Modelling the biochemical and sensory changes of strawberries during storage under diverse relative humidity conditions
Introduction
Fresh fruits and vegetables are among the most perishable food products. Due to their high perishability, they are also among the most frequently wasted foods (Kelly et al., 2018). Food waste has a devasting impact on food security affecting all people regardless of geography or social groupings. With a predicted increase of 1.7 billion in world population between now and 2050, mankind is placing more and more pressure on the shrinking finite resources used to produce our food (Mc Carthy et al., 2018). Yet it is estimated that globally 30–50% of all food produced never reaches its intended destination, i.e. the consumer. Under-addressed supply chain issues are a key cause of commercial food waste (White, 2007). Emphasis has been placed over the years on temperature management that often deviates from optimum during storage, handling and distribution, as one of the major causes of fresh fruit and vegetable overall quality deterioration (van Boekel, 2008, Taoukis et al., 1997; Nunes et al., 2003; Lai et al., 2011
Strawberry is a popular fruit around the world due its physical characteristics and its sweet flavour; but is also considered an important source of bioactive compounds such as vitamin C and diverse phenolic compounds such as phenolic acids, flavonoids and tannins (Cayo et al., 2016; Kelly et al., 2016a, 2016b; Flores-Felix et al., 2018) for human nutrition. At the same time, strawberries are one of the fruits most often discarded throughout the supply chain, due to their short shelf life and high perishability and due to poor management during distribution (Kelly et al., 2018; Nunes et al., 2009; Pan et al., 2014). As the strawberry fruit is metabolically active after harvest, the rate at which its quality will deteriorate depends on both genetic factors and on pre- and post-harvest conditions (Castro et al., 2002). The recommended storage conditions for strawberries are 0 °C and 90–95 % Relative Humidity (RH). However, Nunes et al. (2009) found that during retail storage strawberries are kept at temperatures ranging from 5.8 °C to 7.1 °C and RH conditions ranging from 56% to 93%. This range does not consider the common practice of storing and displaying strawberries outside refrigerated compartments or displays at open markets where temperatures and RH conditions can deviate at a larger scale from the optimum conditions. Furthermore, Lai et al. (2011) studied real strawberry supply chain conditions and their effect on quality and found that RH varies from 33.8% to 87.2% along the different supply chain stages (harvest to retail). Many studies stress the importance of keeping strawberries at 90–95 % RH but the effect of the deviations occurring during the supply chain on the appearance, acceptability and nutritional value of strawberries has not been adequately explored to date. Shin et al. (2007) studied the effect of three RH conditions (75%, 85%, and 95%) and the effect of temperature (0.5 °C, 10 °C, and 20 °C) on the quality and nutritional properties of strawberries and found no effect of RH on these properties. In a later study, Shin et al. (2008) looked at a lower RH condition (65%) and found that weight loss increased, whereas total flavonoids and total phenols decreased in fruit stored at 65% compared to samples stored at 95% RH. Both studies evaluated their nutritional analysis results in fresh weight basis, which could fail to consider the concentration effect due to water loss and hence obscure the effect of RH on these properties.
To better define the effect of storage conditions on deterioration of quality, the kinetics of sensory and chemical properties should be examined. Traditionally, degradation of nutrients and quality characteristics are modelled by 0, 1st or 2nd order kinetic models (Amodio et al., 2013; Oms-Oliu et al., 2009). Alsostudies have emerged that used the Weibull model, the cumulative form of the Weibull distribution function, to study several degradation reactions (Ong et al., 2011; Odriozola-Serrano et al., 2009; Oms-Oliu et al., 2009; Amodio et al., 2013). The Weibull model is flexible due to the inclusion of a shape constant in addition to the rate constant (Oms-Oliu et al., 2009). Odriozola-Serrano et al. (2009) studied the degradation kinetics anthocyanins, total phenols and ascorbic acid in fresh cut strawberries using the Weibull model and used it to explain the changes occurring in anthocyanins and antioxidant capacity of the fruits under storage.
The present study focuses on a broad range of RH conditions (40% - 90%) that can be encountered along the supply chain of strawberries. The main objective is to examine the effect of the different RH conditions during storage on the shelf life, sensory and nutritional quality of strawberries; emphasising on properly describing the changes occurring in both physical and chemical properties of the fruit. This paper aims: i) to examine the effect of RH conditions on the shelf life of strawberries including both the sensory and nutritional quality, ii) to study the kinetics of sensory and chemical changes occurring in strawberries during storage by comparing three kinetic models; and iii) to examine and predict through modelling the waste that would occur depending on the storage conditions.
Section snippets
Plant material and experimental setup
‘Strawberry Festival’ strawberries were harvested twice during 2012 and2013 (March 2012 and February 2013) from a commercial field in Florida, USA. The samples were brought to the USF-Food Quality Laboratory in Tampa, Florida, USA, with minimal delay after harvest (Max delay of 30 min). The fruit were then sorted by colour, size, and absence of defects and packedinto clamshells (capacity ≈ 0.453 kg). In total, 210 clamshells (containing 10 fruit each) were prepared and distributed equally to
Effect of RH and time on weight loss
Weight loss is an important parameter in determining the shelf life of fresh fruits and vegetables, because it is responsible not only for visual appearance deterioration (discolouration, shrivelling etc) but also for objectionable changes in texture, flavour and nutritional value (Nunes and Emond, 2007; Nunes et al., 1998). Robinson et al. (1975) reported previously that the maximum acceptable weight loss in strawberries is 6%, before they become unacceptable for sale. A more recent study (
Conclusions
Strawberries are metabolically active during postharvest storage, and their quality and shelf life is influenced among other things by the storage conditions they experience. Temperature and RH are environmental factors with major impact on strawberries postharvest quality. However, the RH effect on strawberries sensory and nutritional properties has not been explored sufficiently. This study emphasised on studying the kinetics of the physicochemical changes occurring during storage under RH
Acknowledgments
This research was funded by USDA – NIFA Specialty Crop Research Initiative Grant (Project CA-D-PLS-2044-OG). The authors have also received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 70,837.
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