Refrigeration chamber temperature and humidity conditions

Temperature conditions, in the three refrigeration chambers, through the trial period can be observed in Figure 1. The results show very distinct conditions. At local C air temperature shows a high oscillation (σ=3.53 °C) with a mean value of 8.29 °C and a maximum thermal amplitude of 15.5 °C. At local G air temperature shows more stability (σ=0.88 °C) with an average temperature of 5.5 °C and a maximum thermal amplitude of 7 °C. Local L shows the best temperature stability of the air temperature (σ=0.83 °C) with an average value of 0.80 °C and maximum thermal amplitude of 5 °C (Table 1).

Air relative humidity of storage conditions for the 3 farmers were similar, corresponding to 95.5%±5.0% for local C, 89.9%±4.6% for local G and 98.1%±3.5% for local L, close to the range of 90% - 95% recommended by ^{Crisosto et al. (1996}) (Table 1).

Figure 1 Temperature pattern of 3 different refrigeration chambers during the period of study. 

Characteristics of Royal Summer (RS) and Royal Time (RT) cultivars

In this study we used two distinct cultivars with similar harvest time (^{Ferreira et al., 2017}). Although slight differences between cultivars were observed (in weight, firmness and TSS), the main distinctive quality parameter is acidity content. Royal Summer had lower acidity (3.63 malic ac. g/L) than RT (8.45 malic ac. g/L) (Table 2). Quality parameters of peaches at t₀ (harvest time) are shown at Table 2. RS has slight heavier fruits (193 g/fruit) corresponding to A class fruits (155 g/fruit - 195 g/fruit, according to Simões and Ferreira (2015)), a firmness of 6.61 kgf, that is higher than average referred by Ferreira et al. (2017), for peaches of middle season, namely 5.1±1.4 kgf at 2015 and 4.8±1.3 kgf at 2016, and a TSS of 13.6 ºBrix, similar of referred by Ferreira et al. (2017). Peaches of RT cultivar were 182 g/fruit, also class A, a firmness of 5.2 kgf and 11,23 ºBrix. Although there were observed significate differences respecting to fruit weight, fruits of both cultivars belong to the same size classification. Respected to colour parameters (L*a*b*), no significate differences were observed between cultivars at harvest time (Table 2).

At harvest, cv. Royal Summer has 6.61 kgf of firmness which is in the interval of firmness of 6 and 8 kgf usually used by farmers (^{Santos, 2019}) and is very interesting for fruit life postharvest. When fruit acidity is lower, farmers can harvest earlier because consumer acceptance is not negatively affected. ^{Crisosto and Crisosto (2005}) referred that consumers prefer cultivars of low acidity content, and peach high acidity content doesn’t allow the perception of high TSS content.

At harvest, Royal Time firmness was 5.2 kgf, very similar to 5.1 kgf reported by ^{Ferreira et al. (2017}) and ^{Rodrigues (2018}) that work also with RT cultivar. Data from local trials (not published) show that when Royal Time has been harvested at 5.4 kgf firmness the acidity content was >10 malic ac. g/L.

Effect of storage conditions on fruit firmness

Firmness is one of the most important quality parameters for peaches because it affects appearance and manipulation during postharvest process and commercial chain (^{Crisosto, 1994}). Refrigeration is the process used to delay softening and slow down respiration and maturation process. Firmness evolution and decay along storage period, for the two cultivars, is shown in Figure 2 (local C, local G and local L). Figure also shows the weekly average air temperatures in refrigeration chambers.

Figure 2 Peach firmness evolution, for cultivar Royal Summer (dashed line, - - -) and Royal Time (dotted line, ……) related to average weekly temperature (full line) at local C, local G and local L during conservation period. 

At local C, weekly average air temperature was in the range 5°C to 9 °C, which is much higher than -0,5ºC to 1ºC indicated by ^{Girardi et al. (2005}), ^{Ceretta et al. (2000}) and, as expected, peach firmness decreases quickly. At 14 days, firmness is 3 kgf for RS and bellow 2 kgf for RT, which are near the values of ready to eat (1 kgf) according to ^{Crisosto et al. (1996}), but are inappropriate for postharvest processing at warehouse. Temperature conditions in local C are inappropriate for peach storage and for extending shelf life period.

At local G, weekly average temperature was between 5 °C and 7 °C, just inside the range of 2.2 °C to 7.6 °C considered killing temperatures because of its high correlation with chilling injury incidence (^{Crisosto et al., 1996}; Crisosto et al., 2002; ^{Lurie & Crisosto, 2005}; ^{Rodrigues et al., 2020a}). Different behaviour of peach firmness was observed between RS and RT cultivars. Fruits from RS cultivar maintain firmness around 6 kgf during 14 days and decrease to 4 kgf at 21 days. For RT cultivar fruit firmness decrease regularly from harvest (t₀), reaching 4 kgf before 14 days. By the end of storage, in local G, firmness differences are less evident between cultivars.

At local L, characterized by weekly average air temperature between 0 °C and 2 °C, fruit firmness is maintained near the harvest values for 35 days, with similar behaviour for both cultivars.

For storage conditions of local C and G, we observed a faster decay of firmness of Royal Time on the initial period, which is a negative characteristic of RT compared to RS.

Considering that the farmers at Beira Interior region usually use refrigeration for a short time of storage due to high diversity of cultivars and short distance of commercialization, the conditions of local G can be looked with high interest in terms of low energy consumption, a desirable international goal for sustainable food production (FAO, 2011), if other quality parameters would not be negatively affected. Costs with refrigeration chambers at warehouse are the most important fraction of energy consumption in life cycle of peach production (^{Gaspar et al., 2018}), representing 2/3 of total energy consumption in MJ/t of peach. Taking only into account fruit firmness, a storage temperature between 0 °C and 2 °C will represent an additional cost for producers if peaches will stay only for a period of 7 days. On the other way, ^{Liu et al. (2015}) refers that for short conservation period the range of 4 to 6 °C is favourable for obtaining higher content of antioxidants. Although the interest of this range of storage temperature represents in terms of shortening costs, the farmers must be aware of the risk if the fruits must remain for a longer period, especially for RT cultivar. Results of ^{Rodrigues (2018}), working with RT cultivar show that Chilling Injury was only present after 21 days of storage, for a range of storage temperature of 4 ºC to 10 ºC.

Effect of storage conditions on weight fruit loss

After harvest fruit continues its process of respiration which results in weight loss, pectin and acid degradation (^{Crisosto et al., 1996}; ^{Du et al., 2017}). Weight losses leads to money loss and sometimes a point of conflicts and complains in the commercial chain. Figure 3 (local C, local G and local L) presents the fruit weight losses at different sampling dates. Evaluations were done just after storage exit and after 2 days at room temperature (22 ºC - 25 ºC, for warm summer of this region of Beira Interior).

In terms of weight losses RS and RT behave differently. RS show, in all conditions, less weight losses compared to RT. This may be due to a lower respiration rate related with its higher firmness, an indicator of maturation.

Considering t₇ of storage weight losses of RS was in average 2%/week for local C, 1,7%/week for local G and 1%/week for local L, observing a constant and linear decrease of fruit weight until t₂₈, which reflects a clear effect of storage temperature on weight loss. For RT cultivar and t₇, weight loss was in average 6%/week for local C and G, and 5%/week for local L. Considering the short storage period of 7 days, RS cultivar clearly advantageous in terms of less weight loss.

For local C, and t₃₅, RS reached 7% of weight loss and there are significate differences in weight losses between all the times of storage. After 2 days at room temperature RS presents a decrease of 1% of weight compared to the storage exit day for t₇, t₁₄ and t₂₁, but no additional weight loss for t₂₈ and t₃₅. For the same conditions of local C, RT cultivar presents 6% of weight loss for t₇ and 10% for t₂₁ and, for a storage period longer than 21 days, a high percentage of rotten fruits was observed (Table 3) which is impeditive of further quality analysis.

At local G and RS cultivar weight loss until t₂₈ presents a linear behaviour, corresponding to an average of 0.22%/day compared to an average of 0.28%/day for RT. During the last week (t₂₈ to t₃₅) the weight loss presents a higher decrease of 5% for RS and 6% for RT, probably corresponding to a senescence period. Considering t_i+2, (t₇ to t₂₁), after 2 days at room temperature, RS presents 1% of weight loss (compared to t_i) and RT presents 2% of weight loss. The last week results indicate that fruits are senescent and the conservation conditions are inappropriate for such a long period.

At local L, characterized by storage temperatures between 0 and 2 °C, loss of weight for RS and t₁₄ was 2% and can be observed a 0.14%/day of weight loss until t₂₈ with a linear decrease of weight loss. Significate differences are observed between all times of storage. Comparatively, for the same t₁₄, RT presents 6% of weight loss and reached 10% of weight loss for t₂₈. After 2 days at room temperature (t_i+2), and for the period of t₇ to t₂₁, RS cultivar presents 1% of decrease compared to t_i time of storage. Comparatively RT cultivar presents 1,5% to 2% of weight loss after the period at room temperature (Figure 3, local L).

Figure 3 Percentage of weight loss over time for Royal Summer (dashed - - - and dotted …… lines) and Royal Time (long dashed -- -- -- and dash-dotted -.-.- lines) cultivars just after refrigeration period (dashed - - - and -- -- -- lines) and after 2 days at room temperature (dotted …. and dash-dotted -.-.- lines), related with different temperature conditions (full line) at local C, local G, and local L. Different letters correspond to significant differences through the time period (p<0.05). 

Effect of refrigeration conditions on rotten fruits and chilling injury

Percentage of rotten fruits and chilling injury (CI) for each local, time of storage and cultivar are presented in Table 3.

The results suggest that storage conditions of local C and G are not appropriated for fruit quality maintenance for t_>7. For local C, the RT cultivar 25% of fruits were rotten at t₇₊₂, making them not suitable for trade intervenient or consumers.

Local L allows a longer period of storage. Fruits stored until 14 days showed the quality required at consumer level. At time t₂₁₊₂, 25% of rotten fruits were observed for RT and 8% for RS.

The higher percentage of CI was observed for local L at t₃₅. At local C and G the rotten fruits incidence overcome the problem of CI, which indicate that a proper use of fungicides near harvest is highly recommended to avoid this problem as suggested by ^{Nanos and Mitchell (2019}).

CI first appeared in Local G at t₂₁₊₂ and an incidence of 33.3% was observed (Table 3). ^{Rodrigues et al. (2020b}), in a similar study, also found the first CI symptoms after t₂₁₊₂.