Modelo de datos

7.2.1 Impact of moisture loss on postharvest firmness

The results of this experiment indicate the existence of a causal relationship between moisture loss and postharvest firmness for blueberries. The different firmness responses obtained under different weight loss regimes (Figure 5-3), the confirmation of a high correlation between these two parameters (Figure 5-4) as well as the indications of different water loss patterns for firming and softening as observed in MRI analysis (Figure 5-33) provide evidence that fruit moisture loss plays a major role in determining firmness responses of blueberries. Since softening is one of the

most important problems for the blueberry export industry, this relationship of causality between moisture loss and firmness open new possibilities in terms of postharvest managements oriented to improve blueberry quality at the marketplace.

There is a clear potential for industry to benefit from this relationship by minimising moisture loss during the postharvest chain as a way to improve firmness retention of blueberries. Maintaining weight loss levels below 8-15% would minimise excessive softening of blueberries. Firmness might be improved along the commercial chain by considering technologies and materials which limit weight loss such as palletised modified atmosphere bags, less vented clamshell designs, or edible fruit coatings. Furthermore, relative humidity (RH) within the storage or shipping environments should be maintained as high as possible over the whole postharvest chain, in order to reduce the rate moisture loss. Nevertheless, any postharvest management leading to an increase of the RH of the environment in direct contact with the fruit surface should be complemented with fungicidal treatments to avoid the enhancement of decay development. For instance, the use of palletised modified atmosphere bags alone or the combination with extra fungicidal postharvest treatments (such as sulphur dioxide, chlorine dioxide, ozone and ultra violet radiation) could become an interesting option for blueberry exporters to replace conventional containerised CA systems. Finally, postharvest managements could also be oriented to minimise the steps of the supply chain which enhance blueberry moisture loss such as delays in cooling and cold chain breakages.

The relationship between moisture loss and postharvest firmness could be also used as a practical way to monitor the firmness status of blueberry during the postharvest chain. Firmness evolution during postharvest could be easily estimated by monitoring the weight losses through the process, potentially taking opportune correction measures or redirecting fruit batches to less exigent or less distant markets to avoid firmness levels below commercial standards from occurring. In addition, the firmness condition at arrival or in retail conditions could be approximated from knowledge of the common weight loss that blueberries undergo during the export period to a particular market. Consequently, quality control systems could improve their accuracy and efficacy in order to ensure good blueberry firmness standards at the marketplace and consumer level.

The technique utilised in the present experiment of manipulating weight loss by modifying air flow rate and therefore to successfully induce softening and firming responses in blueberries could be adopted in future experiments. This technique can be utilised as a way to either increase or decrease firmness of stored blueberries at a given storage temperature, relative humidity or atmosphere composition. These experimental settings could represent a simple, low cost but effective option to induce variable firming behaviour in this crop which might be used in future studies focussed on the moisture loss and firmness relationship in blueberries.

7.2.2 Future work

Even if this experiment confirms the existence of a causal relationship between moisture loss and postharvest firmness for blueberries, the mechanism explaining this relationship still needs to be clarified. More research including the direct assessment of critical variables (such as turgor, skin toughness, and cell wall modifications) may elucidate the processes determining the relationship between moisture loss and firmness. The utilisation of other methodologies for analysis of fruit microstructure and water content such as scanning electron microscopy and near infrared radiation, respectively, could also deliver some useful information about the mechanisms involved.

Although weight loss levels below 8-15% would minimise blueberry softening during postharvest, the high genetic variability among species and cultivars makes it necessary to validate this limit for the main blueberry cultivars. Fruit characteristics such as amount of waxes covering the skin and the picking scar diameter might potentially modify the patterns for moisture loss, altering the weight loss ranges at which blueberry firming and softening are expressed. Additionally, to elucidate if specific microstructural differences between blueberry cultivars, such as the number or distribution of stone cells, may influence how moisture loss and firmness responses are related is question which should be addressed in future studies.

Due to the important implications of the causal relationship between moisture loss and postharvest firmness for blueberry quality, future breeding programs for new cultivars should consider anatomical improvements able to impact positively on firmness retention by limiting the moisture loss of harvested blueberries. Blueberry cultivars with smaller stem scars and a higher amount and endurance of epicuticular waxes may result in much firmer blueberries at the end of the export process. Also the facilitation of the peduncle detachment during harvesting could be oriented to favour a small stem scar exposure that quickly dries to minimise water loss from blueberries.

Efforts should be also made in order to standardise firmness assessments in both experimental and industry conditions. Although compression firmness is largely preferred in research as the methodology to measure this quality attribute in blueberries (Angeletti et al., 2010; Buran et al., 2012; Cantín et al., 2012; Duan et al., 2011; Li et al., 2011; Lopez et al., 2010; Yang, 2009), there are other studies utilising other techniques such as puncture tests (Duarte et al., 2009) or durometer readings (Alsmairat et al., 2011) which deliver firmness outcomes which are difficult to compare to other experiment results. Even for the case of compression tests, there is not complete agreement in terms of the outputs to use from this test. While peak force at a compression distance of 1-2 mm is the main preference, the slope for a similar deformation is also used in some cases (Marshall et al., 2008). Furthermore, more research is required to correlate instrumental firmness measurements to the sensorial firmness perceived by customers and traders in industry. Very limited information is available in this matter (Beaudry et al., 1998; Ehlenfeldt and Martin, 2002), which is probably one of the reasons why the industry prefer sensorial evaluation (‘touch firmness’) as the method of assessment in quality control (Forney et al., 1998). Additionally, validation of techniques to evaluate blueberry firmness with a higher portability and at lower costs is still needed in order to establish an objective standard for firmness in the industry context.

Although it is known that transpiration through the fruit epicuticular waxes (Albrigo et al., 1980) and the stem scar (Cappellini and Ceponis, 1977; Moore, 1965) are the main pathways for moisture loss from harvested blueberries, it is not known in what proportion these factors determine water loss from this crop. As the stem scar of

blueberries has been shown to dry during the fruit exposure to warm temperatures (8h at 18°C) (Ehlenfeldt, 2002), it would be useful to evaluate how much moisture can be lost through each pathway during the postharvest chain. This information could lead to practical recommendations to apply by blueberry growers and traders, such as considering an initial period of stem scar curing or even promoting the harvesting to be made with pedicel.