2009_Book_FoodChemistry

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 tendency
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 a π-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 polyphenols.
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 glycosidic
precursors (cf. 5.3.2.4), oxidation, water
addition, and cyclization of individual compounds
(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):
18.1.2.6.2 Grapes
(18.35)
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 responsible
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 variety Valencia late by dilution
analyses are shown in Table 18.28.
On the basis of high orthonasal aroma values,
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-octen-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-