Biosynthesis of the flavour precursors of onion and garlic

Abstract

Onion (Allium cepa), garlic (A. sativum) and other Alliums are important because of the culinary value of their flavours and odours. These are characteristic of each species and are created by chemical transformation of a series of volatile sulphur compounds generated by cleavage of relatively stable, odourless, S-alk(en)yl cysteine sulphoxide flavour precursors by the enzymes alliinase and lachrymatory-factor synthase. These secondary metabolites are S-methyl cysteine sulphoxide (MCSO, methiin; present in most Alliums, some Brassicaceae), S-allyl cysteine sulphoxide (ACSO, alliin; characteristic of garlic), S-trans-prop-1-enyl cysteine sulphoxide (PECSO, isoalliin; characteristic of onion), and S-propyl cysteine sulphoxide (PCSO, propiin; in onion and related species). Information from studies of the transformation of putative biosynthetic intermediates, radiolabelling, and from measurements of sulphur compounds within onion and garlic have provided information to suggest a biosynthetic pathway. This may involve alk(en)ylation of the cysteine in glutathione, followed by cleavage and oxidation to form the alk(en)yl cysteine sulphoxide flavour precursors. There is also evidence that synthesis of the flavour precursors may involve (thio)alk(en)ylation of cysteine or a precursor such as O-acetyl serine. Both routes may occur depending on the physiological state of the tissue. There are indications from the effects of environmental factors, such as the availability of sulphur, that control of the biosynthesis of each flavour precursor may be different. Cysteine and glutathione metabolism are discussed to indicate parallels with Allium flavour precursor biosynthesis. Finally, possible avenues for exploration to determine the origin in planta of the alk(en)yl groups are suggested.

Biosynthesis of plant‐derived flavor compounds

“…Sulfur-containing flavor compounds originating from methionine and cysteine are responsible for the odor of garlic (methanethiol, dimethyl disulfide, S-methyl thioacetate), onions (propanthial S-oxide), boiled potato (methional) and cooked cabbage (methanethiol) (Jones et al, 2004). In Arabidopsis, the cleavage of methionine is catalyzed by methionine c-lyase, resulting in the production of methanethiol, 2-ketobutyrate and ammonia (Ré beillé et al, 2006).…”

“…In Arabidopsis, the cleavage of methionine is catalyzed by methionine c-lyase, resulting in the production of methanethiol, 2-ketobutyrate and ammonia (Ré beillé et al, 2006). In onion (Allium cepa) and garlic (A. sativum), a series of volatile sulfur compounds is generated by cleavage of odorless S-alk(en)yl cysteine sulfoxide flavor precursors catalyzed by the enzymes allinase and lachrymatory-factor synthase (Jones et al, 2004;Lanzotti, 2006). The biosynthetic pathway involves alk(en)ylation of the cysteine in glutathione, followed by cleavage and oxidation to form the sulfoxides or (thio)alk(en)ylation of cysteine or O-acetyl serine.…”

“…Because the levels of the flavor precursors amount to 1-5% dry weight in certain Allium species, it is supposed that they play a major role for the plant. Two roles that have been ascribed are defense against pests and predation, particularly in the over-wintering bulb, and carbon, nitrogen and sulfur storage and transport (Jones et al, 2004).…”

Thin layer convective air drying of wild edible plant (Allium roseum) leaves: experimental kinetics, modeling and quality

“…They are proposed to be used for the treatment or the prevention of numerous diseases, including cancer, coronary heart disease, obesity, hypercholesterolemia, diabetes type 2, hypertension, cataract and disturbances of the gastrointestinal tract (Corzo-Martinez et al 2007;Powolny and Singh 2008;Boivin et al 2009). These various biological effects are related to their important content of volatile sulfur compounds, typical of the Allium plants, which are also responsible of their characteristic pungent aroma and taste (Mondy et al 2002;Jones et al 2004;Li et al 2007). These molecules are the products of the enzymatic reaction of alliinase, which is a C-S-lyase present in the vacuoles with S-alk (en) yl-l-cysteine-S-oxide precursors, located in the cytoplasm.…”

Comparative analysis of the complete plastomes of garlic Allium sativum and bulb onion Allium cepa

“…Пищевая ценность как чеснока, так и лука репчатого определяется, прежде всего, углеводным составом листьев и луковиц, а также наличием специфических вторичных метаболитов (Jones et al, 2004). Образование первичных углеводов происходит в хлоропластах в процессе фото-синтеза.…”