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



3. Factors affecting edibility

Ergot

Ergot is a fungal parasite (Claviceps purpurea) that infects cereals and grasses at the flowering stage, ergot bodies growing in place of kernels. Ergot contains alkaloids, which are toxic to animals, poultry and humans (Lorenz 1979).

Strict tolerances for ergot are universally required when marketing durum wheat because of toxicity of associated ergot alkaloids. Published studies on retention of ergot alkaloids in wheat milling fractions and end-products have focused primarily on common wheat (Dexter and Edwards 1998 and references therein), but similar considerations apply to durum wheat. The concentration of ergot alkaloids will depend on extraction rate, milling technique (grinding conditions and mill flow) and the component streams of a given milled product. Using common wheat flour, Fajardo et al. (1995) showed that retention of ergot alkaloids is not influenced by pasta drying temperature, and that 40 to 60% of ergot alkaloids are lost during cooking due to the combined effects of thermal denaturation and leaching into cooking water (Table 1).

Table 1 – Concentration of ergot alkaloids (ppb) in pasta dried by low temperature (LT) and high temperature (HT), and in cooked pasta and cooking water
Product LT spaghetti HT spaghetti
Spaghetti:
Dried 280 260
Cooked 180 120
Cooking water 30 26

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

Ergot in durum wheat has no detectable effect on semolina extraction rate, physical dough properties or pasta cooking quality (Dexter and Matsuo 1982). However, at levels as low as about 12 kernel size pieces of ergot per 500 g, the upper limit allowed in No. 3 Canada Western Amber Durum (CWAD), semolina was significantly speckier, although pasta brightness and hue were not affected.

Specks in semolina attributable to ergot are a serious aesthetic defect because they are dark and highly visible in pasta. Ergot bodies are difficult to remove on the basis of size because they assume the kernel dimensions of infected wheat as they develop in spikelets. Ergot bodies are low in specific weight, and can be removed effectively by gravity tables (Dexter et al. 1991).

Fusarium Head Blight

Fusarium head blight (or scab) has captured attention recently because of outbreaks in Canada and the United States in recent years. Fusarium head blight outbreaks are a safety concern because of mycotoxins in Fusarium-infected grain.

Numerous studies have concluded that the most prevalent mycotoxin, deoxynivalenol (vomitoxin, DON) is stable during milling and secondary processing, although it becomes partitioned in varying concentrations among screenings and mill products (Dexter and Edwards 1998; Pomeranz 1990). Nowicki et al. (1988) determined the DON levels in mill fractions and pasta from severely infected durum wheat. They found that DON concentration in semolina was about 80% of that of cleaned wheat, and that concentration was reduced further by 50% following cooking. Dexter et al. (1997) reported DON concentrations in semolina declined an average of 50% for ten less severely infected durum wheat cultivars harvested in Manitoba in 1995.

Safety remains the primary concern with Fusarium head blight outbreaks, but there can be serious effects on wheat milling and processing (Dexter et al. 1996, 1997). Moore (1994) reported that Fusarium damage in durum wheat imparts poor pasta color. Dexter et al. (1997) found that levels of infection around 2%, the maximum tolerated in lower milling grades of CWAD, adversely affected semolina yield and refinement (color and speck count) (Table 2). That caused a trend, readily discernible by eye, towards redder (higher a*) and duller (lower L*) pasta. Fusarium damage caused slightly weaker gluten, but pasta cooking quality was not affected. They concluded that the strict Fusarium damage tolerances for No. 1 CWAD (0.25%) and No. 2 CWAD (0.5%) were warranted on the basis of processing potential.

Table 2 – Effect of Fusarium damage on some quality properties of Kyle and Hercules amber durum wheat
Property Kyle Hercules
CL AS CL AS
Wheat:
Fusarium damage, % 0.04 2.4 0.2 3.8
DON, mg/g 0.7 2.1 1.2 3.9
Test weight, kg/hL 79.3 78.8 78.9 78.4
Protein, % 15.2 14.8 15.2 15.2
Semolina yield, % 69.8 68.2 71.3 71.1
Semolina:
Ash, % 0.73 0.75 0.76 0.76
AGTRON color, % 44 43 53 48
Specks per 50 cm2 93 121 66 69
Mixograph:
Mixing time, min 3.50 3.42 3.50 3.25
Peak height 67 64 70 65
Spaghetti:
Color:
     L* 69.4 68.7 72.1 70.4
     a* 6.0 6.7 5.1 7.5
     b* 55.7 55.7 56.5 55.1
Firmness, kg/10 strands 1.22 1.23 1.12 1.12

Source: Dexter et al. (1997). Analytical data expressed on 14% moisture basis.

Abbreviations: CL = cleaned by hand removal of obviously damaged kernels; AS = as harvested; DON = deoxynivalenol (vomitoxin).