BRI Australia Ltd., North Ryde, NSW, 1670, Australia
One of the most important and early tests for wheat quality is test milling. This is a crucial step in the evaluation of crossbred performance because not only is flour yield determined but the physical characteristics of the flour which influence dough properties and end product performance are set.
A GRDC supported study, National Comparison of Test Milling (BRI103), found that there was a lack of uniformity in test milling methods. For the same sample, wheat was being conditioned to different final moisture contents before milling. The test mills ranged in age from 5 to 30 years and the condition of the rolls was variable. There was no consistent approach to test mill room ambient conditions, warm up procedures, feed rates, roll gaps and use of additional milling equipment across laboratories. These different methods produced a wide range of flour yields for a given wheat sample and consequently a range in flour quality as measured by differences in flour bran content and starch damage. The greatest variability across all laboratories occurred for milling soft wheat and this variability could not be explained by differences in the use of additional milling equipment. In summary, each quality testing laboratory was producing a different flour from the other laboratories after milling the same sample of wheat.
Uniform adoption of a test milling method and technical ability is required to ensure that the same flour quality is produced by different laboratories from the same wheat sample. The most recognised way to attain technical proficiency is to obtain accreditation for test milling from the National Association of Testing Authorities (NATA). NATA accreditation applies only to individual tests for which accreditation is sought. It is not a blanket accreditation across an organisation or laboratory. Accreditation for test milling would involve proving to the assessors that the laboratory method used for test milling produces reliable and reproducible results and is thoroughly documented. Individual technicians are also assessed for their own performance and competence through regular blind testing of samples and collaborative "ring tests".
Currently, there is no NATA accreditation or equivalent for test milling in Australia. This places outputs of wheat breeding at risk because quality information for processors and end users on new crossbreds is dependent on test milling.
Milling trials were conducted to determine the influence of wheat final conditioning moisture, roll gaps, feed rate, roll wear and temperature, additional milling processes (bran finishers and pin mills) and sieve sizes on flour yield and quality for hard and soft wheat types. Milling performance was measured by the Milling Quality Index (MQI) developed to index flour quality against bran contamination (Southan et al. 2000). Starch damage and flour colour were measured to monitor the effect of each variable on flour quality. NATA accreditation for test milling is underway and will be assesssed against the results described here.
Results and discussion
Roll gap settings, particularly break roll gap settings have a large impact on flour yield and flour quality. By carefully adjusting 1st break, 3rd break, 1st reduction and 3rd reduction rolls independently, the optimal roll gaps were determined for a combination of yield and quality as measured by the Milling Quality Index, flour colour and starch damage for hard and soft Australian wheat (Figure 1).
Figure 1. Test Mill performance as measured by Milling Quality Index for 1st and 3rd break rolls over a range of roll gap settings. The Buhler Test Mill manual roll gap settings are indicated by the arrows labeled B1 and B3.
The optimal roll gaps were found to be different from those described in the Buhler manual. This information will assist laboratories measuring test mill performance of Australian varieties to increase the discrimination between samples based on milling performance. Buhler test mills have a recommended roll gap clearance described in the manual. However, the data generated in this project indicates that these gaps are not necessarily optimal for Australian wheat.
The repeatability of flour yield for any given sample varies depending on the operator, sample size and particularly the method for cleaning remaining flour and stocks from the test mill. The cleaning process after a sample has passed through the mill influences the flour yield and total product recovery. Flour was found to collect in “catch points” inside the mill affecting mill performance. Two Buhler test mills have been stripped, pulled apart and reassembled with some modifications which have improved the efficiency of the milling by reducing the cleaning time between samples and increasing sample recovery and flour yield repeatability. Pictures of mill improvement are shown in Figure 2.
Figure 2. Silicone material used as sieve cleaners. This material was found to give greater sieve cleaning than chains because of its highly elastic and non-abrasive nature.
Documentation for NATA accreditation has been completed, internally audited by a NATA assessor and submitted to NATA for external accreditation. Assessment of the standardised test milling protocol will be undertaken by independent assessors. NATA requires demonstration of technical competence so that the operator understands the operating performance of the mill and monitors the performance of test milling to ensure the results are within upper and lower limits. A Memorandum of Understanding exists between the Commonwealth and NATA such that Commonwealth Funds are allocated for technical testing with laboratories with NATA accreditation where possible.
Southan, M.D., Quail, K.J., Osborne, B.G. (2000) Cereals 2000. Proc. 50th Aust. Cereal Chem. Conf., 498-501.