Agriculture Reference
In-Depth Information
Organic acid content
Organic acids per
g fresh weight in apples increase until about mid-July in
the northern hemisphere and then decline progressively, following a smooth
curve through the final stages of growth and through maturation, ripening and
storage (Table
) until the following April. In pear the decline is continuous
from the early fruitlet stage to harvest (Ulrich,
.
). In apple both citric and
malic acids show the mid-season rise in concentration but quinic acid falls
from the beginning of the season.
The immediate precursors of organic acids are generally other organic
acids or sugars. These could accumulate through metabolism of imported
carbohydrate and amino acids and by fruit CO
fixation.
Organic acids provide the main substrate for respiration. The utilization of
malate as a respiratory substrate is mediated by malic enzymes catalysing the
reductive decarboxylation of malate to pyruvate, allowing carbon from malate
to be fed into the Krebs tricarboxylic (TCA) cycle. Citrate can feed directly into
this. The metabolism of malate by tissue slices increases as apples pass through
the climacteric (Hulme and Rhodes,
). The utilization of or-
ganic acids during the post-harvest period is the main reason for the increase in
sweetness in originally high-sugar, high-acid apples or insipidity and blandness
when sugar and acid concentrations are initially low (Lott,
; Knee,
).
Pigments, lipids and flavour compounds
Anthocyanin synthesis occurs during fruit growth whereas colour changes dur-
ing ripening depend mainly on the disappearance of chlorophylls
a
and
b
(see
Chapter
). Carotene declines during ripening but xanthophylls
increase as mono- and diesters, mainly with palmitate and oleate. The appear-
ance of these esters may precede the climacteric rise in ethylene synthesis.
Galactolipidsandassociatedlinolenylmoieties,whicharetypicalchloroplast
membrane constituents, are lost on ripening but phospholipids and fatty acyl
groups remain constant or increase slightly. Lipid turnover increases. Traces
of farnasene, thought to be implicated in the development of scald, are present
on the surface of pre-climacteric fruits and the amounts increases rapidly on
ripening.
The organic compounds on which apple and pear aroma and flavour de-
pend are synthesized during the climacteric phase.
, Figure
.
Readiness for harvest
Many of the factors that change as apples and pears mature are used as criteria
to determine the optimal date of harvesting.
