Weight loss
The interaction of storage time and treatments had significant (P = 0.01) effect on weight loss. This trait increased by time. Two chemical pesticides caused higher weight loss at both storage times comparing control. Refinery micronized sulfur at both concentrations (R30 and R60) caused higher wight loss than control on 25th day but was similar to it after 50 days (Fig. 1). Research shows that sulfur may have adverse effects on fruit weight loss, which is concentration dependent. For example, in blueberry cultivars ‘Brigitta’ and ‘ONeal’, fumigation with high (250–350 µL L− 1 h) concentration of SO2 caused higher weight loss than control after 45 days of cold storage, but lower concentrations (50–200 µL L− 1 h) were similar to control (Rivera et al. 2013). In this study, 30 and 60 kg/1000 L of refinery micronized sulfur showed adverse effects on weight loss but mineral sulfur does not. Mineral sulfur at its lower concentration (M30) even showed less weight loss than control on day 50. This result may be due to the longer persistence of refinery micronized sulfur on fruit surface. In a study on grapes, residue concentrations generally were lower for a wettable powder of elemental sulfur versus a micronized formulation applied at the same rate and timing of the last application before harvest (Kwasniewski et al. 2014). On the other hands, our results indicated that the negative effects of sulfur on water loss was eliminated sooner than that of movento and confidor. It is well known that chemical pesticides have varied degrees of phytotoxicity along with their benefits. They can destruct the plant cell walls and membranes leading to elevated transpiration and water loss (Parween et al. 2014).
Firmness
The interaction of storage time and treatments had significant (P = 0.01) effect on hull firmness. This trait decreased by time. Movento and all sulfur treatments showed a reduction in hull firmness comparing control at harvest time, but not during storage (Fig. 2). The simple effects of treatments and storage time were significant on kernel firmness (P = 0.01). Day 50 and sulfur treatments caused a reduction in kernel firmness (Tables 1 and 2). Firmness is a major quality indicator for fruit marketability. Normally, when fruit water content decreases over time, a reduction in firmness is expectable. The lower firmness of sulfur-treated samples at harvest time indicates the lower water content comparing control. Comparably, application of sulfur (100–150 µL L− 1 h) on blueberry through SO2 pads inside the packaging, resulted in lower firmness than control (Rodrigues and Zoffoli 2016).
Table 1
The effect of chemical pesticides and different concentrations (kg/1000 L) of refinery micronized (B) and elemental (Z) sulfur on some physical and chemical properties of hull and kernel of pistachio cultivar Ahmadaghaei
Treatment | Kernel firmness (KgF) |
Control | 3.21 ± 0.19 a |
Movento | 2.88 ± 0.18 ab |
Confidor | 3.00 ± 0.26 ab |
R30 | 2.55 ± 0.18 |
R60 | 2.65 ± 0.13 b |
M30 | 2.71 ± 0.20 b |
M60 | 2.77 ± 0.21 b |
Table 2
The effect of storage time on some physical and chemical properties of hull and kernel of pistachio cultivar Ahmadaghaei
Storage time | Kernel firmness (KgF) | Hull WA (aw) | Kernel WA (aw) | Kernel phenolic content (mg in 100 g FW) |
At harvest | 3.05 ± 0.10 a | 0.970524 ± 0.000001 a | 0.971333 ± 0.000001 a | 0.1402 ± 0.0001 a |
Day 25 | 3.14 ± 0.11 a | 0.973143 ± 0.000002 a | 0.971571 ± 0.000001 a | 0.0892 ± 0.0010 b |
Day 50 | 2.28 ± 0.10 b | 0.956952 ± 0.000001 b | 0.960190 ± 0.000002 b | 0.0928 ± 0.0100 b |
Water activity
The simple effect of storage time was only significant of the water activity of hulls and kernels. This trait displayed a reduction on 50th day comparing control (Table 2). The growth of pathogens in foods is markedly affected by water activity (Romero et al. 2007). The water activity of both hulls and kernels was above 0.9 aw which is high and suitable for the growth of most pathogens. The treatments did not influence the water activity because it is mostly dependent on temperature (Slade et al. 1991). All the samples had relatively the same temperature at harvest time and also at cold storage. Although the water content of a food does not necessarily determine the water activity, the severe water loss on 50th day resulted in a reduction in water activity.
Phenolic content
The interaction of storage time and treatments was significant (P = 0.01) on the content of phenolics of hull. In control, this trait remained statistically stable, but sulfur-treated samples revealed an increasing trend over time so that R30, R60 and M60-treated samples had 57.3%, 38.4% and 40.3% higher phenolic content of hull respectively, on 50th day comparing harvest time (Fig. 3). Pistachio hulls are a rich source of phenolic compounds being ranked among the first 50 food products highest in antioxidant potential (Tomaino et al. 2010). The increasing effect of sulfurs on phenolic content of hulls is somehow related to the higher water loss which causes a higher drought stress. The phenolic content of wheat plants significantly increased after they were fumigated by 0.06 ppm SO2 for 8 h daily from germination to grain maturity (Agrawal and Deepak 2003). But SO2 fumigation did not influence the phenolic content of blueberries (Cantin et al. 2012). Bolling et al. (2010) found that total phenolics of almond skins stayed constant or increased progressively during 15 months storage at 4 or 23°C.
The simple effect of storage time was significant (P = 0.01) on phenolic content of kernels. The phenolic content of kernels was totally lower than that of hulls which has been reported in previous studies (Tomaino et al. 2010). Phenolic content of kernels declined at both storage times comparing harvest time, which indicates the difference in metabolic status between hulls and kernels (Table 2). In another study, the phenolic content of pistachio kernels measured by DPPH method, showed a 12.2% decrease after high water loss (Tsantili et al. 2011).
Antioxidant activity
The interaction of storage time and treatments was significant (P = 0.01) on the antioxidant activity of hulls and kernels. Pesticides-treated hulls were similar to control, however, sulfur-treated hulls had lower antioxidant activity than control at all storage times (Fig. 4a). In kernels, sulfur treatments had lower antioxidant activity than control, while movento showed a more similar effect with control (Fig. 4b). These results are not in agreement with the results of phenolic content and imply that in sulfur treatments, a significant decrease must have occurred in the content of other molecules with antioxidant properties, probably antioxidant enzymes, vitamins or carotenoids (carotenoid content was lower in R30 and R60 sulfur treatments (Fig. 7)). In another study, the antioxidant activity of 10 different fruit extracts was mostly correlated to the content of vitamin C and carotenoids rather than phenolics (Olsson et al. 2004). Similar to hulls, the antioxidant activity of kernels remained stable over time for most samples except the samples that were treated with confidor and R30, in which a decrease was observed on 50th day comparing harvest time. The stability of antioxidant activity over time is probably due to the low temperature (4°C) during storage in which the antioxidant enzymes have low activity. The antioxidant activity of strawberry fruits remained stable during storage as a result of cold temperature (Jin et al. 2011).
Anthocyanin
The interaction of storage time and treatments was significant (P = 0.01) on anthocyanin content of hulls and kernels. Refinery micronized sulfur at both concentrations (R30 and R60) and mineral sulfur at high concentration (M60) showed a higher anthocyanin content of hulls than other samples. A slight decreasing trend for hull anthocyanin content was observed during storage (Fig. 5a). Research shows various effects of SO2 on anthocyanin content of fruits. For example, when blubbery cultivar ‘Emerald’ was stored in controlled atmosphere after SO2 fumigation, it revealed higher content of anthocyanins comparing controlled atmosphere alone (Cantin et al. 2012). Timberlake and Bridle (1967) believe that SO2 directly reacts with anthocyanins rendering them colorless and stabilizing them against degradation. Another study indicated that the content of anthocyanins in longan pericarp decreased after SO2 fumigation (Han et al. 2001). The highest anthocyanin content of kernels was related to harvest time for control. Some treatments revealed a decreasing trend over time (Fig. 5b). Most of the kernel anthocyanins are present in its testa which is a thin layer on the green seed. The testa dries rapidly by time and the anthocyanins get out of the vacuoles and may be degenerated by polyphenol oxidase.
Chlorophyll
The interaction of storage time and treatments was significant (P = 0.01) on chlorophyll content of hulls and kernels. Refinery micronized sulfur at both concentrations (R30 and R60) showed the highest chlorophyll content than other samples. Chlorophyll content of some samples had a slight decrease during storage (Fig. 6a,b). Since all the pistachios were completely ripe at harvest time, the high chlorophyll content in sulfur treatments is not due to the difference in ripening process. Chlorophylls are responsible for harvesting light quanta and transferring energy during photosynthesis. The way that the plants response to SO2 stress depends on the specific physiological state of them and their need to energy (Li and Yi 2020). Usually, the studies report the negative effect of SO2 on photosynthesis and chlorophyll content, but some reveal the stimulation of chlorophyll synthesis under SO2 treatments. For example, the total chlorophyll content was significantly increased in the leaves of Arabidopsis plants exposed to 30 mg/m3 SO2 for 72 h (Li and Yi 2020). In green gram, 200 µg (active ingredient) kg− 1 of sulfosulfuron stimulated the chlorophyll synthesis in fresh leaves (Khan et al. 2006).
Carotenoid
The interaction of storage time and treatments was significant (P = 0.01) on carotenoid content of hulls and kernels. Refinery micronized sulfur treatments (R30 and R60) showed a lower carotenoid content of hull at harvest time. The carotenoid content of hulls showed a decreasing trend over time for most samples, fallowed by a slight increase on 50th day (Fig. 7a). Similar to hulls, R30 and R60 caused a lower carotenoid content of kernels at harvest time. The carotenoid content of kernels remained stable during storage for some samples while some others showed a slight decreasing trend (Fig. 7b). Apart from antennae function, carotenoids also act as natural defense under stress condition (Parween et al. 2014). The reduction in carotenoid content by SO2 has been reported in other plants such as rice (Agrawal et al. 1982) and strawberry (Muneer et al. 2014). Lower carotenoid content in R30 and R60 treated hulls may show the decoloring property of sulfur as well. In figs cultivar ‘Brown Turkey’, 50–100 (µL/L) h SO2 caused bleaching of fruits with a concentration-dependent trend in cold storage (Cantin et al. 2011). Movento-treated hulls had the highest carotenoid content at harvest which can be related to the higher water loss and more phytotoxic effect of this pesticide.
Color indicators
The interaction of storage time and treatments was significant (P = 0.01) on the color indicators of hull. At harvest time, refinery micronized sulfur-treated samples (R30 and R60) had higher L* value but lower a* and b* values of hull than others (Fig. 8a,b,c). This means that R30 and R60 treated hulls were lighter and greener than others. In coordination, these two treatments had different hue (basic color) and chroma (color intensity) values (Fig. 8d,e). Lower hue again shows the greener color and less red and yellow pigments, while higher chroma value reveals more intense color of these two treatments. This is coordinated with the results of chlorophyll and carotenoid content. Similarly, SO2 treatment caused a greener color in Arabidopsis leaves (Li and Yi 2020). Totally, L* value of hull showed an increasing trend in 25th day fallowed by a slight decrease in some samples. This increasing trend can be due to the gradual synthesis of cuticle on hull epidermal cells and also the gradual removal of sulfur particles. Likewise, fresh cut apples treated with 50% sodium sulfite were lighter than control after 5 days of cold storage (Ortzi et al. 2018). a* and b* values revealed a decreasing trend during storage which is in agreement with the decreasing trend of carotenoids. In a similar way, the hue value of most samples increased on 50th day which shows the gradual loss of red and yellow pigments. The chroma value revealed a reduction over time that again shows the loss of pigments and loss of color intensity.
The interaction of storage time and treatments was also significant (P = 0.01) on the color indicators of testa of kernel. The results of testa were very similar to the results of hulls. So that, R30- and R60-treated samples had higher L* and hue values but lower a*, b* and chroma values of testa at harvest time (Fig. 8f,g,h,i,j) which means that they were lighter and greener with less red and yellow pigments. These treatments provided a more intense color. L* and hue values showed increasing but a*, b* and chroma values showed decreasing trends over time. We are of this opinion that sulfur particles do not penetrate to the kernel if the hull is intact. But the kernels are certainly influenced by the physiological status of hulls and get signals from them through metabolic pathways. This can be the reason why the kernels changed their color in a way that the hulls did.
Sensory evaluation
The interaction of storage time and treatments was significant (P = 0.01) on taste, aroma and appearance of pistachios. As expected, all three parameters displayed a decreasing trend during storage, but in some cases, this decrease was not statistically significant. R30 treatment could maintain the taste and aroma of pistachios even after 50 days (Fig. 9a,b). Sulfur, in certain concentrations and application times, is applied as a preservative in food technology because it keeps the quality of food constant, meeting market requirements (Rossello et al. 1994). For example, in wine industry, small doses of SO2 is required to extend the sensory quality (Wyk et al. 2018). Herein, the panelists did not record any off-flavor for the treatments. Comparably, no off-flavor was developed after SO2 fumigation in blueberries (Cantin et al. 2012). All sulfur treatments showed a higher degree of appearance at harvest time (Fig. 9c), which is related to the antimicrobial effect of sulfur, keeping the fruit surface safe from disease damages. The lighter and more intense color of sulfur-treated hulls were other reasons for the higher degree of appearance in these treatments
Ethylene production and respiration rate
The interaction of storage time and treatments was significant (P = 0.01) on both ethylene production and respiration rate of pistachios. All sulfur treatments had higher respiration rate than control, which might be the result of high energy consumption for protection mechanisms activation under SO2 stress. The respiration rate declined by time as the result of lowered metabolism under cold temperatures (Fig. 10a). The respiration rate of wheat plants increased by 0.06 ppm SO2 fumigation for 8 h daily. It was claimed that the detoxification reactions against SO2 are ATP mediated which is provided by respiration (Agrawal and Deepak 2003). In the present study, a probable toxification to hulls resulted in respiration elevation.
Sulfur treatments at lower concentration (R30 and M30) showed higher ethylene production than others. After 25 days, no significant difference was observed between the samples (Fig. 10b). Tomato plants fumigated by SO2 for 1 h showed enhanced ethylene production and when the treatment prolonged for 4 h, ethylene production was also enhanced. It was concluded that ethylene might play an important role in SO2-induced plant injuries at relatively short terms of SO2 fumigation (Gong Young et al. 1995). Alfalfa plants fumigated by 0.7 µL/L SO2 for 8 h, increased ethylene production 10 times greater than control plants (Peiser and Yang 1979). Ethylene production of bean plants treated with elemental sulfur increased before full bloom and before fruit set, and flowering occurred earlier because a slow oxidation of elemental sulfur in air produced SO2 which enhances ethylene evolution from leaves (Recalde-Manrique and Diaz-Miguel 1981).