160 Produce Degradation: Reaction Pathways and their Prevention
Norterpenoids, such as the marmelo lactones and marmelo oxides, are important
aroma components of quince [25].
6.2.3.5 Furanones
The predominant furanones include 2,5-dimethyl-4-hydroxy-3(2H)-furanone
(DMHF), commonly referred to as strawberry ketone or Furaneol®^ (registered trade-
mark of Firmenich S.A, Geneva, Switzerland), and its derivatives, which are widely
distributed in both fruits and vegetables [26]. DMHF is especially important in
strawberries [27,28], pineapples [29], muscadine grapes [30], and tomatoes [31]. At
low concentrations DMHF has a fruity or strawberry-like note and at higher con-
centrations its aroma becomes caramel- or burnt sugar-like [26]. DMHF occurs
naturally in its free form (aglycone), as 2,5-dimethyl-4-methoxy-3(2H)-furanone
(mesifuran), and in two glycosidically bound forms (DMHF glucoside and DMHF
malonyl-glucoside) [26]. Two other DMHF-like furanones include 5-methyl-4-
hydroxy-3(2H)-furanone (norfuraneol) and 4,5-dimethyl-3-hydroxy-2(5H)-furanone
(sotolon).
The DMHF glycosides are themselves odorless and are considered the probable
precursors to DMHF. Biosynthesis of DMHF, via its glucoside, from deoxyketoses
by strawberry callus cultures has been demonstrated [26,32,33]. Isotopic precursor
studies [34] and radiotracer studies [35] with detached ripening strawberries were
used to demonstrate the biosynthesis of DMHF and its glycosides from D-glucose
or D-fructose, via D-fructose-6-phosphate as an intermediate. Mesifuran is formed
in strawberry by enzymatic O-methylation of DMHF by O-methyltransferase, with
S-adenosyl-L-methionine providing the methoxy group [36–38].
6.2.3.6 Sulfur-Containing Compounds
Sinigrin and other glucosinolates (thioglucosides) are major precursors of sulfurous
aroma components of cruciferous vegetables, such as broccoli, cabbage, brussels
sprouts, and others. Upon disruption of the plant tissue, the thioglucoside sinigrin
is hydrolyzed by the enzyme myrosinase to form allyl isothiocyanate, allyl thiocy-
antate, allyl cyanide, and 1-cyano-2,3-epithiopropane [39,40]. In both wasabi and
horseradish several thioglucosides are hydrolyzed by myrosinase to yield numerous
volatile compounds, such as isothiocyanates, thiocyanates, nitriles, and others [41].
Isothiocyanates are characteristic aroma compounds in wasabi and horseradish [41].
The main component, allyl isothiocyanate, is believed to be primarily responsible
for the pungency of both plants, but other isothiocyanates are important as well.
Phenethyl isothiocyanate is a characteristic aroma component of horseradish, while
unique and differentiating green notes of wasabi are due to a number of ω-meth-
ylthioalkyl and ω-alkenyl isothiocyanates [41].
S-Alkyl- and S-alkenylcysteine sulphoxides are important nonvolatile aroma
precursors of the genus Allium, which includes onion, garlic, leek, and shallot [4,42].
Alliinases (C-S-lyases), released upon tissue disruption, catalyze the hydrolysis of
the nonvolatile precursors to yield pyruvate, ammonia, and a range of volatile sulfur-
containing constituents with pungent and characteristic aromas [43].