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The Th flavour fl
seesaw: balancing sugars and acids
Helen Boldingh, Annette Richardson and Ken Marsh
HortResearch: Ruakura, Kerikeri and Mt Albert
Figure 1: Balancing kiwifruit sugars and acids to enhance consumer appeal.
Who likes a sweet fruit that has no “ZING”? Or eating lemons? Great
kiwifruit fl avour comes from balancing sweetness with acidity.
Consumers consistently rate fl avour as one of the most important
reasons for buying a particular fruit, infl uencing market demand
and therefore grower revenue. In this article, we examine
accumulation of acids in kiwifruit and consider how climate,
management techniques and storage modify the acidity and fi nal
fl avour of kiwifruit.
COMPOSITION OF KIWIFRUIT
Producing kiwifruit with fl avour superior to that of our competitors’
fruit is the basis of the successful Taste ZESPRI TM marketing
programme. Kiwifruit fl avour is closely approximated by the fruit dry
matter content (DM) (Harker 2004). Fruit DM is the sum of the
components that remain after the fruit are dried to remove the
water. It is made up predominantly of sugars and acids, with lesser
amounts of protein, pectin, fi bre, vitamins (largely A and C) and
minerals. However, the amount of each component can vary; for
example, the acid content of ripe fresh fruit can range between 0.9
and 2.0 per cent titratable acidity (TA). The two main acids found in
kiwifruit are citric and quinic acid, each at around 40-50 per cent of
the total acids, with lesser amounts of malic acid (10 per cent). The
effect of each acid on the acidity of the fruit can vary, with citric
being the most acidic and quinic the least.
FLAVOUR OF KIWIFRUIT
ZESPRI TM GREEN (GREEN) Kiwifruit are described by consumers as
having fresh, sweet-acid fl avours, while the fl avour of ZESPRI TM
GOLD (GOLD) fruit is described as sweet, slightly tropical and fruity
(Marsh et al. 2006). These descriptions sum up extremely complex
interactions between many components such as soluble sugars,
acids and volatile compounds that are rapidly released from the
cells when we bite into the fruit. It is the volatile compounds that
give the characteristic GREEN or GOLD fl avour to the fruit, but
consumers can also taste relatively small changes in levels of key
components like sugars and acids. It is not just the total amount of
each compound that infl uences our liking for a fruit, but also the
balance between components. For example, kiwifruit with high DM
have both higher sugar and higher acid levels than those with low
DM (Woodward and Clearwater 2007), but consumers consistently
prefer high DM fruit as they have more fl avour (Harker 2004).
We have studied the interaction between sugars, acids and other
fl avour compounds using a kiwifruit pulp system (Figure 2). Addition
of any of the main sugars increased consumers’ perception of
sweetness of both GREEN and GOLD kiwifruit equally (Marsh et al.
2006). Sometimes increasing sugar levels also reduced perceptions
of acidity in fruit. Adding small amounts of acid to GREEN fruit pulps
always reduced consumer’s perception of sweetness and increased
those of acidity. In GOLD fruit pulps, adding quinic acid to raise TA
levels by about 0.3 per cent, had a slightly greater impact on fruit
taste than similar additions of citric and malic acid. The same study
also showed that increasing sugar or acid levels in fruit pulps altered
both the measured amount of aroma volatiles and consumers’
perceptions of banana and lemon-like fl avours.
ACID CONTENT OF ACTINIDIA SPECIES IN THE
KIWIFRUIT GERMPLASM COLLECTION
We have also studied fruit from a range of Actinidia selections in the
HortResearch germplasm collection, looking for fruit with unusual
acid combinations that may contribute to unique fl avours in future
kiwifruit cultivars. Ripe fruit from most of the species tested
contained some citric, malic and quinic acid; however, the amount
and proportions of these acids varied (Figure 3). Fruit from species
‘four’ selections had the highest total concentration of acid, but the
proportions of each acid were similar to those in GREEN and GOLD