Introduction
The kiwifruit was bought over, from China to New Zealand in 1906 and became immediately popular. Since then the kiwi fruit has develop into an important export product for New Zealand, due to its profile as the most nutrient dense fruit, followed by papaya, mango and orange (California Kiwifruit). The season generally runs from early May till around early July. This short season produces large quantities of Kiwifruit, so large in fact that the demand has become lower than the quantity of fruit available, leading to a need for a more diversified product range. It was found that by extending the shelf life of the kiwifruit it allowed full advantage of the leftover fruit, also decreasing the amount of large waste product that had been traditionally encountered.
The kiwifruit industry emerged in the 1970s and immediately experienced complications. These included changes in flavour, pigments, colour and formation of hazes and precipitates in liquid products. Many of these problems do not only occur in the kiwifruit industry but also occur in other fruit processing industries. Therefore past and future research into fruit processing provides large benefits for the industries concerned and also consumers. We are also seeing a breakthrough in many new and emerging technologies such as Ohmic heating, pulsed electric fields, edible coatings and pathogen-specific bacteriophages. These have been competing and replacing traditional processing which cause much larger quality losses. Traditional processing methods include drying and related processes (removing water), refrigeration (chilling and freezing), radiation (irradiation using gamma rays , light pulses, ultra violet), thermal processing (pasteurisation, blanching, cooking, sterilisation), high hydrostatic pressure (including extrusion), oscillating magnetic fields and recombination processes (homogenisation), separation processes (filtration, membranes, centrifugation), ultrasound and pulsed electric fields.
Although there seems to be a surplus of methods to preserve kiwifruit, nutrient losses during processing are still being encountered. This is a major downfall as kiwifruit is highly nutrient dense. It has the highest level of vitamin C (3-5 times more than citrus fruits) and it is a good source of dietary fibre, folic acid, vitamin E and minerals (P, K, Ca, Mg, Mn). It also has an impressive antioxidant capacity, contains a wealth of phytonutrients (including carotenoids, lutein, phenolics, flavonoids and chlorophyll). Epidemiological studies have concluded that consumption of fruits and vegetables imparts health benefits, e.g. reduced risk of coronary heart disease and stroke, as well as certain types of cancer (Gilbert, 1997). These benefits are of major importance to consumers, hence adding additional pressure to the kiwifruit industries.
Today kiwifruit that does not meet export standards and not sold in local market are processed into various products, primarily canned slices in syrup, frozen pulp and slices, juices and wines (VENNING, 1989). However, there has been an increasing market demand for minimally processed fruits and vegetables due to their fresh-like character, convenience, and human health benefits. Processing techniques such as ethylene absorbers, antimicrobial techniques and advances in packaging have helped make this product possible, at the same time traditional techniques such as thermal processing are becoming less desirable.
Minimally Processed Kiwifruit products
Minimally processed foods are products such as fresh slices and fresh fruit salad. Processing technology is needed in order to maintain the fruits qualities and lengthen the shelf life of the product once processed. The principal problem for fresh-cut fruit industries is the relative shorter shelf-life of minimally processed fruit compared to the intact product. Minimal processing includes grading, washing, sorting, peeling, slicing, chopping and then packaging; resulting in quality deterioration related to water loss, softening, microbial contamination, increased respiration and transpiration, increased ethylene, increased enzymatic activity and cut-surface browning (Rolle, 1987). Many new and emerging technologies have been developed to increase the shelf life of the packaged product to a few days. These include processes such as the addition of an edible coating, dipping the cut fruit in different solutions, the use of volatiles compounds, the use of essential oils, preventing enzymatic and non enzymatic chlorophyll reactions, the addition of citrus acid and improved packaging. Such modified atmospheres can be achieved either actively, by filling the packages with specified gas mixtures, or passively, as a consequence of respiratory O2 uptake and CO2 evolution of packaged produce and gas transfer through the packaged films (L. Jacxsens, 2003).
Kiwifruits like all fruits and vegetables are living commodities and their rate of respiration is of key importance to maintenance of quality. Respiration and transpiration lead ultimately to water loss and senescence, therefore it can be observed that the greater the respiration rate the shorter the shelf life. The first step to processing the kiwifruit is the fresh-cut process (peeling, cutting, dipping), this causes a loss of compartmentalisation allowing for the mixing of previously sequestered metabolise of the ethylene generating system, generating increased production of ethylene (Mazliak, 1983). Ethylene is not only detrimental by itself but it influences and promotes enzymatic activities, softening and ripening. Because these compounds occur naturally in the fruit and the process of ethylene is inevitable researchers have found a way to not stop the production of ethylene, but instead to inhibit ethylene action. Such inhibitors include 1-Methylcyclopropene (1-MCP), which is a nontoxic antagonist of the ethylene hormone that binds to the ethylene receptor after cutting. Although this prevention method is already a commercial product in some countries, it is yet to be used in New Zealand. The introduction would be suitable to not only decrease respiration and transpiration rates in kiwifruit, but also for other non climatic fruit such as bananas. Along with cutting prevention methods, dipping may also be used. Including ascorbic acid and calcium chloride helps to preserve the phenolic substances present by inhibiting or hindering enzymatic browning as well as other harmful reactions influencing food aroma, colour and texture (D.A.C. Rodrigues, 2008).
There are not only pre-treatment methods available to control quality degradation but postproduction processes such as packaging. Modified atmosphere packaging is based on the fact that low concentrations of O2 and moderate to high concentrations of CO2 around the fruit, can bring about beneficial chemical changes in fruit tissue generating an extended shelf life with the retention of quality attributes. Whereas traditional packaging is meant for mechanical supporting of otherwise non-solid food, and protecting food from external influences such as heat, light, the presence or absence of moisture, oxygen, pressure, enzymes, spurious odours, microorganisms, insects, dirt and dust particles, gaseous emissions, and so on (Robertson, 2006). On the other hand, new food packaging technologies developed during past decades as a response to consumer demands towards mildly preserved, fresh, tasty and convenient food products with prolonged shelf-life and controlled quality (Lagaron, Català, & Gavara 2004). Active packaging has been developed to allow specificity towards a product and its attributes to allow it to have a longer shelf life with minimal quality losses. Active packaging includes additives that are capable of scavenging or absorbing oxygen, carbon dioxide, ethylene, moisture, odour and flavour taints; releasing oxygen, carbon dioxide, moisture, ethanol, sorbate, antioxidants and/or other preservatives and antimicrobials; and/or maintaining temperature control (Table 1). The wide diversity of active packaging devices have specific applications to individual food products for which the shelf-life can be extended substantially, so long as the food's unique spoilage mechanisms are understood and controlled. Modified atmospheric and active packaging have allowed the kiwifruit industry to meet consumers demand for fresh, quality products without having to add anything to the original product. This has been achieved by the use of ethylene absorbers and through the control of gases. Gases are best controlled by the use of sheet active packaging. Its great diversity also means that it shows full potential to be used for other products.
Table 1. (Restucciaa D., 2010)
Examples of active packaging applications for use within the food industry.
Absorbing/scavenging properties
Oxygen, carbon dioxide, moisture, ethylene, flavours, taints, UV light
Releasing/emitting properties
Ethanol, carbon dioxide, antioxidants, preservatives, sulphur dioxide, flavours, pesticides
Removing properties
Catalysing food component removal: lactose, cholesterol
Temperature control
Insulating materials, self-heating and self-cooling packaging, microwave susceptors and modifiers, temperature-sensitive packaging
Microbial and quality control
UV and surface-treated packaging materials
Consumer demand has increased to more natural ready to use or ready to eat products. These do not generally contain preservatives or antimicrobial substances and rarely undergoes any heat processing before consumption. Therefore, a variety of pathogenic bacteria, such as Listeria monocytogenes, Salmonella, Shigella, Aeromonas hydrophila and Staphylococcus aureus, as well as some Escherichia coli strains may be present on fresh fruits and in the related minimally processed refrigerated products (Gould, 1992). In addition to the problems linked to microbial proliferation, the activation of enzymatic systems can lead to changes of colour, softening, flavour modification and loss in nutritional value (Nicoli, 1994). To prevent this treatment such as freezing, high pressure heat treatment and filtration may be undertaken. Traditional and more common thermal treatments, such as pasteurisation and sterilisation, all requiring high heat which in turn damages nutrients, colour, flavours, texture and volatiles. Research efforts towards non thermal food preservation techniques have been well underway and been growing in the last decade. Non thermal processes, like supercritical carbon dioxide (SC-CO2), may offer better retention of natural flavour and nutrients in treated foods compared to the traditional thermal processes (Kincal, 2006). Microbial and enzymatic inactivation with SC-CO2 has been mostly applied to liquid food products, such as kiwi fruit juices; in particular, SC-CO2 has been shown to be a promising alternative for pasteurisation of fruit juice. The study of Garcia-Gonzalez et al. (2009) found CO2 efficiently induced microbial inactivation as well as preserved more quality attributes, including aroma compounds, than thermal treatment. This provides not only future benefits for the kiwifruit processing industry but those processing other fruits, vegetables, wine and milk.
Through pre-processing treatments during cutting and dipping, and post-processing treatments such as packaging, consumer demand for minimally processed kiwifruit products with an extended shelf life is able to be met. Although there have been vast advances in the past decade, more research efforts are continuing towards a longer, cheaper, efficient and more optimal way to preserve and process kiwifruit.
Processed Kiwifruit Products
Traditional processing methods are very much still in use today. One of the major processes being used today is freezing preservation. Although this has been used for thousands of years, its ease and high product quality continues to make it a successful processing technique. Generally speaking the quality of frozen food products is not only due to the freezing process but also the thawing process. Kiwifruit quality is influenced by the rate of freezing and the formation of small ice crystals, these are critical to minimise tissue damage and drip loss during thawing. Although freezing does not affect flavour and colour, it also does not inactivate enzymes and hence can still affect the proteins present once thawed. Storing in a freezer will also not stop significant degradation of chlorophyll to pheophytins; this must be controlled as over half of the kiwifruit used in processing is made into frozen products, such as pulp, in New Zealand. Chlorophyll in plant tissue is protected from acidic plant constituents by its linkage with proteins of the thylakoid membrane (Cano M.P. 1992). The chlorophyll-protein linkage is easily broken during freezing. Freezing releases intercellular acids and encourages enzyme action. The solubilised chlorophyll is converted into the olive brown pheophorbide by low pH and chlorophyllase. Freezing may also cause macroscopic problems, other than enzymic activations. It may cause dramatic textural losses provoked by increasing ice formation causing cryoconcentracion phenomena. This promotes membrane denaturation and cell wall degradation mechanisms causing the membrane to rupture. The membrane rupture results in enzyme and/or chemical activity that also contributes to the mechanical damage (C. Fuster, 1994). Possible strategies proposed by various authors for controlling chlorophyll degradation include (Heaton and Marangoni 1996): inactivation of chlorophyllase with minimal conversion of chlorophyll to pheophytin; addition of Mg2+ salts to prevent the loss of magnesium from chlorophyll; addition of Cu2+ or Zn2+ salts, which form a new bright green complex chlorophyll ion metal and control of the pH, temperature and ionic strength of food products. Browning may also be caused by non enzymatic reactions such as the Maillard browning reactions and ascorbic acid browning reactions. Reactions involving ascorbic acid were found to contribute more too overall browning than did Maillard browning (Reid, 1998). 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. This is a major problem in the processing industry not just for kiwi fruit but for their fruits such as bananas, as the brown colour indicates quality deterioration and is not visually accepted by consumers. Therefore previously discussed methods are a must to increase consumer purchase of processed kiwifruit.
Another studied method was the pre-treatment of osmotic dehydration before freezing. This causes a reduction of the freezable water content and to the possible specific function determined to impregnated solutes in membrane protection. It has been proven to improve the texture characteristics of thawed fruits and vegetables (M. Robbers, 1997), decreases enzymatic browning (Conway, Castaigne, Picard & Vovan, 1983) and reduces structural collapse and drip loss during thawing (Forni, Torreggiani, Crivelli, Maestrelle, Bertolo & Santelli, 1990). Osmotic dehydration is operated at room temperature and atmospheric pressure with minimal thermal and mechanical damage, making it an efficient, worthwhile process. It works by dehydrating the kiwifruit to desirable moisture content and then freezing; hence although large ice crystal formation is inevitable, less water is available to be frozen, thus lowering refrigeration load. It involves removing water from the feed by a solution (concentrated brine) flowing downstream a microporous, hydrophobic membrane. The hydrophobic nature of the membrane prevents penetration of the pores by the aqueous solution, creating air gaps within the membrane. The difference in solute concentration, and consequently in water activity between the two sides of the membrane, induces a vapour pressure difference causing a water vapour transfer across the pores from high-vapour pressure phase to the low one (Gostoli, 1999). Osmotic dehydration is a useful technique for the concentration of fruit and vegetables, realised by placing the solid food, whole or in pieces, in sugars or salts aqueous solutions of high osmotic pressure. It gives rise to at least two major simultaneous counter-current flows: a significant water flow out of the food into the solution and a transfer of solute from the solution into the food. Successful applications of dehydrofreezing on fruits and vegetables have been reported. Samples of fresh kiwifruit were immersed in 68% (w/w) aqueous sucrose solution to dehydrate for 3 h, then frozen in an air-blast freezer with an air velocity of 3 m/s at about −3 °C (Spiazzi et al., 1998). It was clearly observed that freezing starts at a lower temperature in the dehydrated product and the temperatures of dehydrated samples went down to −18 °C in 19-20 min, 20-30% faster as compared with untreated kiwi which required the freezing time of 23-24 min.
Kiwifruit juice has a large number of enzymes present, increasing the number of factors supporting quality deterioration. Juice clarification, stabilisation, depectinization and concentration are typical steps where membrane processes as microfiltration, ultrafiltration, nanofiltration and reverse osmosis have been successfully utilised. Although all fruit juices are made similarly the standard technique used is not as efficient when working with kiwifruit juice. Suspended solids in fresh kiwi fruit juice can be completely removed with ultrafiltration, but the resulting clarified juice has a lower viscosity and negligible turbidity. Microfiltration has lately appeared 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. The high amount of haze is due to the kiwi fruit having higher protein content in comparison to many other fruits. Many researchers have look into reducing the haze by lowering some of the protein content. Conventional methods of processing the juice such as a HTST treatment, in which heat-labile protein is coagulated followed by addition of fining agents to absorb heat-stable soluble protein, have been tested and refined for this problem. 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.
kiwifruit processes.GIF
Traditionally kiwifruit products underwent various preservation methods, followed by packaging into cans or glass jars, being hermetically sealed and then heated to a specific temperature for a specified time to destroy disease-causing microorganisms and prevent spoilage. Canning has however proved to lead to a loss of nutrient value in foods, particularly of the water-soluble vitamins. This produces kiwifruit with a much milder flavour, softer texture and lighter colour than fresh kiwifruit. Traditional methods included pasteurisation and blanching of the kiwifruit. Which are designed to destroy pathogenic vegetative microorganisms and plant based enzymes to increase the shelf life of the product. Because these are only mild heat treatments they may be used for minimal processed foods as well. High pressure is able to inactivate enzymes or microorganisms and achieve the pasteurization of food. Moreover, this technology allows the preservation of precious natural food properties like vitamins or natural aroma following treatment (Hendrickx & Knorr, 2002). This is not only relevant for the kiwi fruit industry but may also be used in other fruit and vegetable processing industries to help maintain quality and the nutrient profile of the food.
The Future of the Kiwifruit Processing Industry
The future of the kiwifruit processing industry continues to grow in New Zealand as consumers become more aware of the health benefits kiwifruits supply to them. This increased demand has put added pressure on researchers to develop new and improved ways to keep kiwifruit quality the same or similar yet increase the shelf life.
The use of edible coatings has been developed for minimally processed fruits. Edible coatings provide a semi-permeable barrier against oxygen, carbon dioxide (CO2), moisture, and solute movement; thereby reducing respiration, water loss, gas exchange, decrease nutrient loss, prevent microbial growth and oxidation reaction rates (Park, 1999). They are also useful as a carrier of food additives such as antibrowning agents. Proposed coatings of fresh fruits and vegetables with a semi-permeable composite film comprise of the sodium salt of carboxymethylcelluose (CMC) and sucrose fatty acid esters. This concept allows producers to supply fresh looking kiwifruit products with minimal processing time and energy expenditure needed. It is not only a useful technology for kiwifruit but also for the rest of the fruit and perhaps vegetable industry.
Rapid industry expansions in other technology fields are also evident. Methods such as ultrasound, pulsed electric fields and pulsed magnetic fields are currently being used and trialled. The principles behind these technologies rest on their ability to excite water or solute molecules inside food. Rapidly alternating electric or magnetic fields are used to re-orientate molecules, creating friction causing them to heat rapidly or modify the organoleptic properties. This heating technology is termed as Ohmic heating or electro-heating. In the food industry, more attention has been paid to the application of Ohmic heating on aseptic processing and pasteurisation of particulate foods. In comparison with microwave heating, Ohmic heating is more efficient because nearly all of the energy enters the food as heat and Ohmic heating has no limitation of penetration depth. Therefore using Ohmic heating to thaw frozen foods is an innovative method. Fuchigami, Hyakuumoto, Miyazaki, Nomura, and Sasaki (1994) investigated the effects of electrostatic thawing on texture and amount of cell damage. Light microscopy and transmission electronic microscopy for frozen carrots revealed that drip, cell damage and softening were prevented by Ohmic thawing. The use of an alternating electric field would be more beneficial because an electrostatic field would cause electrolysis and thus require expensive electrodes. Although this technology is not very evident in the kiwifruit processing industry today it has a high potential for future incorporation into the industries.
The application of pulsed electric field (PEF) is one of several emerging non-thermal methods.
The application of sufficiently high electric fields results in pore formation and breakage of cell membranes. Electrical breakage may involve various mechanisms which in turn produces dielectric breakdown within a biological structure, making the cell membrane become more permeable to electrical current and to solutes (Bouzrara, 2000). Pulsed electric field is an attractive process in cell disruption, metabolite release and food preservation. It has potential to be used as a pre-treatment on mass transfer during Osmotic dehydration as a first step in the drying of fruit. Food texture is closely related to the hydrated condition of the food. Water in food is affected by PEF treatment since there is large dipole moment altering the behaviour of water molecules. Electrostatic interactions (between water and proteins, lipids, polysaccharides, etc.) in biological systems confer specificity and play a role in retaining the particular conformation of the molecules. Therefore this technology is suitable for pre-treatment of processed kiwifruit before drying and freezing. It is an optimal process not just for kiwifruit but for other fruit to. Although this process is appealing the cost would remain very high and hence further investigation into consumers' evaluation of the two products before and after treatment would be recommended to be done.
Food irradiation is commercialized in about 40 countries. Identification of irradiated over non-irradiated foods is highly recommendable to confirm both compliance with existing regulations and beneficial effects of irradiation treatment. For fresh fruits and vegetables, irradiation is approved at doses ranging from 1 to 3 kGy in different countries for quarantine treatment, microbial control, delay of ripening, and shelf-life extension (Deokjo J., 2008). The radiation of interest in food preservation is ionizing radiation. These shorter wavelengths are capable of damaging microorganisms such as those that contaminate food or cause food spoilage and deterioration. Although this may seem as dangerous it has proven to be a highly potential processing method, however even though it is successful it still does not mean consumers will trust and be happy with this product, due to the negative media about radiation.
The major cause in shelf life deterioration is the microbial harm that may be caused to the consumer if the product is microbiologically unsafe. Recently efforts have been focused on using pathogen-specific bacteriophages to achieve the control of pathogens in produce. Other bio control options include the use of protective cultures, typically psedomonads or lactic acid bacteria, or natural antimicrobial compounds such as bacteriocins, naturally occurring in plant volatiles and non volatile essential oils. Application of natural antimicrobial substances (such as bacteriocins) combined with novel technologies provides new opportunities for the control of pathogenic bacteria, by improving food safety and quality. Bacteriocin-activated films and/or in combination with food processing technologies (food irradiation, pulsed electric fields) may increase microbial inactivation and avoid food cross-contamination. Application of bacteriocins, bacterial protective cultures, or bacteriophages, can also decrease the incidence of food-borne pathogens in livestock, animal products and fresh produce items, reducing the risks for transmission through the food chain.
Microbes elicit a variety of mechanisms that facilitate colonization and prevalence in ecological niches. These include adherence, competition for available nutrients, production of toxic metabolites, and secretion of dedicated antimicrobial substances such as antibiotics and bacteriocins. The wise exploitation of these mechanisms of microbial interference can be beneficial to human and animal health, and economy.
Conclusion
It can be seen that although the kiwifruit processing industry has been running for over 40 years, improvements are still being made and developed all the time in order to get a fully optimised product that consumers and producers are happy with. As technology continues to increase we will continue to see future advances in processing and preservation methods not just for kiwifruit but for the whole fruit and vegetable processing industry. Not only will this benefit already existing products such as preserved whole fruit or segments in glass or cans, wine, liquor, a highly aromatic jam, marmalade and fruit paste but may lead to development of new products. More specifically, local uses of the fruit, particularly in New Zealand, have included that of gourmet chocolates, beauty products and freeze dried powder. It can therefore be seen that were there is a consumer need and demand for a product the processing industry is relied on to develop methods and techniques to meet these. Kiwifruit has high potential for future processing success.
