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

3. Factors affecting edibility


Ergot is a fungal parasite (Claviceps purpurea) of cereals and grasses (Lorenz 1979). Infection takes place at the flowering stage, the ergot body growing in place of the kernel. Ergot contains alkaloids, which may be toxic when ingested by animals, poultry or humans (Mantle 1977a, b).

In recognition of its toxicity, strict tolerances for ergot are universally required when marketing wheat. However, there have been relatively few studies on the retention and stability of ergot alkaloids in flour mill streams and processed wheat end-products. Scott et al. (1992) reported that low levels of ergot alkaloids are prevalent in Canadian cereal products, with rye flour being the most contaminated.

Fajardo et al. (1995) determined the distribution of ergot alkaloids in individual millstreams from a pilot-scale milling of Canada Western Red Spring (CWRS) wheat containing 0.03% ergot (the maximum allowed for the No 3 grade is 0.04%). As seen in Table 1 they showed that ergot alkaloids are partitioned in variable concentrations among millstreams. Ergot is more plastic than hard wheat endosperm, hence it is flattened during smooth roll grinding. As a result, the ergot alkaloids tend to concentrate in late reduction streams and shorts derived from the reduction system. Ergot alkaloids are quite stable during end-use processing (Fajardo et al. 1995). Processing of flour into pasta and Oriental noodles has little effect on levels of ergot alkaloids, and a substantial proportion of alkaloids are still present after cooking. Processing flour into pan bread has a minimal effect on alkaloid levels, although less alkaloids are present in the crust than the crumb.

Table 1. Concentration of ergot alkaloids (ppb) in some millstreams from ergotic CWRS wheat.
Stream Ash, % Alkaloids,  ppb
Red dog 2.72 4700
Bran 6.28 33
Middling 6 1.24 900
Middling 3 0.51 44
Bran flour 2.83 44
Break 3 0.58 27
Break 1 0.48 10
Middling 1 0.34 8

Source: Fajardo et al. (1995). Ash and alkaloid contents expressed on 14% moisture basis.

These results demonstrate the complexity of predicting the concentration of ergot alkaloids in wheat flour and end-products. The concentration will depend on extraction rate, milling technique (grinding conditions and mill flow) and the component streams of a given divide flour. The variable stability of the alkaloids among final end-products further underline the complexities of establishing safe tolerance limits for ergot.

Fusarium Head Blight

Fusarium head blight (or scab) occurs worldwide on small grain cereals (Parry et al. 1995). Fusarium head blight outbreaks are a health concern because of the mycotoxins found in Fusarium-infected grain. Accordingly, there have been numerous studies focusing on the level of mycotoxins, particularly the trichothecene deoxynivalenol (DON, vomitoxin) in infected wheat, flour, and processed products (Pomeranz et al. 1990 and references therein, Tkachuk et al. 1991a, Trigo-Stockli et al. 1996). There is general agreement that DON is stable during wheat milling and secondary processing, although it becomes partitioned in varying concentrations among screenings, mill feed and flour streams (Table 2).

Table 2. Concentration of vomitoxin (µg/g) in CWRS wheat, milled products and bread.
Product Wheat A Wheat B
Dirty wheat 7.1 1.4
Dockage 16.7 1.4
Clean wheat 4.6 1.8
Bran 4.2 1.8
Shorts 7.4 2.1
Red dog 5.6 2.0
Straight-grade flour 4.0 1.5
Bread 4.0 1.1

Source: Wheat A data from Scott et al (1983); wheat B data from Scott et al (1984). Bread data are expressed on equivalent flour basis.

In addition to edibility problems, fusarium damage (FD) has a detrimental effect on the processing quality of wheat (Dexter et al. 1996 and references therein). According to Bechtel et al. (1985) F. graminearum, the most prevalent species in the Great Northern Plains of North America, is an aggressive invader destroying starch granules, storage proteins, and cell walls. Boyacioglu and Hettiarachchy (1995) found that moderate F. graminearum infection causes significant compositional changes in carbohydrate, lipid, and protein in American hard red spring wheat.

FD reduces the milling performance of wheat (Tkachuk 1991a, Dexter et al. 1996). As seen in Table 3, FD results in lower test weight because of the shrunken nature of FD kernels. Flour yield and flour ash are affected slightly by FD, but the major impact on milling is a strong negative effect on flour brightness.

The effect of FD on baking quality is significant, but appears to be dependent on environment and variety. Seitz et al. (1986) concluded that scab levels up to 3% do not significantly affect hard red winter wheat baking quality. In contrast, Dexter et al. (1996) found that within the levels of FD encountered in southern Manitoba in 1994 there were highly significant effects (Table 3). One variety, Roblin, exhibited unacceptable baking performance even after fusarium-damaged kernels were removed. Baking strength index (Tipples and Kilborn 1974) which measures loaf volume potential at constant protein content is less than 80% of normal for Roblin even when severely FD kernels are removed.

Food safety remains the primary concern with FD wheat. However, the impact of FD on wheat processing potential cannot be ignored, and must be considered when establishing FD limits for wheat milling grades.

Table 3. Effect of Fusarium damage on some quality properties of Grandin and Roblin hard red spring wheat.
Property Grandin Roblin
Fusarium damage, % 1.5 7.3 0.2 5.9
DON, mg/g 1.7 8.0 1.2 9.5
Test weight, kg/hL 79.0 76.7 77.9 75.0
Protein, % 14.0 13.7 14.8 14.7
Flour yield, % 75.0 74.7 74.3 73.5
Ash, % 0.50 0.52 0.49 0.52
Grade color, units 0.1 1.2 -0.1 0.9
Absorption, % 62.4 62.0 62.5 62.0
DDT, min 4.5 4 6.5 5.75
Stability, min 8 7 9.5 0
Absorption, % 60 58 61 61
Loaf volume, cc 920 855 690 520
BSI, % 106 99 72 55

Source: Dexter et al. (1996). Analytical data expressed on 14% moisture basis.