Oxidative flavour chemistry and biochemistry in parsley
Deterioration of flavour quality during processing and storage is often brought about by oxidative processes. These typically involve oxygen or an active form of oxygen in effecting transformation of a wide range of volatile and non-volatile compounds, including key quality chemicals, flavour precursors and antioxidants. To investigate the nature of the chemical and biochemical change within vegetables and herbs, unblanched frozen parsley was selected as a suitable tissue. The chemical status of parsley during technological processing was determined using novel analytical protocols (SNCV A/SNCNV A) implemented as part of a unified strategy for the quantitative analysis of volatile and non-volatile species. The analysis utilized a single stabilized solution produced from plant tissue, under a regime which minimized isolation stress and artifact formation. On frozen storage (-10OC) the principal volatiles of parsley, myrcene, beta-phellandrene and menthatriene were extensively degraded to non-volatile products at differential rates. p-Cymenene and the tentatively assigned menthatriene diepoxide were formed as minor volatile oxidation products. Myristicin remained largely unchanged. Under similar frozen storage, chlorophyll 'a' displayed significant degradation with only minor amounts of chlorophyllide 'a', pheophytin 'a' and 13[superscript]2 hydroxychlorophyll 'a' formed. Ascorbic acid was extensively degraded in timescales preceding monoterpene and chlorophyll loss. Thermal blanching of parsley extensively prevented the degradation of the monoterpenes, suggesting that endogenous enzymes were responsible for the changes. Elimination of oxygen, in the absence of blanching, prevented volatile loss, confirming the requirement for oxygen. The hypothesis that peroxidase can operate in a co oxidative couple with the flavonoid, apigenin-7-glucoside and hydrogen peroxide, as proposed by Yamauchi (1985), was investigated to establish its potential role in the degradation of terpenoids and chlorophyll. In model experiments, using horseradish peroxidase, menthatriene and chlorophyll showed extensive degradation only when all components of the couple were present. In addition the requirement for oxygen was also established. Naringenin and umbelliferone have been shown to behave similarly to apigenin, as co-substrates for peroxidase. Lycopene, with some structural similarity to menthatriene, was also susceptible to co-oxidation. Polyphenol oxidase, proposed to operate in a similar fashion to peroxidase with mono- and di-phenols as substrates (Montedoro et al. 1995), in model experiments did not cause the degradation of chlorophyll. The co-oxidative role of lipoxygenase in parsley is believed to be of minor significance, however, it is likely to be responsible for the production of low levels of hexanal observed during thawing of frozen parsley. From this thesis it is concluded that the aroma and colour quality loss in frozen unblanched parsley probably results from the oxidative degradation of the unsaturated monoterpenes and chlorophyll 'a' respectively via an oxidative cascade initiated by the action of peroxidase.