The first obvious attraction of kiwifruit to the consuming public comes when the fruit is sliced in two and the internal beauty of its cell structures is exposed to the sight. No other fruit has such a complex and aesthetic-looking appearance in this form. The array of radial lines emanating from the centre of the fruit combined with the small black spots created by the seeds offers an abstract presentation which pleases the eye. The dimension of colour, in this case green, adds a final touch to the spectacle which completes the consumers' appreciation of what kiwifruit is all about. The phenomenon of taste follows this initial exposure, but by then our minds are set to expect a fulfilling experience. Thus, the green colour of kiwifruit helps identify this fruit as being separate and distinct from others.

A Dismal Brown

Food processors have attempted to produce products which show this green coloration to advantage. However, when the fruit is processed and subjected to heat, the colour changes into a dismal-looking brown. Some processors have kept the green colour for an extended period by avoiding heat-treatment of the juice or other liquid products of kiwifruit and freezing the product soon after production. Others have simply added food dyes such as green dyestuffs or combinations of yellow with blue to obtain a colour close to that of the original flesh of the fruit.

Heat treatment of liquid food products is commonly done to achieve a reduction in the microbial population of the product and/or to destroy the activity of certain enzymes which would cause problems if left unaffected. It is therefore desirable to use heat treatment processes if high costs of freezing, distribution and storage are to be avoided and if the product is designed to have a usable shelf-life.

Chlorophyll

The problems behind this loss of green colour from kiwifruit lie in the fact that the green material is a chemical called chlorophyll which is also present in many other types of plant material, such as grass, leaves, and vegetables. But all these other plant materials have low acidity compared with that found in kiwifruit. In whole kiwifruit, the chlorophyll pigment is carried within transparent cells which protect it from the harsh acid environment of the kiwifruit juice cells. Once the fruit is macerated, the chlorophyll is flung into contact with the surrounding acidic environment and susceptible to degradation, a process which is rapidly accelerated by the application of heat.

The research of the scientists in HortResearch has led to a process designed to maintain the natural green appearance of the processed liquid product without resorting to the addition of extraneous dyestuffs. The resultant liquid may be heated to normal food processing temperatures and the fresh green appearance will remain. This new process will allow kiwifruit juice to be processed on a large commercial scale, while still retaining all the sensory, nutritive and aesthetic attributes of the product. This breakthrough is in the process of being protected by Patent Application but, in the meantime, HortResearch is pressing on studying other ways in which to improve the New Zealand kiwifruit industry's position as a supplier of the best in processed kiwifruit products.

One other major problem with liquid products from kiwifruit is that the delicate aroma and flavour of kiwifruit are also severely affected by heat treatment. The changes have been described as "cooked cabbage", "cooked gooseberry" or "hay-like" but whatever description is accepted it is clear that the sensation created is somewhat undesirable. Research is well underway at HortResearch (see article titled "The Kiwifruit Sensation" on p. of this issue) to stabilize the aroma and flavour of the kiwifruit juice and develop a new process which will also eliminate the undesirable off-flavours formed during pasteurization. It is anticipated that the resultant product will possess a smooth mouth-feel comparable to a pulpy nectar-type texture. The high natural vitamin C in the juice will be preserved as will the high mineral content, especially potassium which is found in abundance in kiwifruit.

Reducing Acidity

Finally, the high acidity of single strength kiwifruit juice renders it unacceptable as a drink without some adjustment of its sweetness level. This is commonly done by the addition of sugar and subsequent dilution to taste - but the product then moves out of the "juice" category and into the "cordial" category. An alternative method has been developed in which the natural sugars of the juice remain at the same original levels but the acidity of the juice is reduced. By assiduously targeting a preferred acidity level, the balance of sugar to acid attains a value which is generally acceptable amongst consumers.

Thus, with heat-stable green coloration, original aroma and flavour character and acceptable sugar-acid balance combined with good storage capabilities, New Zealand will be able to provide the world's first kiwifruit juice which looks and tastes like something akin to the succulent green flesh of our famous kiwifruit.