Food Chemistry

18.1 Fruits 837

complexes which are bluish-gray or bluish-black
in color. Al^3 + and Sn^2 + also form intensely
colored complexes. Leucoanthocyanins, when
heated in the presence of an acid, are converted
into anthocyanins. The red color of apples and
pears, which is formed during cooking, is derived
from leucoanthocyanins.
Phenolic compounds can also form complexes
with proteins. These complexes increase the
turbidity of fruit juices, beer and wine. The ten-
dency to form complexes of this type increases
with increasing degree of polymerization of the
phenols; even dimeric procyanidins are active,
e. g., procyanidin B2 (epicatechin-epicatechin)
in apple juice. Based on model experiments, it
is thought that especially the amino acid proline
should be involved in complex formation, its ring
system forming aπ-complex with that of the
phenols. Hydrogen bridges are also supposed to
contribute to stabilization of the complexes. In
the pH range 4.0–4.2, the amount of precipitate
is maximum, being 7 times higher than at pH 3.0.
In a similar manner as proteins and peptides,
polyvinylpolypyrrolidone (PVPP) binds polyphe-
nols. Therefore, it is especially suitable for the
separation of haze active polyphenols.

18.1.2.6 Aroma Compounds

Aroma compounds contribute significantly to
the importance of fruits in human nutrition.
The aroma substances of selected fruits will be
outlined below in more detail. The structures and
synthesis pathways of common aroma substances
are explained in Chapter 5.
The aroma of fruits can change on heating due
to the liberation of aroma substances from gly-
cosidic precursors (cf. 5.3.2.4), oxidation, wa-
ter addition, and cyclization of individual com-
pounds (cf. 5.5.4).

18.1.2.6.1 Bananas

The characteristic aroma compound of bananas
is isopentyl acetate. Some esters of pentanol,
such as those of acetic, propionic and butyric
acids, also contribute to the typical aroma of
bananas, while esters of butanol and hexanol
with acetic and butyric acids are generally fruity

in character. An important contribution to the

complete, mild banana aroma is supposed to be

provided by eugenol (1), O-methyleugenol (II)

and elemicin (III):

(18.35)

18.1.2.6.2 Grapes

The compounds responsible for the typical

aromas of different grape varieties has not been

clarified in each case. Esters contribute to the

fruity notes. The flowery-fruity aroma note

of American grapes (Vitis labrusca) is based

on 2-aminobenzoic acid methylester (methyl

anthranilate), which is not found in European

varieties, 2-Isobutyl-3-methoxypyrazine is re-

sponsible for the green paprika-like aroma note

of Cabernet Sauvignon grapes.

18.1.2.6.3 Citrus Fruits

The aroma of the most important citrus fruit, the

orange, has been analyzed in detail. The potent

aroma substances identified in the freshly pressed

juice of the varietyValencia late by dilution

analyses are shown in Table 18.28.

On the basis of high orthonasal aroma val-

ues, it is expected that (S)-2-methylbutyric

acid ethylester, ethyl butyrate, (Z)-3-hexenal,

isobutyric acid ethylester, acetaldehyde and

(R)-limonene are especially important for the

aroma of orange juice (Table 18.28). Based

on the retronasal odor threshold, the group of

important compounds is enlarged to include 1-oc-

ten-3-one, trans-4,5-epoxy-(E)-2-decenal and

ethyl caproate. A mixture of the odorants listed

in Table 18.28, in which only wine lactone

was missing, reproduced and aroma of orange

juice. Omission experiments showed that the key

aroma substances of the orange are acetalde-