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Figure 2: The kiwifruit pulp system used to study interactions between sugar, acid and fl avour compounds.
Figure 3: Acid concentrations in different Actinidia species.
fruit. While the total amounts of acid in
species ‘one’ fruit were similar to those in
GREEN and GOLD, quinic acid made up 80
per cent of the total acid, with some malic
acid but very little citric acid. Low acid levels
were found in both species ‘two’ and
species ‘three’ fruit. Both species had very
little quinic acid, and species ‘two’ also had
low citric acid levels. These differences in
acid levels would contribute to very different
fl avour profi les. This suggests that there is
potential to use between-species variation
in acidity to infl uence novel fl avours for new
commercial cultivars.
SEASONAL CHANGES IN ACID
LEVELS IN GREEN AND GOLD
FRUIT
We measured the individual acid
composition of both GREEN and GOLD fruit
over a season from fl owering, throughout
fruit development until commercial harvest.
The total concentration of acid in GREEN
fruit increased rapidly during the fi rst 60-80
days after fl owering, coinciding with the
rapid growth of fruit (Figure 4A). Total acids
were also high during the early development
of GOLD fruit. Thereafter levels declined
slightly, reaching constant levels during fruit
maturation.
Quinic acid is the main acid present during
the early development of both GREEN and
GOLD fruit and is mostly produced in the
60 days after fl owering (Figure 4C). In
contrast, citric acid (Figure 4B) only
accumulates in fruit during fruit expansion,
with concentrations staying stable or
increasing slightly prior to fruit maturation
and harvest. This suggests that quinic acid
and citric acid production are not linked, but
controlled separately (Cheng et al. 2004).
In both GREEN and GOLD fruit, levels of
malic acid vary widely before fi nally
increasing prior to harvest (Figure 4D).
EFFECTS OF CLIMATE AND
MANAGEMENT ON FRUIT ACIDS
Climate and management techniques are
likely to affect fruit acidity and taste, but
there have been few concerted efforts to
record the effects of climate or preharvest
treatment on kiwifruit acidity. Recent studies
have shown that acidity levels in kiwifruit
fruit are strongly correlated with fruit DM
levels (Woodward and Clearwater 2007).
Therefore we expect changes in both the
environment and management that alter
fruit DM content will also affect fruit acidity.
For example, high temperatures during fruit
expansion resulted in a major reduction in
both fruit DM and total acid levels of GREEN
fruit at harvest (Richardson et al. 2004).
However in the same study, heating vines
during fruit maturation reduced fruit DM,
but did not affect fruit acidity. This suggests
that in some cases DM and acid levels may
respond differently to either management
techniques or environmental factors.
Therefore it may be important to measure
acid levels during studies targeted at
increasing DM.
The combination of both growing conditions
and storage conditions can also affect fruit
acidity and fl avour. At harvest, acid levels of
kiwifruit grown in warm conditions in
California and Italy (Crisosto and Crisosto
2001; Tombesi et al. 1993) are around
2.5-3.5 per cent TA, compared with about
1.4 per cent TA in New Zealand fruit
(MacRae et al.1989). However, acid levels
in fruit grown in warmer climates can drop
by 50 per cent during storage at 0°C, while
there is little change in the acidity of New
Zealand fruit during storage. In Italy, Tombesi
et al. (1993) found that GREEN fruit which
was artifi cially shaded had higher acidity
levels at harvest than fruit grown in standard
conditions, but the difference was lost in
storage. In New Zealand, studies have
shown that malic acid levels in fruit can
increase and soluble solids levels decrease
when fruit are stored at 4°C, but not at 0° or
10°C (Marsh et al. 2004). Malic acid levels
can also vary in fruit stored in different CA
environments (Harman and McDonald
1989). Therefore, effects on fruit acid levels
may explain some of the differences in
fl avour between fruit grown in different
countries or stored under different
conditions.
SUMMARY
The fl avour of fruit has a large impact on
consumer acceptance. Kiwifruit fl avour is
largely determined by both the amount and
balance between sugars, acids and volatile
NZ KIWIFRUIT JOURNAL JANUARY / FEBRUARY 2008
43