KIWIFRUIT RESEARCH

A need for processing commonly occurs as a result of a glut in fresh fruit production. This was the case with kiwifruit, and fruit culled from export inspection lines was taken by processors. There was a demand to develop a diversified product range for the consumer if full advantage were to be taken of the quantity of excess fruit available.

In order to relate scientific findings to commercial reality, it was necessary to clearly identify the most common starting material which would be used by the processing industry. In the case of kiwifruit it was a single cultivar of an Actinidia species, Actinidia deliciosa (A. Chevalier) C. F. Liang et A. R. Ferguson var. deliciosa (syn. A. chinensis Planchon var. hispida C. F. Liang). The fruit commonly

rejected from the fresh fruit exports and purchased by the processing industry is, in most cases, sound and of good quality but may have been rejected because of defects of size, shape or due to some aesthetic fault such as exterior markings.

The kiwifruit processing industry emerged in the 1970s but immediately experienced difficulties. It was found that kiwifruit was a difficult fruit to process, not behaving as do many other fruits. Although researchers had anticipated some of the processing problems, there was still a considerable lack of information on many of the fundamental properties of kiwifruit. Industry assisted in bringing many of the problems which they encountered to the attention of the scientist. These included changes in flavour, loss of the green colour, development of brown pigments, formation of hazes and precipitates in liquid products, and the sensation of an irritation ("catch") in the throat when eaten. Many of the technical problems appeared inter-related and programmes were initiated into researching these areas.

Colour Changes

During processing kiwifruit loses its green colour and changes to a dull, olive-brown coloration. This is due mainly to conversion of chlorophyll to pheophytin (plus other degradation products) and occurs in both liquid and solid processed products. Kiwifruit juice concentrate develops a deep brown colour during storage and consequently must be stored at or below -18°C to minimise the change.

DSIR developed a successful approach to the maintenance of green colour in dehydrated kiwifruit and this is currently the matter of a patent application. However, the creation of a green, heat-treated kiwifruit juice evades solution. CSIRO developed a method for the extraction of chloroplasts from kiwifruit juice and their incorporation into a recombined juice with short shelf-life. Pasteurisation or sterilisation of this recombined juice results in conversion of chlorophyll to olive-brown pheophytin and other brown chlorophyll derivatives, as there is still no protection offered to the exposed chlorophyll.

Research into developing a new process for stabilising the natural green colour in juices and pulps is well advanced at the Food Science and Technology Section of DSIR Fruit & Trees and is partially funded by the New Zealand Kiwifruit Marketing Board. Initial results are extremely encouraging.

Nonenzymic browning of kiwifruit concentrate is a complex system which was investigated at DSIR. Five synthetic model concentrates were studied in order to explore the contributions of different nonenzymic browning reactions to colour changes in stored kiwifruit concentrate. Two different classes of nonenzymic browning mechanisms were found to be of significance: the Maillard browning reactions and ascorbic acid browning reactions. Reactions involving ascorbic acid were found to contribute more to overall browning than did Maillard browning. Kiwifruit juice concentrate produced under conditions of reduced oxidation leads to less conversion of ascorbic acid to dehydroascorbic acid and other reaction products which are capable of further reactions forming brown pigments.

Flavour Changes

• Aroma.Most changes to kiwifruit flavour during processing into juice are referred to as `detrimental'. The natural flavour of fresh, ripe kiwifruit is very complex and subtle. There is no single flavour component which provides the characteristics of kiwifruit flavour and it is believed that a great deal of synergism is involved between flavour components.

  Immediately after cutting and mashing the fruit, hydrogen sulphide is evolved and this soon gives way to an aroma note described as "fruity" or "estery". The flavour is susceptible to further changes and this may be influenced by an indigenous lipoxygenase. Scientists at DSIR showed that ethyl butanoate, hexanal and E-hex-2-enal are important contributors to the aroma of kiwifruit and found that increasing ripeness is associated with a rapid increase in the levels of aroma volatiles, especially esters. Increasing storage time prior to ripening is accompanied by a decrease in the amount of aroma volatiles.

  A study at DSIR looked into the strong unpleasant aroma notes (described as "hay", "old cut grass" and "cooked gooseberry") which are formed in processed kiwifruit juice and other products. E-hex-3-enal was shown to be a major contributor to this undesirable aroma. Methyl butanoate, ethyl butanoate and E hex-2-enal exert a positive influence on the favourable characteristics of kiwifruit flavour. Esters appear to have a positive influence on the perception of sweetness and this aspect of flavour chemistry is under investigation. We are also studying the optimum system for recovery of a high quality kiwifruit essence from single-strength juice and making an attempt to relate quality to the presence of known aroma compounds.

• Taste. Kiwifruit has a unique taste which is particularly susceptible to changes in the sugar-acid balance. In addition to the fundamental taste of the juice, consumers experience an unusual "cloying" sensation not far removed from astringency when drinking the product. The cause of this is uncertain but it may originate from the mix of non-volatile organic acids. The mix is unusual due to the presence of quinic acid in significant quantities in addition to ascorbic, citric and malic. Typical levels in mature fruit at harvest are 6-14mg/gfw for citric, 3-8 mg/gfw for quinic and 1-2mg/gfw for malic.

  The major soluble sugars found in kiwifruit are glucose, fructose and sucrose. The quantities of these sugars in whole, ripened fruit are : fructose 8-14 mg/gfw; glucose 9-16 mg/gfw; sucrose < 2mg/gfw. Inositol is present at 1-2 mg/gfw.

  The effect on taste of the interaction of different sugars in combination with varying amounts of acids requires more research effort. The subject is further complicated by the superimposition of aroma compounds on taste sensation.

• Texture. Juices containing varying amounts of cloud particles offer the consumer the experience of a different mouth-feel (texture) compared with clarified fruit juices. Cloudy juices have been available on the world market for a number of years, however, recent developments in enzyme technology have led to a significant improvement in the consistency and quality of cloudy juices and concentrates. The production of a high quality cloudy kiwifruit juice is being researched at DSIR.

• Enzymes: As with many forms of juice processing, commercial enzymes have been used successfully to aid juice extraction from kiwifruit pulp. A mixture of polygalacturonase and pectinase enzymes are added to assist with juice release from pulp. Despite a high degree of depolymerisation achieved by enzymic treatment, the juice of kiwifruit does not flow freely during pressing. In order to create capillaries along which juice can travel away from the pulp, a "press-aid" is commonly added. Cellulose acts in this manner, creating channels for juice exudate, and is stirred into the pulp prior to pressing at 2% w/w. The use of enzymes permits the production of juices and concentrates of sparkling clarity once the pectins are sufficiently degraded.

• Irritation. The presence of a throat irritant in processed kiwifruit products was first

  reported in 1981. All dehydrated and "worked" kiwifruit products (such as nectars and pulps), where shear forces have acted on the cells of the fruit tissue, are known to generate this irritation. The irritant effect was investigated at DSIR and found it was due to calcium oxalate crystals in the form of needle-shaped "raphides".
  Figure 1: Needle-like raphides

  The raphides (Figure 1) occur in idioblast cells and are most abundant in the inner pericarp adjacent to the seeds of the fruit. As the raphides are embedded in mucilage, consumption of fresh fruit does not normally cause an irritating effect. Apart from the irritation, the ingestion of the calcium oxalate is harmless.

Haze Formation

Kiwifruit is high in protein compared with many other fruits and this causes protein-haze problems in clear juice processing and storage. Analysis of fresh, ripe, Hayward kiwifruit shows it contains 0.75% w/w protein; freshly pressed kiwifruit juice has been reported to contain 0.6% w/w protein. Conventional methods of processing the juice use a HTST treatment in which heat-labile protein is coagulated followed by addition of fining agents to absorb heat-stable soluble protein. The most commonly used fining agent for this purpose is bentonite but care must be exercised in its use so that flavour is retained and juice yield is maximised.

DSIR scientists investigated the effect of SO₂ addition to concentrates and reported a reduction in haze formation over a 4 months' storage period. Other workers at DSIR studied the influence of protein content on kiwifruit juice stability and the effect of bentonite treatment on specific protein fractions in relation to haze and sediment formation. Heat treatment reduced the soluble protein content by approximately 70%, primarily due to the precipitation of actinidin, the kiwifruit protease. Very large amounts (3500 -4500 mg/L) of bentonite were required to remove all the heat-stable protein fractions.

Concentration and Aroma Recovery

Methods are under investigation to separate the aroma from kiwifruit juice before any form of treatment which could conceivably modify the flavour characteristics, such as heat treatment or enzymic hydrolysis. Protection of components from oxidation may also be an important consideration for certain fruit species. Our current approaches to the separation and concentration of kiwifruit essence involve vacuum distillation at ambient temperatures combined with the trapping of highly volatile aromatics in liquid nitrogen.

Dehydration

Dehydration of kiwifruit is fraught with problems relating to colour and flavour changes. At DSIR, a method has been developed for maintaining the green colour of dehydrated kiwifruit slices and this procedure has recently been successfully patented. The process uses components which occur naturally in the fruit and the system does not employ preservatives or added colouring.

In addition to maintaining the fresh green colour of the dehydrated slices, the loss in vitamin C content during the drying cycle and subsequent storage is minimised.

Figure 2: Loss of Vitamen C (ascorbic acid) during storage

In juice from Hayward variety, vitamin C is found at a level of 100mg per 100mL. During storage of dehydrated products, the amounts of ascorbic and dehydroascorbic acids suffer a decline (Figure 2). Juice concentrates also suffer a decline in Vitamin C and must be stored at or below -18°C in order to minimise the effect.

Sensory Evaluation

An essential component of our work on fruit flavour encompasses Sensory Evaluation. All of the information on the aroma, taste, texture and irritant effects of processed kiwifruit products has been supported by a professional, well-equipped sensory evaluation unit. Trained and consumer panels are used to determine the sensory profile and acceptability of products.

The research focus for this group is based on determining the relative contribution of the physico-chemical characteristics to the sensory properties of aroma and flavour of kiwifruit and apples. Initial work determined the role of the major sugars and organic acids to the sweetness and acidity of fruit. Recently the effect of the major esters, alcohols and aldehydes on the aroma of kiwifruit juice has been determined. This has enabled the key compounds contributing to the unique kiwifruit flavour to be identified.

MEMBRANE FILTRATION

Ultrafiltration and reverse osmosis are standard techniques for clarification and concentration of fruit juices but both of these techniques have limitations when applied to kiwifruit juice. U.F. and R.O. have been used in pilot-scale processes to extract and concentrate the kiwifruit protease known as actinidin but the process lacks economic viability.

Microfiltration using membranes with porosities of 0.20-0.45mm appears encouraging for the production of clarified juices from partially clarified kiwifruit juices whilst permitting aseptic packaging of the product. Unfortunately protein haze has still been evident in kiwifruit juice and alternatives to this method are being researched.

One of the more interesting methods receiving our attention is membrane technology for the direct osmotic concentration of fruit juices. This principle uses strong solutions of sodium chloride or high fructose corn syrup to establish an osmotic pressure gradient across a semi-permeable membrane and thus remove water from a single strength fruit juice. A major benefit of this system is that it does not require any costly energy input except that associated with pumping fluids along pipes.

There is also the prospect of sterilising liquid products at ambient temperatures using high hydrostatic pressures. By combining suitable membrane techniques for various treatments there is the possibility of processing kiwifruit juice at low temperature thereby maintaining much of the original flavour and colour characteristics of the fruit.

"FOURTH GENERATION" PRODUCTS

There is a demand for food products which are convenient to use and have a reasonable shelf-life but which have not been subjected to flavour-modification due to heat treatment or other non-physical procedures. These products are known as "Fourth Generation" foods and are those which have been minimally processed in order to maintain much of their original fresh character. Heat or preservatives are not applied. These products are not sterile and have a limited shelf-life.

In our research on kiwifruit, we are studying "Fourth Generation", minimally-processed fruit slices and attempting to control both physiological and microbiological changes in order to extend the shelf-life of the slices. This involves a study of the gaseous environment surrounding the sliced fruit, the nature of the packaging material and other non-detrimental physical treatment of the fruit. The optimal volume of package in relation to the mass of fruit must be defined and a study made of the real situation under which packages are stored throughout the distribution cycle.