Canadian Grain Commission
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The implications of frequently encountered grading factors on the processing quality of common wheat



4. Factors affecting processing performance

Orange wheat blossom midge

The orange wheat blossom midge (Sitodiplosis mosellana Géhin) is a prevalent pest in the wheat-growing areas of Europe and Asia. Periodic midge outbreaks are also common in the Great Northern Plains of North America, most recently in 1996.

Barnes (1956) has detailed the biology and life cycle of the orange wheat blossom midge. Eggs are deposited on the floret during heading and flowering, and the larvae feed on the developing grain. Severely damaged kernels are very light, and are lost during harvesting and grain cleaning. Lightly damaged kernels have a distorted shape, and often exhibit a split in the pericarp that gives the kernels a sprouted appearance.

Serious midge outbreaks have a devastating effect on crop yield. There are also reports that midge damage has serious effects on wheat milling and baking performance (Fritzshe and Wolffgang 1959, Miller and Halton 1961, Dexter et al. 1987). Grain from midge-damaged wheat exhibits unusually high protein content, reduced flour yield, dark flour color, high flour ash, weak sticky dough properties and poor bread quality.

Although midge damage could lead to serious wheat quality problems in infested areas, it is unlikely to pose a significant problem to the overall quality of a wheat harvest. Midge outbreaks are localized and generally of short duration. There is a strong incentive for insecticide treatments to protect crop yield, and if timely, insecticide treatments can significantly reduce the extent of wheat quality deterioration (Table 4). In areas where midge damage is extensive, the SDS-sedimentation test (Axford et al. 1978), a rapid simple estimate of gluten strength, can be used to screen for the adverse effect of midge damage on gluten properties (Dexter et al. 1987).

Table 4. Effect of aerial spraying of Lorsban (37.4 L/ha) on some quality properties of midge-infested Neepawa hard red spring wheat.
Property Unsprayed Sprayed
Wheat
Midge damage, % 8.7 0.4
Protein, % 14.4 12.1
SDS, mL 56 62
Flour yield, % 72.4 74.2
Flour
Ash, % 0.56 0.48
Grade color, units 0.8 -1.0
Farinograph
Absorption, % 65.0 63.4
Stability, min 3.25 5.25
Bread
Absorption, % 59 59
BSI, % 74 92

Source: Dexter et al. (1987).

Hard vitreous kernels

Hard vitreous kernel (HVK) content is a widely used specification in the grading and marketing of hard wheats. The CGC defines hard vitreous kernels as those having 'a natural translucent coloring which is an externally visible sign of hardness'. Kernels having a starch spot of any size are considered to be non-vitreous (also known as starchy, yellow berry or mealy kernels).

Several factors influence the degree to which wheat becomes non-vitreous, including weather conditions, soil fertility and heredity (Phillips and Niernberger 1996). It is generally accepted that the primary effect of HVK on wheat quality is a direct relationship between vitreousness and protein content (Pomeranz et al. 1976, Simmonds 1974). Pomeranz et al. (1976) showed that protein quality is not affected because HVK and loaf volume are unrelated when protein content is held constant.

A secondary effect of HVK is a positive correlation to kernel hardness (Pomeranz et al. 1976) The softer nature of nonvitreous wheat is due to a less extensive gluten protein matrix which results in weaker protein-starch adhesion within the endosperm (Simmonds 1974). Phillips and Nierberger concluded that degree of vitreousness has no effect on milling yield. Work done in our laboratory confirmed that for hand-picked samples exhibiting variable degree of vitreousness there is an effect on kernel hardness as evident by a change in particle size index (based on the concept of break release) measured as described by Williams and Sobering (1986) (Table 5). The effect is so slight that intrinsic hardness differences among wheat classes are readily discernible for piebald (partly vitreous) kernels. There is some overlap in hardness between classes when kernels are fully starchy, but for North American hard wheat fully starchy kernels rarely comprise a major proportion of commercial wheat samples.

Protein content of wheat can be easily, precisely and objectively measured. In contrast, HVK determination is tedious and subjective. Increasingly protein guarantees are a prerequisite for marketing wheat which may make HVK a redundant quality factor.

Table 5. Protein content and particle size index (PSI) of some Canadian wheat classes when fully vitreous, partly vitreous (piebald) and fully starchy.
Wheat class Protein, % PSI, %
Durum
Vitreous 10.8 34.2
Piebald 9.1 35.3
Starchy 7.9 48.7
Hard Red Spring
Vitreous 12.7 43.9
Piebald 9.9 43.2
Starchy 8.8 48.4
Hard Red Winter
Vitreous 11.5 52.0
Piebald 9.7 52.8
Starchy 8.5 57.6
Canada Prairie Spring
Vitreous 12.1 55.4
Piebald 9.4 54.3
Starchy 8.3 58.3

Source: Durum data from Dexter et al. (1989); other data from Edwards and Dexter (unpublished).

Frost damage and immaturity

The short growing season in western Canada and the northern United States makes frost damage a common grading factor. The severity of the quality effects of frost damage depends on the maturity of the grain when exposed to frost, the temperature to which the grain is exposed and the duration of exposure (Preston et al. 1991).

Severe frost damage is one of the most serious quality defects associated with wheat quality (Dexter et al. 1985 and references therein). Severe frost damage reduces flour milling value due to the combined effects of lower flour yield and poorer flour refinement (higher four ash and darker color) (Table 6). In addition, severely frosted wheat is extremely hard, resulting in higher energy consumption during flour milling. Mill balance is also disrupted because the extreme hardness of middling stock results in a greater proportion of stock migrating to tail-end reduction passages. The extreme hardness of severely frosted wheat results in high flour starch damage.

Severely frosted wheat exhibits unsatisfactory physical dough properties (Table 6). Bread volume, appearance, crumb structure and crumb color deteriorate progressively as degree of frost damage increases.  

Table 6. Effect on milling and baking quality of various proportions of frost-damaged Canada Feed wheat admixed with No 1 CWRS wheat.
Property 100% Feed 30% Feed 15% Feed
Wheat  
Grinding energy, Whr/kg 30.6 26.6 24.1
Flour yield 69.7 73.9 73.6
Flour
Ash, % 0.56 0.52 0.48
Grade color, units 4.5 2.5 1.7
Protein, % 12.0 13.2 13.5
Starch damage, FU 50 35 32
Farinograph  
Absorption, % 67.7 65.2 64.5
DDT, min 2 6.5 6.25
Bread  
Baking absorption, % 63 64 64
Loaf volume, cc 705 920 940
BSI, % 90 105 105

Source: Dexter et al. (1985). Analytical data expressed on 14% moisture basis.

Sprout damage

Pre-harvest sprouting due to damp harvest conditions has little impact on milling properties, but can have serious adverse effects on bread quality (Chamberlain et al. 1983). Sprout damage is detrimental to bread quality because of the action of the starch degrading enzyme alpha-amylase which is present in very high levels in sprouted wheat (Kruger 1994).

As alpha-amylase degrades starch during mixing and fermentation, the water holding capacity of starch is reduced. Baking absorption must be reduced, lowering the number of loaves of bread obtained from a given weight of flour, an important economic consideration to bakers (Tipples et al. 1966, Tkachuk et al. 1991b). Loaf volume often is not affected by sprout damage, and can actually increase due to more rapid gas production during fermentation (Ibrahim and D'Appolonia 1979). Sprout damage leads to sticky dough which causes handling problems, a more open coarse crumb structure and gummy crumb (Buchanan and Nicholas 1980, Moot and Every 1990). Gummy crumb causes build-up on slicer blades and interferes with effective bread slicing (Dexter 1993) (Figure 1).

Bread waste: build-up on slicer blades due to gummy crumb of bread baked from sprouted wheat flour.

Fig. 1. Potential bread waste from
build-up on slicer blades due to gummy
crumb of bread baked from sprouted
wheat flour. Top, center and bottom
loaves prepared from control,
3% germinated and 5% germinated
wheat respectively (Dexter 1993).

 

All of the effects of alpha-amylase are exaggerated for baking processes with long fermentation times because alpha-amylase continues to degrade starch during the fermentation stage. In the case of Oriental noodles dough is prepared at lower water absorption, preparation time is much less, and alkaline additives are often present which raise the dough pH well outside the optimum of most cereal enzymes. As a result the effects of sprout damage on noodle quality are so slight that they do not preclude the noodles from being marketable (Kruger et al. 1995).

 

Visual estimation of sprout damage gives only a rough indication of end-use quality effects because of the very heterogeneous distribution of alpha-amylase within individual wheat kernels and inconsistent retention of alpha-amylase activity among wheat exhibiting comparable degree of damage (Kruger and Tipples 1980). Alpha-amylase originates in the outer layers of the wheat kernel, so the enzyme tends to concentrate in high ash streams during flour milling (Kruger 1981). As a result, the best prediction of end-use quality for a sprouted wheat sample is obtained when tests such as falling number, amylograph viscosity and assays for a-amylase activity are determined directly on the flour.

Heat damage

A problem frequently associated with wet harvests is heat damage caused by improper storage of damp grain or by artificial drying at too high a temperature. Damp grain may heat during storage, causing loss of gluten functionality. In extreme cases kernels turn black and emit a charred odor (binburnt kernels).

 

Binburnt grain does not pose a serious threat to wheat marketing because it is readily detectable visually. However, artificial drying at too high a temperature may damage gluten functionality with no visual evidence of heat damage. Heat damage has little effect on milling properties, but may have serious adverse effects on physical dough properties and end-product quality (Table 7).

 

Table 7. Quality evaluation of artificially dried CWRS wheat.
Property Undamaged Heat damaged
Wheat
Test weight, kg/hL 77.7 77.5
Flour yield, % 74.8 74.5
Flour
Protein, % 12.7 11.3
Wet gluten, % 36.7 30.5
Wet gluten/protein 2.89 2.70
Ash, % 0.48 0.47
Grade color, units 0.4 0.3
Farinograph
Absorption, % 65.2 64.7
DDT, min 9.75 1.75
Bread
Baking absorption, % 64 61
Loaf volume, cc 820 595

Source: Preston et al. (1989).

Composites of commercial rail carlots visually qualified for No 3 CWRS. The damaged carlots were downgraded to Canada Feed by the GRL heat-damage monitoring program.

Fortunately the wheat growing areas of North America are usually favorable for drying grain naturally in the field. Occasionally wet harvest conditions necessitates hot-air drying to reduce moisture content to levels acceptable for safe storage. To protect wheat processors rapid sensitive tests are needed to detect heat damage.

Physical dough properties are very sensitive indices of heat damage. The GRL uses changes in mixograph properties to detect heat damage in commercial samples during years of wet harvest. Sample pairs taken before and after drying are milled by a rapid procedure, and mixograph curve shapes are compared (Kilborn and Aitken 1961, Preston et al. 1989). Any difference in mixograph curve shape indicates a change in gluten functionality, and provides a signal that dryer temperature must be reduced. Samples dried by farmers, private operators and terminal elevators are monitored.

Monitoring heat damage by physical dough properties is highly effective, but is relatively time consuming and requires expensive equipment and considerable technical expertise. More rapid procedures are needed to screen for heat damage in individual lots being discharged at a mill or storage facility.

Preston and Symons (1993) refer to a number of more rapid heat damage tests available based on enzyme inactivation, dye binding capacity, turbidity and protein extractablity, but all still require technical expertise and can be sensitive to cultivar and protein content. They have proposed a simple rapid test based on the formation of protein fibrils when a sample of ground grain is wetted. Seckinger and Wolf (1970) were the first to observe that wetting of thin sections or particles of wheat endosperm results in rapid formation of microscopic protein fibrils. Preston and Symons (1993) found that the extent of fibril formation viewed by bright field microscopy was highly sensitive to degree of heat damage (Figure 2). Using their procedure all samples identified as seriously heat damaged by the mixograph procedure showed little or no fibril formation. The procedure shows great promise because it requires little technical training, and an inexpensive low-power microscope gives satisfactory resolution.

Mixograms and corresponding bright field photomicrographs of flour obtained from No.1 CWRS

Figure 2. Mixograms and corresponding bright field photomicrographs of flour obtained from No.1 CWRS 12.5 wheat treated for (a) 0 hours, (b) 4 hours and (c) 16 hours at 70°C in sealed glass jars after tempering to 16.5% moisture for 16 hours. Adapted from Preston and Symons (1993).

Mildew

Mildew fungus (Cladosporium) is often associated with wet harvests. Mildew damage is characterized by grey tufts of spores at the distal ends of damaged kernels. Subjective estimation, based on overall sample appearance, is the only means of estimating moderate mildew damage.

Because mildew is associated with weathering and sprout damage, it is difficult to fully differentiate quality effects from those due to sprout damage. Severely mildewed kernels, which become blackened and rotten, are readily quantified, and should be tolerated in low amounts because of negative impact on flour brightness. At the GRL the effects of moderate mildew damage on soft wheat from Ontario demonstrate that as mildew increases test weight and Falling Number decrease, indicative of increased sprout damage (Table 8). Flour milling performance is lower because flour is darker. Effects on other quality factors such as alkaline water capacity, alveograph dough properties and cookie quality are slight.

Mildew does not pose a toxicological hazard. Although moderate mildew damage is apparently not a major quality factor, it serves as a very useful flag for wet harvest conditions and potential sprout damage. The discoloration of the seed coat also can be an aesthetic detriment to food applications like breakfast cereals.

Table 8. Effect of moderate mildew damage on the quality of Canada Eastern White Winter (CEWW) wheat at maximum levels tolerated for various grades.
Property No 1 CEWW No 2 CEWW No 3 CEWW
Wheat
Test weight, kg/hL 78.2 77.2 75.9
Falling number, sec 255 170 105
Flour yield, % 73.7 73.1 72.6
Flour
Ash, % 0.43 0.41 0.41
Grade color, units -1.6 -0.7 -0.3
Protein, % 7.8 7.9 7.8
AWRC , % 65 66 65
Alveograph
P (ht. X 1.1), mm 21 22 21
Length, mm 160 134 136
W X 103 ergs 62 60 56
Cookies
Spread, mm 83 82 81
Ratio, spread/thickness 9.4 8.7 8.7

Source: Dexter (unpublished data). Analytical data expressed on 14% moisture basis.

Smudge and black-point

The fungi Alternaria alternata and Helminthosporium sativum are common sources of infection in wheat which give kernels a dark-brown or black discoloration. When the infection is confined to the germ end it is referred to as "black-point". When the infection progresses along the crease discoloring more of the kernel it is referred to as "smudge". In western Canada Alternaria alternata is by far the most prevalent cause of infection.

Smudge is considered a serious quality factor because at that stage the infection has penetrated the endosperm causing discoloration of the flour. Studies at the GRL have concluded that the effect of black-point on wheat flour milling and baking quality are minimal, even at levels sufficiently high to down-grade wheat to Feed (Table 9). An Australian study reached similar conclusions (Rees et al. 1984).

Black-point and smudge pose no toxicological danger. Although black-point is a minor quality factor, it remains an important aesthetic grading factor because the poor appearance of wheat with black-point impedes marketing to demanding end-users. The discoloration of the germ and bran associated with the infection also are a detriment to production of germ and breakfast cereals.

Table 9. Effect of black-point on the quality of Canada Western Red Spring (CWRS) wheat.
Property No 1 CWRS No 2 CWRS No 3 CWRS
Wheat
Black point, % 0 5 40
Test weight, kg/hL 79.3 79.6 79.8
Flour yield, % 74.0 73.2 73.0
Flour
Ash, % 0.48 0.48 0.47
Grade color, units 1.0 1.2 1.2
Protein, % 14.9 14.9 15.3
Farinograph
Absorption, % 64.8 64.9 65.5
DDT, min 5.5 5.25 5.75
Bread
Baking absorption, % 68 68 68
Loaf volume, cc 985 1000 1010

Source: Dexter and Preston (unpublished data). Analytical data expressed on 14% moisture basis.