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Fresh produce quality deteriorates rapidly under high temperature environments. As such, moving product quickly from the field to the packhouse and to cooling as soon as possible extends postharvest life (Wills et al., 2007). The period of time elapsed from harvest until the produce reaches the cool storage is referred to as a cooling delay, which usually includes harvest handling, transport and packing (Ferraz et al., 2001; Tetteh et al., 2004). Longer cooling delays and higher temperatures during this period can decrease the product storage life and affect the final quality of many fresh commodities (Thompson et al., 2001). Alternatively, rapid cooling of fresh commodities such as forced air, hydro or vacuum cooling are utilised for many perishable fruit and vegetables to quickly remove the field heat and hence to contribute to improve their shelf life (Wills et al., 2007).

Cooling delays affect the quality and postharvest life of fresh blueberries in subsequent storage (Thompson et al., 2001), hence considerable effort has been made in the blueberry industry to improve the cooling management (Boyette, 1993). The optimisation of postharvest logistics by lowering prepacking temperatures and by reducing supply chain times is considered a primary challenge for blueberry

growers and marketers (Ferraz et al., 2001; Forney, 2009). The effect of cooling delays on blueberry quality is dependent on the holding temperature and total time of the delay period (Ferraz et al., 2001). In addition, each of the quality attributes varies in its response to cooling delay (Tables 1-4 and 1-5). The studies evaluating the residual effect of cooling delays on fresh blueberries are reviewed in this section, in pursuance of providing the necessary information to analyse the storage delay results of this study.

Experimental results on the residual effect of cooling delays on decay and chemical parameters are consistent. When highbush and lowbush blueberries were held from 2 to 48 h at temperatures close to 20°C, a progressive increase of pathogen development after the subsequent storage was observed as delay periods were longer (Table 1-4) (Ceponis and Cappellini, 1979; Jackson et al., 1999). Likewise, Ceponis and Cappellini (1982) reported a positive correlation between longer delays and higher incidence of decay after storage when highbush blueberries were subjected to holding periods at 30°C (Table 1-4). In all studies, the levels of total soluble solids and titratable acidity are not affected after storage by cooling delays (Table 1-4).

Table 1-4. Residual effect of different cooling delays on decay and chemical parameters of blueberries after storage

Cultivar

Period (h) Temp (°C) Period (d) Temp (°C) Decay Chemicals (specie) Comments Reference

% Incidence Not Evaluated after

2 20 4 1,5 7.4 a not reported 3 extra days Ceponis & Cappellini, 1979

48 20 4 1,5 28.5 b measured (Highbush) at 21°C

% Incidence

2 30 14 2 2 not Bluecrop Evaluated after Ceponis & Cappellini, 1982*

48 30 14 2 6,9 measured (Highbush) 1 extra day

72 30 14 2 10,8 at 21°C

Count (log10 g-1) TSS, TA Not

3 19 21 0 5.1 a reported Packing

9 19 21 0 5.3 b NS (Lowbush) temperature Jackson et al., 1999

21 19 21 0 5.4 c not reported

48 19 21 0 5.5 d

TSS, TA

0 30 28 2

2 30 28 2 not developed NS Bonita Evaluated after Ferraz et al., 2001

4 30 28 2 (Rabbiteye) 1 extra day

6 30 28 2 at 20°C

8 30 28 2

*No statistical analysis reported for this data. NS: non-significant

Table 1-5. Effects of cooling delays on weight loss and firmness of blueberries at storage

Longer holding periods and higher temperatures before cooling impact both moisture loss and firmness of fresh blueberries during subsequent storage. Increasing delay at 20°C from 0 to 24 h lowered firmness in rabbiteye blueberries (cv. Brightwell) after a subsequent 10 d storage at 4.5°C (NeSmith et al., 2002), while delays of 8 and 24 h at 32°C generated weight loss of 6.1% and 8.5%, respectively, after 3 d of subsequent storage at 4°C in rabbiteye (cv. Tifblue) (Tetteh et al., 2004). Nevertheless, when the duration of the cooling delay has been shorter than 9 h, its effect on weight loss and firmness after storage has not been consistent. Ferraz et al. (2001) obtained progressive increase of weight loss after storage as the delay period increased, but no differences in terms of firmness (Table 1-5). In contrast, Paniagua (2012) reported no firmness differences after storage between ‘Maru’ blueberries previously held at 10°C for 4 or 8 h (Table 1-5). Accordingly, evidence shows that cooling delay effect on firmness of blueberries is clearer when the period of delay lasts minimum for 24 h, for holding temperatures of at least 10°C. Moreover, the influence of delay increasing water loss during storage is more consistent, especially at higher holding temperatures.

Cultivar

Period (h) Temp (°C) Period (d) Temp (°C) Weight loss (%) Firmness (specie) Comments Reference

Compression (N)

0 30 28 2 5.2*

2 30 28 2 5,8 Bonita Evaluated after Ferraz et al., 2001

4 30 28 2 6,3 NS (Rabbiteye) 1 extra day

6 30 28 2 6,8 at 20°C

8 30 28 2 7,2

8 32 3 4 6,1 not Tifblue Tetteh et al., 2004*

24 32 3 4 8,5 measured (Rabbiteye)

Firmness loss (%)

0 20 10 4,5 not 10 a Brightwell Compression NeSmith et al., 2002

24 20 10 4,5 measured 16 b (Rabbiteye) Firmness

Compression (N)

4 10 14 0 7.87* 0.9 a

8 10 14 0 7,55 1.0 a Maru Delay at 10°C Paniagua et al., 2012

20 10 14 0 9,52 0.7 b (Rabbiteye) applied after

4 10 21 0 8.83* 0.8 a 6 h at 20°C

8 10 21 0 9,46 0.8 a

20 10 21 0 11,51 0.6 a

*No statistical analysis reported for this data. NS: non-significant

Figure 1-8. Weight evolution of rabbiteye blueberries cv. Tifblue during different cooling delay treatments and throughout subsequent storage at 4°C and 95% RH. Adapted from

Tetteh et al. (2004)

The effect of the delay duration on water loss at storage seems to vary directly with to the period of exposure to the moisture content gradient between the blueberry fruit and environment during the holding period, which increases at higher temperatures. This would lead to a residual effect on product moisture loss at storage in terms of magnitude, without affecting the rate of water loss during this period, as shown by Tetteh et al. (2004) after comparing the residual effect of cooling delay periods at 21°C, 27°C and 32°C on blueberry weight during subsequent storage at 4°C for 4 d (Figure 1-6). Furthermore, this effect on moisture loss seems to be influencing to some extent the residual effect of delays on blueberry softening at storage, although some degree of inconsistency indicates the participation of other factors as well. Moreover, cooling delay effect on blueberry decay during storage seems to be exclusively explained by a direct influence of holding period and temperature on the growth rate of pathogens.