Abstract/Summary

Increased consumption of Brassica vegetables such as cabbage (Brassica oleracea) reduces the risk of cardiovascular diseases (CVD) and cancer mainly because of the bioactive compounds they contain. Cabbage contains high amounts of glucosinolates (GSLs) which, when hydrolysed, yield several products (such as isothiocyanates, thiocyanates, nitriles and epithionitriles), depending on the conditions of the hydrolysis process. Isothiocyanates (ITCs), one group of hydrolysis products formed from myrosinase enzyme action on glucosinolates (GLSs), are responsible for many of the health promoting properties of cabbage. In addition, GSLs, ITCs and other sulfur-containing compounds are also responsible for the bitter taste and sulfurous aromas of cabbage, which is reported to reduce consumer liking and consumption of cabbage. This thesis investigates the effect of variety and domestic cooking methods on phytochemical and volatile composition of cabbage with subsequent impact on sensory profile and consumer acceptability. The effect of variety, growing conditions and domestic cooking on the GSL-myrosinase system of cabbage was investigated. The results presented highlight significant differences in the myrosinase activity and stability, GSLs and glucosinolate hydrolysis products (GHPs) of the different cabbage types and varieties studied. Field grown cabbages with lower growing temperatures resulted in higher myrosinase activity and GSLs. The severity of the cooking method influenced the types and amounts of GHPs formed. Steaming led to denaturation of epithiospecifier protein (responsible for GSL hydrolysis to nitriles and epithionitriles) but retention of active myrosinase resulting in the formation of more beneficial ITCs than nitriles or epithionitriles. The highest concentrations of beneficial ITCs were observed in steamed white (WC1) and red (RC3) cabbage varieties. In addition to cabbage GSL-myrosinase system, other non-volatiles (amino acids, sugars and organic acids) and flavour volatiles were also analysed and their influence on sensory profile and consumer acceptability explored. Cabbage type/variety and domestic cooking influenced the types and amounts of phytochemical compounds formed. Sulfides, responsible for undesirable sulfurous aromas of cabbage were the main volatile compounds identified in raw cabbage. Cooking reduced the perception of bitter taste, and amounts of sulfurous volatiles produced, with consequent increase in consumer liking and acceptance of cabbage. Black kale was perceived to be more bitter than red cabbage even though red cabbage contained twice the amount of GSLs found in black kale. The difference in bitterness perception was related to the ratio of bitter-tasting GSLs to sweet-tasting compounds such as sugars and amino acids. The GSL-sugar/amino acid ratio for black kale was 1:4 and that of red cabbage was 1:8. The results suggest that higher amounts of sweet-tasting compounds have a masking effect on bitterness perception. To understand individual differences in bitter taste perception and consumer liking of cabbage, consumers were genotyped for their TAS2R38 and gustin rs2274333 genes which influence taster status. TAS2R38 had a significant effect on bitter taste perception and liking but the effect was not as expected and was mostly driven by the TAS2R38 rare genotype group. Gustin rs2274333 influenced bitter taste perception and liking in black kale varieties but differences were not clearly defined. Overall, it was observed that, irrespective of bitter taste genotype, cooking was the main driver of bitter taste perception as all genotypes perceived cooked cabbage significantly less bitter than raw cabbage. In conclusion, the results of this study provide helpful insights into the relationship between cabbage phytochemical composition and sensory characteristics. Breeding of cabbage varieties with high amounts of sweet-tasting compounds such as sugars without reduction in the concentration of beneficial GSLs may be a viable way of improving cabbage consumption. The study demonstrates that not only does mild cooking of cabbage enhance formation of beneficial ITCs, it also improves consumer liking and acceptability regardless of variety or bitter taste genotype.