.0 – 83.0

10.9 – 12.2 11.7 – 13.2 12.0 – 13.8

Normal Normal Normal

10 15 20

39.8–43.2 40.0–49.4 50.2–54.0

49.6 – 52.1 51.8 – 60.8 59.6 – 64.5

65.6 – 71.9 68.5 – 74.0 72.1 – 80.0

6.4 – 9.7 8.3 – 10.3 9.8 – 10.3

High amylosea High amylose High amylose

10 15 20

42.0–42.2 47.0–50.2 50.0–52.5

55.1 – 55.7 57.0 – 62.0 61.7 – 64.8

71.8 – 75.5 77.0 – 79.1 75.1 – 82.8

6.4 – 7.5 8.0 – 8.9 7.3 – 8.1

Adapted from [2]. To, Tp and Tc are onset, peak and conclusion temperatures of amylopectin gelatinisation and DH represents the transition enthalpy. a It is difficult to determine the gelatinisation temperatures for high amylose starches with a high degree of precision in view of the relatively flat peaks.

to parameters measured by DSC) also increase for potato, sweet potato and soy bean seedling starch as a function of increasing environmental temperature [22]. The mechanisms regulating the crystallisation of starches and the influence of environmental conditions on these mechanisms are uncertain, but it is hypothesised by this author that elevated growth temperature directly enhances crystallite formation in vivo and this is comparable to annealing in vitro. Double helix formation during starch synthesis is probably driven by thermodynamic forces alone (which has similarities with the physical processes which operate during retrogradation in food systems) and will, therefore, be operating in conjunction with, but not driven by, specific synthetic steps active during the deposition of starch. If we assume that double helix formation is spontaneous upon starch synthesis because this is the most thermodynamically favourable state, it seems unlikely that growth temperature increases the number of double helices but probably more likely influences optimisation of crystalline register by packing the double helices together. This might involve conformational reorganisation within both the crystalline and amorphous zones as a consequence of growth temperature to facilitate the association of the

double helices. However, it is not yet certain if environmental conditions, which influence starch synthesis by regulating biosynthetic enzyme activity, directly affect the formation of double helices or more probably the association of these double helices into crystalline regions. Apart from the effects of environment on the crystallinity of barley starches, it is also important to recognise that although amylose content is not greatly affected by growth temperature, the amount of lipid does increase (as previously mentioned) and this is associated with an increase in the gelatinisation temperature. Morrison et al. [5] calculated regression equations to define Tp for 12 waxy and six normal field grown barley starches which take into account the amount of amylopectin crystallisation, FAM and LAM. Accordingly, they proposed that: Tp(°C)= 54.67+ (1.407 LAM−0.375 FAM). The Tp of a completely waxy granule would, therefore, be 54.67°C. Although the equation does not necessarily apply to barley starches grown under different environmental conditions than those employed by Morrison et al., it can be seen that an increase of LAM does favour higher gelatinisation temperatures whilst an increase in FAM has the opposite effect.

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Table 4 Effect of annealing temperature on the gelatinisation temperature of waxy, normal and high amylose barley starches Genotype

Growth temperature (°C)

Control

Tp annealing temperature (°C) 45

55

Waxy Waxy Waxy

10 15 20

52.1 57.5 63.1

63.3 64.1 65.8

69.6 69.0 69.6

Normal (G. Promise) Normal (G. Promise) Normal (G. Promise)

10 15 20

49.6 51.8 62.8

63.5 63.4 67.7

None 70.0 71.9

Normal (Triumph) Normal (Triumph) Normal (Triumph)

10 15 20

49.8 52.6 64.6

62.4 62.2 67.9

None 68.3 72.8

High amylose High amylose High amylose

10 15 20

55.1 57.0 64.8

65.9 65.0 68.8

70.4 70.4 72.7

Adapted from [2]. Tp is the peak temperature of amylopectin gelatinisation.

Regardless of the mechanisms controlling the formation of double helices and subsequent crystallisation of amylopectin, it is evident that reliance on a particular cultivar for a given food application (because of perceived quality attributes of the starch) is of limited value, unless the different environmental effects experienced during starch deposition are documented and understood. Environmentally induced variations in the crystallisation of amylopectin, amount of free and lipid complexed amylose and associated properties of starch granules (from similar genotypes) are probably as big as any influences exerted genetically. This contradicts the traditional view of others [25] who have considered that genotypic variation is bigger than variation caused by environment. It has been suggested by Tester and Morrison [10] that Tp represents a measure of starch crystallite perfection whilst DH represents the amount of crystalline amylopectin, although some authors would contend [27] that the gelatinisation process primarily involves hydrogen bond rupture within double helices and it is these double helices which are measured by DSC. Theoretically, if Tester and Morrison were correct, starches extracted from

plants grown at 10, 15 or 20°C (presented in Table 3) would have the same gelatinisation characteristics if further crystallised under optimum conditions. Annealing experiments at both 45 and 55°C of these starches (Table 4) has shown that although partial gelatinisation occurs (in that annealing temperature is greater than To) at 55°C for some starches (extracted from grains grown at 10 and 15°C), almost constant values for Tp (ca. 70°C) were approached. Maximum crystallinity must have, therefore, been attained which supports the hypothesis of Tester and Morrison. Similar results have been obtained for waxy rice starches [28]. In association with effects of environmental temperature on the gelatinisation properties of barley starches, there is a marked effect on swelling properties. The increase in onset temperature of gelatinisation as a function of growth temperature is associated with an increase in the onset temperature of swelling (Table 5). Growing the barley plants at 20 rather than 10°C causes the onset temperature of swelling to increase by at least 10°C, in some cases more. The swelling factor at 60°C (which is around the middle of the gelatinisation endotherm by DSC for most

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Table 5 Effect of growth temperature on the swelling onset temperature and higher temperature swelling factors of waxy, normal and high amylose barley starches Genotype

Growth temperature (°C)

Onset temperature of swelling (°C)

Swelling factor measured at (60°C)

(80°C)

Waxy Waxy Waxy

10 15 20

40.0 45.0 50.0

10.0 10.5 7.5

15.0 17.0 13.5

Normal (G. Promise) Normal (G. Promise) Normal (G. Promise)

10 15 20

40.0 40.0 60.0

5.0 4.5 1.0

9.0 8.0 4.0

Normal (Triumph) Normal (Triumph) Normal (Triumph)

10 15 20

37.5 40.0 50.0

5.5 4.5 3.0

10.0 9.0 7.0

High amylose High amylose High amylose

10 15 20

40.0 45.0 55.0

2.5 3.5 1.2

4.0 6.0 2.5

Adapted from [2].

starches, Table 3) tends to decrease as a consequence of elevating growth temperature (Table 5). This is primarily because the gelatinisation temperature has increased and higher temperatures are necessary to cause crystallite dissociation and facilitate swelling in water. In reference to the previously described equation where: SF= 41.1(AP+0.8×FAM)(1 −2.7 LAM0.485), it is clear that if an increase in growth temperature has no effect on the amylose to amylopectin ratio but increases the amount of lipid and hence LAM, the contents of the second brackets decrease and hence so does SF. One would, therefore, expect there to be less effect of lipid on SF for waxy than normal genotypes, which themselves would be less affected than high amylose genotypes. This does tend to be reflected in Table 5, although in practice the normal cultivar ‘Golden Promise’ has a growth temperature dependent reduction of approximately 50% in SF at 80°C which is proportional to the decrease identified for the high amylose (Glacier Pentlandfield) cultivar.

4. Industrial significance In comparison to wheat, maize, rice and potato starches, barley starch is not commonly used as a food ingredient. Barley is, however, the prime source of starch and hence glucose for the production of alcoholic beverages in most industrialised countries. When malted barley is mashed, a process whereby starch is gelatinised in water and hydrolysed by amylases present in the malted grain, it is important that temperatures sufficiently high to cause complete gelatinisation of the starch without inactivating amylases are used. If gelatinisation is restricted during this process, incomplete conversion to dextrins, sugars and ultimately alcohol will occur. Taking a typical mashing temperature range of 64–65°C in reference to Table 3 it is clear that starch within grain grown at 10°C will be much more effectively gelatinised and swollen in this temperature range than starch within grain grown at 20°C. Consequently the starch produced at 20°C is less easily hydrolysed by amylases and hence ultimately conversion to alcohol is restricted. Brewers therefore potentially lose in two ways if growth temperature is increased during starch biosynthesis in barley.

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Firstly, because the proportion of starchy endosperm in grain is reduced there is less substrate available for alcohol production. Secondly, the starch may not be fully gelatinised and this might limit hydrolysis by amylases and conversion to alcohol. There is, therefore, a need for maltsters and brewers to address the role of environment in modifying malt quality and for brewers to optimise alcohol production based on a knowledge of environmental conditions present during grain filling. More research is necessary on this area to improve both the financial and quality aspects of alcoholic beverage production.

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[27] Cooke D, Gidley MJ. Loss of crystalline and molecular order dunug starch gelatinisation: Origin of the enthalpic transition. Carbohydr Res 1992;227:103 – 12. [28] Tester RF, Morrison WR. Swelling and gelatinisation of cereal starches. I. Effects of amylopectin, amylose and lipids. Cereal Chem 1990;67(6):558 – 63.