2009_Book_FoodChemistry

842 18 Fruits and Fruit Products
(cf. 16.2.6) and the concentration of linalool increases
due to the hydrolysis of the corresponding
glycoside (Table 18.32). Since the C 6 -aldehydes
and nonadienal decrease simultaneously, the
fruity-flowery aroma notes are enhanced and the
“green” notes diminished.
The important compounds in plums are linalool,
benzaldehyde, methyl cinnamate, and γ-decalactone
together with the C 6 -aldehydes. Benzaldehyde,
nonanal, and benzyl acetate contribute
to the aroma of canned plums.
18.1.2.6.12 Litchi
The compounds acetic acid isobutylester,
guaiacol, cis-rose oxide, 2-acetylthiazoline, (E)-
β-damascenone, 4-hydroxy-2,5-dimethyl-3(2H)-
furanone, linalool, geraniol and 2-phenylethanol
exhibit the highest aroma activity.
18.1.2.7 Vitamins
Many fruits are important sources of vitamin C
(Table 18.33). Its biosynthesis in plants starts
from hexoses, e. g., glucose. It is postulated that
following C-1 oxidation and cyclization to 1,4-
lactone (II), the 5-keto compound (III) appears as
an intermediary product which is oxidized to the
2,3-endiol (IV) then reduced stereospecifically to
L-ascorbic acid (V) (cf. Formula 18.40).
Table 18.33. Ascorbic acid in various fruits (mg/100g
edible portion)
Fruit Ascorbic Fruit Ascorbic
acid
acid
Apple 3–35 Currant, 177
Pear 1–4 black
Apricot 5–15 Orange 50
Cherry 8–37 Grapefruit 40
Peach 5–29 Lemon 50
Plum/prune 2–14 Acerola 1000–2000
Blackberry 17 Pineapple 25
Strawberry 60 Banana 7–21
Raspberry 25 Guava 300
Currant, red 40 Melons 6–32
(18.40)
Industrial-scale production of ascorbic acid also
starts with glucose. The sugar is first reduced to
sorbitol (VI) and then oxidized with Acetobacter
suboxidans to L-sorbose (VII) which after cyclization
and conversion to the diisopropylidene
derivative (VIII) is oxidized to the corresponding
derivative of L-2-oxogulonic acid (IX). After
removal of the protecting isopropylidene groups,
L-ascorbic acid (vitamin C) is obtained via L-2-
oxogulonic acid (X; cf. Reaction 18.41).
The synthesis can be shortened with a genetically
modified strain of the bacterium Erwinia
herbicola which directly converts D-glucose to
L-2-oxogulonic acid (X).
β-Carotene (provitamin A) occurs in large
amounts in apricots, cherries, cantaloups and
peaches. B-vitamins present in some fruits
(apricots, citrus fruits, figs, black currants and
gooseberries) are pantothenic acid and biotin.
Other B-group vitamins occur at levels of no
nutritional significance. Vitamins B 12 and D
and tocopherols are found in no more than trace
amounts.