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 signscant 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.

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The effect of sulphate nutrition on the flavour components of three members of the family Cruciferae (radish, cabbage and white mustard) has been investigated. The plants were grown in a glasshouse in sand culture at six concentrations of sulphate in the nutrient medium and their flavour characteristics determined by sensory and gas-chromatographic methods. The latter were based on (a) headspace samples from the raw and cooked vegetables and (b) solvent extracts chromatographed at 200 "C. Sensory tests showed that the flavour strengths of the vegetables increased from low values at extreme sulphur deficiency to maxima with increase of the sulphate available to the plant. For radish and mustard, there was no correlation between total volatile components (determined by the headspace method) and sulphur content but a highly significant correlation was demonstrated between the latter and less volatile flavour components present in solvent extracts. In the case of mustard seed, p-hydroxybenzyl isothiocyanate was determined colorimetrically and shown to be correlated with sulphur nutrition. With cabbage, ally1 isothiocyanate and total volatile content were determined by a headspace method and each was found to be significantly correlated with sulphate nutrition.It is suggested that the flavour of these plants consists of at least two types of components : (a) compounds which impart a bland, generalised "vegetable" flavour to the food and (b) a specific component or components which are perceived as a burning sensation on the tongue sometimes accompanied by acharacteristic odour. The former are independent of sulphur nutrition whilst the concentrations of the latter are highly correlated with sulphur nutrition.The effect of sulphate nutrition on growth response, visual deficiency symptoms and mineral composition has been reported.

Investigation of the relationship between the swlphate nutrition of garlic (Allium sativum) and wild onion (A. vineale) and their flavour strength is now reported. The plants were grown in a glasshouse in sand culture at two concentrations ofsulphate in the nutrient medium and their flavour strength determined by sensory, biochemical and gas chromatographic methods. These observations have furnished further examples of environmental control of flavour components. The garlic plants in the two groups differed significantly in their fresh weights per plant. Their sulphur contents also showed that deficiency of this nutrient had been established in spite of the relatively large amount of sulphur introduced in the original cloves. The ratios of total sulphur content in the two groups ofplants and of their flavour strengths as determined by totalpyruvate values and taste threshold concentrations, respectively, were approximately constant (about 5). Total peak areas in the gas chromatograms increased with increasing (sensory) flavour strength. The garlic plants grown in sand culture were morphologically atypical, possibly because of the high moisture content of the growth medium, but their odour, taste and gas chromatograms were qualitatively indistinguishable from those of field-grown garlic which grew normally. Similar but more pronounced differences of flavour strengths were obtained with A. vineale and in this case the difference in flavour strengths between the deficient and normal plants was about 13-fold. The gas chromatograms of the deficient plants were almost completely lacking in the peaks which are characteristic of Allium sp.The discrepancies between initial and final sulphur contents of the deficient plants are discussed.

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