The academic definition of shelf life is the amount of time a product can be stored without deterioration (Sewald and Devries, 2003). However, this concept creates a misconception for horticultural products, as quality deterioration starts immediately after harvest. However, for horticultural products, storage life is also defined as the time a product can be stored before reaching a quality standard, which is unacceptable, by the consumer. Accurate storage life prediction is desirable during the supply chain process to minimise the cost of product repacking. Usually storage life prediction tests
are designed to validate the length of the time a product can remain as ‘acceptable’
(Kilcast and Subramaniam, 2000) and/or have no change in desired sensory characteristics over the remaining entire life of a product.
Shelf life testing under accelerated conditions is widely adopted by food producers
(Sewald and Devries, 2003). This shelf life testing is known as ‘accelerated shelf life testing’. This method involves extrapolation of results from an accelerated environment to the expected results at optimal conditions. A few days or weeks at accelerated
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conditions required to reach a quality limit may correspond to months of shelf life at optimal conditions. For this form of testing, extreme environmental conditions (e.g. higher temperature and humidity) are used to accelerate the deterioration processes. Rise in storage temperature to create accelerated conditions is often assumed appropriate and interpreted by using Arrhenius model (Kilcast and Subramaniam, 2000). However, the pattern of deterioration (cause of product failure) of a product can differ between two storage conditions. A key assumption in accelerated shelf life testing is that pattern of deterioration in both conditions is correlated (Kilcast and Subramaniam, 2000). For each product, the correlation between the accelerated and optimal conditions will differ. Therefore, prior to industrial application, this approach requires establishment and validation of the relating correlation between storage and accelerated conditions (Kilcast and Subramaniam, 2000).
Accelerated shelf life testing of horticultural products differs from manufactured or processed food products (e.g. milk products) as many physiologically driven bio- chemical processes occur during storage. Another key feature is that one needs to perform this method for every batch of product. In the processed food industry, shelf life date is determined only once to print on the box. A carefully designed method of accelerated shelf life testing to predict the storage performance of fruit has potential to be adopted in the kiwifruit supply chain. However, application of this technique to fruit requires storage and monitoring of sub-samples of each batch of fruit in accelerated conditions. This approach may be referred to as accelerated fruit library monitoring.
2.5.1 Accelerated fruit library
An accelerated fruit library (AFL) is a technique to predict the storage life of fruit and assist in inventory management by identifying and enabling segregation of good or poor storing batches. AFL is different from conventional operational library monitoring, where fruit samples experience the same optimal storage conditions as the stored stock. The purpose of an operational library monitoring is to measure fruit loss at regular intervals to assess the amount and rate of loss in a whole lot (Benge and Kay, 2003). However, operational libraries do not predict the amount or rate of loss in advance. AFL may reflect loss patterns of a lot prior to the loss occurring during optimal storage, allowing mitigating actions to take place before the stock becomes unacceptable. By
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doing this AFL can facilitate in making efficient inventory decisions and enable reduction of total storage losses. The collection of losses information at accelerated conditions to correlate with normal storage environment may be possible, by doing careful observations of quality parameters that reflect the keeping quality of kiwifruit. For kiwifruit, flesh firmness is an important quality parameter along with number of rots (Section 2.4). In case of kiwifruit, application of ethylene and high temperature storage (or both) could be used to accelerate the softening losses.
Creation of an AFL requires loss assessment during a monitoring period to capture existing variability between fruit or batches. Data collected in AFL may combine the effect of many pre and at-harvest factors on storage potential. A robust relationship between AFL and storage data is required prior to industrial application (East, 2011). There are several examples of predicting storage potential or performance of fruit by following the pattern of losses at accelerated conditions. Ingle and Morris (1989) reported that softening rate in optimal storage conditions (0 °C) was correlated with softening at-harvest and at ambient conditions (20 °C). Later, after harvest flesh firmness change at 20 °C was used to indicate the storage potential of apple cultivars (Iwanami et al., 2004; Iwanami et al., 2008). East et al. (2008) proposed that rot incidence in mangoes stored at variable temperatures can be explained with Weibull distribution model. In addition, batch dependent model parameters at high temperature can indicate rot incidence in the same fruit at optimal (low temperature) conditions (East et al., 2008). East (2011) also predicted storage performance of strawberry batches as influenced by rot development from data collected in accelerated conditions (higher temperatures). In another example, at-harvest acceleration of degrading process by Mg2+ infiltration was used to predict the incidence of bitter pit in apple after storage (Burmeister and Dilley, 1993; Retamales et al., 2000; Sestari et al., 2009). These examples of AFL suggested that segregation of kiwifruit grower lines for good or poor storage potential may be achievable.
2.5.2 Options to accelerate kiwifruit softening
Both ethylene application and high temperature storage have the potential to accelerate kiwifruit softening and other losses (e.g. rots). Ethylene plays an important role in accelerating fruit ripening processes (Saltveit, 1999; Sfakiotakis et al., 2001; Section
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2.3.2.3). Many researchers studied kiwifruit physiological and biochemical changes with application of ethylene at ambient conditions. Jeffery et al. (1992) stated that exposure of kiwifruit to 5 ppm ethylene at ambient conditions (20 °C) resulted in substantial reduction in firmness from 6.3 to 1.1 kgf within 4 d of treatment. Wills et al. (2001) compared sensitivity of climacteric fruit to ethylene concentrations (i.e. < 0.005, 0.01, 0.1, 1 and 10 ppm). Banana and, kiwifruit were found highly sensitive to ethylene as compared to custard apple, mango, tomato, avocado and peach. Park et al. (2006) exposed kiwifruit to ethylene (100 ppm for 24 h) at 20 °C to study ethylene biosynthesis during ripening and observed a substantial decrease in flesh firmness at 2 d of treatment. Likewise, Korsak and Park (2010) also reported the same effect of ethylene exposure on kiwifruit to accelerate fruit softening. Therefore, ethylene can definitely accelerate the softening of kiwifruit in AFL. This high softening rate in accelerated environment could potentially be correlated with firmness loss in ethylene free low temperature storage. However, application of ethylene would not be as easy to apply in industry. Deliberate application of ethylene may entail a risk of contamination of the remainder of the fruit in the storage facility.
After packaging, kiwifruit are usually stored at low temperature (0 °C) for an extended period (Sale, 1990; Cotter et al., 1991; Burdon and Lallu, 2011) referred as optimal storage. During storage, temperature is of prime importance being a direct influencing factor on fruit quality, particularly loss of firmness (Ferguson and Stanley, 2003). Storage of kiwifruit at higher temperature (e.g. 20 °C) accelerates the ripening process because of the more rapid onset of autocatalytic ethylene production (Antunes and Sfakiotakis, 2000, 2002b). Given the examples of AFL application, use of high temperature could serve the purpose of accelerated fruit library establishment (Ingle and Morris, 1989; Iwanami et al., 2004; Iwanami et al., 2008; East, 2011). High temperature storage to accelerate fruit losses provides an opportunity for expressing uniform environmental conditions for all samples of many grower lines. Application of high temperature during AFL would be easy to adopt by industry with little additional cost or training.
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