Maize Crop Quality – a Look at the 2017/2018 Season’s Results

Bushel weight/test weight is applied as a grading factor in the USA grading regulations but is also routinely done at most intake points locally. Bushel weight in pounds (lbs) was determined on the maize crop samples and the results converted to test weight reported in kilogram/hectolitre (kg/hl), by multiplication with a factor of 1,2872.

Test weight, expressing the specific mass (or grain density) of maize, is influenced by amongst others, factors like cultivar, moisture content, foreign matter, other grain and damaged kernels (such as insect-damaged kernels and immature kernels).

White maize had an average test weight of 77 kg/hl compared to the ­77,2 kg/hl of yellow maize. White maize’s average test weight was 0,7 kg/hl lower than the previous season and that of yellow maize 0,3 kg/hl higher. The test weight in total varied from 59,6 kg/hl to 82,5 kg/hl. The samples with the lowest test weight values originated from production regions mainly in the North West Province, the Free State and Mpumalanga.

The 100-kernel mass, which provides a measure of grain size and density, averaged higher for white maize than for yellow maize (34,3 g versus 31,5 g on ‘as is’ basis), as in previous seasons. As can be expected from these results, the kernel size of white maize, determined by sieving the samples through both 10 mm and 8 mm sieves, was larger than that of yellow maize and also larger than in the previous season.

Yellow maize kernels below the 10 mm sieve were also slightly larger than the previous season. The percentage of yellow maize kernels above the 10 mm sieve were on average 15,1% lower than white kernels and the percentage kernels below the 8 mm sieve 11,2% higher than that of white maize.

Kernels that are too small hamper the separation of kernel fractions in the wet milling process as used by the starch manufacturing industry, resulting in a lower starch yield. A mixture of small and large kernels causes additional problems, as homogeneous steeping cannot be achieved. The dry milling industry also prefers fairly larger maize kernels. However, uniform kernel size is of particular importance to this industry, since too large kernels create problems, especially when mixed with smaller kernels. Kernel size is less important to the animal feed manufacturing industry. Larger kernels are nevertheless preferred, as small kernels are easily lost during the screening stage of processing.

The percentage of stress cracks observed varied overall from 1% to 38%, averaging 11% for white and 9% for yellow maize. These values are on average 3% higher than in the previous season. Stress cracks, an undesirable physical attribute in maize, are internal narrow fine cracks in the endosperm of the kernel, located just below the surface. Small numbers of stress cracks occur naturally in all maize (usually less than 3% due to the natural drying process on the field). High drying and cooling rates during post-harvest handling are, however, the major factors in stress crack development.

When moisture is removed from the kernel too quickly, the structure of the kernel fails and stress cracks form. Kernels with large numbers of stress cracks are more susceptible to breakage, yield smaller grits during dry milling, absorb water too rapidly during wet milling and are more susceptible to insect and mould damage during storage.

The milling index obtained from the SAGL Milling Index 2018 model on the Foss NIT Infratec 1241 – Generation 3 Standard Version Grain Analyser, varied from an average of 78,3 for white maize to 79,4 for yellow maize. The milling index, a dimensionless index value, indicates the relative ratio of total hard endosperm products (B2 grits, B3 fine grits and B3 coarse grits) to offal products (B1 fine flour and total chop/bran) as determined on the Roff laboratory mill. Samples analysed varied from soft (<40) to very hard (>100).

Grit Yield All values, obtained from the same model, averaged 64,4 for white maize and 64,6 for yellow maize. Grit Yield All is linearly correlated with the SAGL MI and indicates the true amount of total hard endosperm that can be extracted from the maize during Roff milling.