A focal point for publicity material prepared for marketing kiwifruit internationally has been the emphasis on the fruit';s high vitamin C content. Besides vitamin C however, there are a multitude of other compounds which contribute to kiwifruit';s nutritional value which we could also draw attention to. Among these are included a class of compounds referred to as amino acids - small (in a chemical sense) nitrogen-containing molecules which form the fundamental units of proteins.

Knowledge of the freely-soluble amino acid content in ripening kiwifruit is important for several reasons. In a processing context these compounds are involved in browning reactions which affect the sensory quality of juices and fruit concentrates, while in the manufacture of fermented beverages, they are the principal nitrogen substrate for yeast (the absence of particular amino acids may lead to "stuck" fermentation). In a completely different setting, amino acid composition is of interest when identifying "smart" foods (see below).

In a previous study (Clark et al. 1992) we established that the concentration of individual amino acids in kiwifruit varied considerably during fruit maturation and postharvest storage. We conclude that investigation by commenting on the changes occurring as fruit removed from storage are finally ripened.

Measurements

Fruit harvested on the 9 May 1991 were cool stored for one month, and then ripened at 18°C for 23 days. During the ripening period, samples were removed every few days for analysis of firmness, soluble solids and free amino acids. The main findings were as follows:

• Twenty-five amino acids were detected - the principal ones being arginine, asparagine and glutamine (Table 1).

• There was no significant change in the concentration of the majority of the free amino acids during ripening. This is illustrated by one of the major components, arginine (Fig. 1A), where concentrations were found to fluctuate but, because of the large amount of variability in the data (indicated by the size of the error bars), no distinct trend in the values could be established over time.

• Compositional changes accompanying ripening were observed for only four of the minor compounds; alanine (Fig. 1B), aspartic acid, glutamic acid and serine. In each case, the concentration increased in the latter half of the ripening period. Changes much beyond this time are probably only of academic interest since the fruit would have been unfit for consumption or processing.

• No useful predictive relationships were found linking fruit firmness, or soluble solids, with any individual amino acid or combination of amino acids.

Implications

• The amino acid (and protein) content of kiwifruit is not especially high. Nevertheless, both supplement our dietary requirement for "essential" amino acids - those unable to be synthesised by the body. Kiwifruit has a greater total concentration of free amino acids compared to fruits like apple and banana (Table 1), and a high proportion of arginine, asparagine and glutamine. The presence and high proportion of these three amino acids is not sufficiently unusual to set kiwifruit apart from other fresh fruits, in the way that its histidine content, for example, distinguishes banana.

  Figure 1. Changes in the fresh weight concentration of arginine (A) and alanine (B) in ripening kiwifruit after one month of cool storage. The length of the error bars are 2 times the standard error of the mean.

• Lack of pronounced changes in amino acid composition during ripening, especially amongst the major components, means that reactions involving amino acids during processing should be independent of the stage of fruit ripeness. The large between-fruit variability in the composition of individual amino acids, however, makes it difficult to predict accurately the nutritive contribution pulped fruit or juice make to processed food products. The latter observation supports Castaldo et al. (1992) who came to a similar conclusion after examining the composition of kiwifruit puree using fruit sampled from different regions from throughout Italy.

Are kiwifruit "smart"?

The amino acids tyrosine and phenylalanine are precursors of neurotransmitters (dopamine and adrenaline) that increase alertness. Other amino acids such as glutamate, the salt of glutamic acid, are found in high concentrations in the brain and central nervous system, or, in the case of g-aminobutyric acid, are associated with sites affecting relaxation and sedation . Foods containing elevated concentrations of these compounds are termed "smart" foods by those who advocate the inclusion of these items in the diet as a practical (but unproven) way to influence alertness, memory retention and other neural functions. Kiwifruit, and apple and banana for that matter, contain minor amounts of phenylalanine and tyrosine. Consequently they are lowly ranked on a "smart" food scale, especially in relation to other foods that contain a greater protein content. No dietary implications of any significance arise from this observation. In the normal course of events we aim for a balanced diet, and the alternative nutritional attributes for which kiwifruit is well-known (eg., vitamin C content, fibre content) ensures that it remains an important product for consumption, regardless of its ranking on any classification list as to whether or not it is "smart" to eat.

References

Castaldo, D. et al. 1992. J. Ag. Food Chem. 594-598.

Clark, C.J. et al. 1992. Scientia Hortic. 52: 85-94.

Gomis, D.B. et al. 1990. Chromatographica 29: 155-160.

Steward, F.C. et al. 1960. Ann. Bot. 24: 117-146.