Effect of sulphate nutrition on flavour components of onion ( Allium cepa )

It is known that the volatile components of raw and cooked onions contain mono‐, di‐ and tri‐sulphides and other flavour‐contributing sulphur compounds. The relationship between the sulphate nutrition of spring (salad) onion ( Allium cepa ) and its flavour strength determined by sensory, biochemical and chromatographic methods has been investigated. The plants were grown in sand culture in a glasshouse and the nutrient solutions contained five levels of sulphate from zero to an adequate amount. In their growth response, total sulphur content and visual symptoms, the plants showed characteristic essential nutrient deficiency. The sensory tests, and instrumental and biochemical assays showed that flavour strength increased approximately in proportion to sulphate concentration in the medium; lachrymatory potency also increased. There were significant coefficients of correlation between sulphate concentration of the nutrient medium and some aspects of flavour strength. The correlation coefficients between total sulphur content (% of the edible portion) and total peak area, n‐propyl disulphide peak area and total pyruvate were also high. These observations suggest the possibility of control of onion flavour strength and lachrymatory potency, under field conditions, by control of sulphate nutrition. The question of loss of maximum potential onion flavour as a consequence of sulphur deficiency in soils is discussed. A headspace method for sampling onion flavour volatiles and for their introduction into gas chromatographs is described.