Effect of sulphur fertilisation on the acrylamide-forming potential of wheat

Abstract

Acrylamide is a contaminant that forms through the Maillard reaction from free asparagine and reducing sugars such as glucose, fructose and maltose during high-temperature cooking and processing. Acrylamide is a Class 2a carcinogen and also affects the nervous system and fertility at high doses. The FAO/WHO Expert Committee on Food Additives has recommended that dietary exposure should be reduced and the European Food Safety Authority issued ‘indicative’ levels for acrylamide in food in early 2011. Wheat products are major contributors to acrylamide in UK and European diets and the development of best agronomic practice to keep wheat’s acrylamide-forming potential as low as reasonably achievable is important.

Free asparagine concentration is the main determinant of acrylamide-forming potential in wheat grain and accumulates to very high levels if wheat is grown under conditions of sulphur deficiency. This makes sulphur availability the most important factor affecting the acrylamide-forming potential of wheat grain. The aim of the present project was to provide data on acrylamide formation in flour from grain samples produced from six field trials in which different levels of sulphur had been applied. The trials comprised four different varieties of winter wheat, grown at three different locations over three harvest years, with five different levels of sulphur fertilisation. Free amino acid concentrations were measured by gas chromatography-mass spectrometry and acrylamide in heated flour was measured by liquid chromatography and tandem mass spectrometry. The data showed a clear and significant effect of sulphur application in reducing the acrylamide-forming potential of wheat in five of the six trials. The exception was a trial at Woburn, Bedfordshire in 2011/12, a year in which heavy rainfall in spring and early summer, followed by a dry mid to late summer, may have affected the outcome of the experiment.

The most appropriate level of sulphur application was deemed to be that which gave a significant benefit compared with no sulphur application, with no further significant benefit with higher levels of application. This ranged from 12.5 kg SO3 per hectare to 50 kg SO3 per hectare (corresponding to 5 to 20 kg of sulphur per hectare) in the five trials in which sulphur application was effective. However, application at the 50 kg SO3/ha rate led to significantly less acrylamide formation compared with the 25 kg SO3/ha rate in two of the trials. Given the necessity of preventing free asparagine accumulation in all conditions, it is therefore recommended that sulphur-containing fertiliser be applied at a rate of 50 kg SO3/ha (20 kg sulphur/ha). This is the top of the range of rates of sulphur fertiliser recommended for wheat in the Fertiliser Manual (RB209).