Detection of heat damage in wheat

Abstract

The aim of this project was to develop a quick, reliable and simple test for detecting heat damaged wheat samples at mill intake. The selected test should be capable of providing results within the standard 20-30 minute turnaround time at mill intake and reliably reject loads of heat damaged wheat which, if included in a bread grist, would have a deleterious effect on breadmaking performance.

Heat damage is caused when wheat, that has been harvested in wet conditions, is dried for prolonged periods using air temperatures above 60C. The most important effect of excessive heat is to inflict damage to functional wheat proteins that are responsible for the production of the visco-elastic gluten network required for bread production. When gluten of the right quality is present, this network entraps air that is introduced during mixing and carbon dioxide developed during fermentation, enabling the baker to produce a loaf of the required volume and texture. Damage to the vital protein framework results in structural weaknesses and hence inability to retain gas bubbles, with consequent reduction in loaf volume and crumb structure, i.e. an overall decrease in breadmaking quality. Heat damage also reduces the germination capacity of a wheat sample.

Within this study, a standard 7-day germination test was used to determine the level of heat damage inflicted on a moist wheat sample when subjected to different temperature/time regimes. The results of germination studies provided the "reference" results for heat damage against which all "potential" heat damage methods were compared. A standard Chorleywood Bread Process (CBP) test was used to determine the effect of differing levels of heat damage on end-use quality. Loaf volume was used to identify the critical point at which heat damage produced a significant effect on breadmaking performance and provide conclusive proof of the serious effect heat damage can have on grain destined for flour milling market.

Several methods for the detection of heat damaged wheat were investigated:

• Near UV absorbance of salt-soluble proteins
• Protein solubility test
• Rapid germination testing (Germograph)
• Rapid visco analyzer
• Turbidity
• Durotest
• Gel protein
• Gluten quality (testing extensibility)
• Mixograph
• Image analysis
• Surface tension measurements

Initally, these were tested by comparing the results of severely heat treated and control samples for a range of wheat varieties. Those methods showing the most promise were subsequently evaluated using a range of heat treated samples to check their sensitivity to milder levels of heat damage.

The Turbidity test gave consistently reliable results under the conditions used for the study and has been shown to relate better to germination test results than the currently used gluten washing technique. The method was modified to use a simpler spectrophotometer (colorimeter) suitable for use in a mill or grain store situation.