Growing interest in the cultivation of Actinidia arguta
((Sieb. & Zucc.) Planch. ex Miq.) in Europe has been observed since the beginning of the 21st century [1
]. It is related to the high nutritional value of the fruit and the high productivity. These fruits, only a few centimeters long, are characterized by a unique taste and aroma, which arouses the curiosity of a growing group of consumers [2
]. The productivity of this creeper was noticed over 1000 years ago in Asia [3
]. The fruits of minikiwi are primarily characterized by a high content of vitamin C and significant antioxidant levels from the groups of carotenoids and flavonoids [4
]. Berries are a rich source of minerals such as phosphorus, potassium, and calcium, as well as other vitamins PP, A, B1, and B2 [6
]. Actinidia arguta
shrubs tolerate the climatic conditions of Central and Eastern Europe due to their high frost resistance [8
]. The dynamically developing minikiwi plantations in Europe require the development of new cultivation technologies.
Until now, research has focused on agrotechnical aspects, i.e., proper fertilization [9
], keeping plants [10
], or searching for new, valuable varieties [11
]. Currently, an important issue is the optimization of the fruit storage conditions and supply. Thus far, assessing the aspects of the physiological ripening of fruits, the appropriate moment of maturity has been determined in which the fruits should be harvested [11
]. However, minikiwi fruits have a relatively low storage ability under refrigerated conditions. The main reason is not fungal or physiological diseases, but the quick softening of fruit stored in a common cold store [14
]. Appropriate storage conditions limit the ripening processes in fruit [16
]. This applies to both the storage temperature and the composition of gases in the environment around the fruit. Inhibition of the respiration process by lowering the oxygen concentration positively affects the inhibition of processes related to fruit ripening, such as ethylene production, loss of firmness, decomposition of polysaccharides to monosaccharides, and degradation of chlorophyll [18
]. Minikiwi is a climacteric fruit; therefore, endogenous ethylene, even in small amounts, activates the enzymes exo-polygalacturonase, endopolygalacturonase, pectin esterase, and endo-1,4-β-d
-glucanase, contributing to the reduction in firmness [18
]. Lack of care for appropriate storage conditions is the cause of the rapid loss of quality by fruit [22
]. Too much carbon dioxide and too little oxygen can seriously damage the skin and flesh of the fruit. Critically low O2
concentrations lead to fruit fermentation. According to Schlie et al. [23
], at an O2
concentration below 0.3–0.6%, the process of anaerobic respiration takes place—alcoholic fermentation. This gas composition causes irreversible damage to the fruit.
Research on the storage of kiwi (A. deliciosa
) in low-oxygen compositions (O2
= 05–1.0%) was carried out at the end of the 20th century [24
]. The results of the research indicated that storing kiwis in 1% O2
quite effectively inhibits the ripening of the fruit. It turned out, however, that oxygen content at the level of 0.5% can cause fermentation in the fruit, which was confirmed by the higher ethanol content in the flesh, especially in fruits that are more mature during the harvest, i.e., harvested with an extract content above 10° Brix. ULO (ultra-low oxygen) or DCA (dynamic controlled atmosphere) technologies allow a reduction in the content of oxygen to a very low level. The practical use of these technologies is justified, as research shows [25
] that lowering the O2
content to 1.5% enables the storage of minikiwi for up to 2 months in cold store conditions.
Lowering the O2
content to the limit just above anaerobic respiration is most effective in inhibiting the ripening of pome fruits such as apples [26
]. DCA technology is based on measurements of the intensity of carbon dioxide released in the processes of respiration, ethanol release, or chlorophyll fluorescence. Fruits react to stress caused by oxygen deficiency and switch from aerobic to anaerobic respiration. This reaction is best determined by monitoring chlorophyll fluorescence or ethanol synthesis. This technology is used only in technologically advanced farms, because improper use may damage the entire batch of stored fruit. On the other hand, the research to date by Hertog et al. [27
] showed that fruit stored in an atmosphere with a low carbon dioxide content, close to zero, had a firmness 15 N lower than fruit stored in an atmosphere with a 5% CO2
concentration. The essence of storage is to determine the composition of the atmosphere with the lowest possible concentration of O2
and the highest possible concentration of CO2
which does not damage the fruit tissue.
The aim of the study was to attempt to assess the influence of variable parameters of O2 and CO2 concentrations on the basis of various storage technologies on the quality of the minikiwi fruits. The study tried to demonstrate the physiological changes occurring in the fruit during storage, caused by ultra-low oxygen concentrations or high concentrations of carbon dioxide. The practical aspect was to demonstrate the most effective storage technology for minikiwi fruit in order to extend the supply of fruit in global markets.
Firmness is one of the most important quality features of minikiwi fruit. The fruit after harvest was significantly different in firmness, and ‘Ananasnaya’ was almost two times harder than ‘Geneva’. The experiment showed a significant effect of the oxygen and carbon dioxide contents in the atmosphere surrounding the fruit on its firmness. The research showed that the high CO2
content was more effective in inhibiting the softening of blueberries than the lowered oxygen content (Table 1
). In both years of research, after 2 weeks, a significant reduction in FF was observed, regardless of the storage technology used. However, in the following days, the softening process was much slower with the CO2
contents of 5% and 10%. In the first year of research, after 8 weeks of storage in DCA and ULO, the FF decreased to the level characterizing the fruit as suitable for consumption (below 5 N), although in the following year, the softening processes in DCA and ULO progressed slightly slower. The fruits stored in CA1 and CA2 retained their firmness for much longer in both years. It is worth noting that even after 12 weeks of storage in a high concentration of CO2
, the firmness of the berries exceeded 2–3 times the firmness acceptable by consumers, especially after storage at a concentration of 10% CO2
. Interestingly, after 12 weeks of storage in CA2, the firmness value for both varieties was at a quite similar level (10–14 N), regardless of the firmness that the fruits exhibited immediately after harvest, i.e., ‘Geneva’ 34–36 N, and ‘Ananasnaya’ 53–54 N.
The harvest of fruits of both cultivars was carried out with a low SSC of 6.1–6.4° Brix for ‘Geneva’ and for ‘Ananasnaya’, 8.6–6.8° Brix. The SSC increased in proportion to the loss of FF during storage (Table 2
). Similarly, in this indicator, a drastic increase in value was noted during the first 2 weeks of storage in fruits of ‘Geneva’, regardless of the concentrations of O2
in the cooling chamber. The increase in the SSC was slightly slower in the fruit of ‘Ananasnaya’, although even here, after 4 weeks, similar levels of SSC were observed as in the case of ‘Geneva’. The process of SSC increase was slower in conditions of higher CO2
concentration, i.e., in the CA2 combination. The analysis of the results showed that the composition of the atmosphere with a high CO2
content favored a slightly slower increase in SSC values, but super-low oxygen conditions (DCA technology) also slightly slowed this process. The highest rate of increase in the SSC was observed during 8–10 weeks of storage in the ULO technology, after which a decrease in the SSC in the fruit was found in the following weeks of storage.
Similarly to the increase in the SSC in the fruit, a decrease in the sucrose content was observed (Table 3
). An intense decrease in the content of this disaccharide was found in both years of research and in both cultivars after 2 weeks of storage. In the following weeks, the decomposition of sucrose into glucose and fructose was much slower, and in the fruit stored in conditions with a high CO2
content (CA1 and CA2), it was at the border of statistical significance. The sucrose degradation was faster in the berries stored in DCA and ULO. However, there was no significant difference between DCA and CA1 in 2018 (6.52 and 6.85, respectively). An interesting fact is that ‘Ananasnaya’ fruit is characterized by a lower sucrose content than ‘Geneva’, but a quite similar content of sugars. As expected, the content of glucose and fructose increased during the storage of the fruit (Table 4
and Table 5
). In this case, such a drastic increase in the content of monosaccharides was not observed during the first weeks of storage, but the increase was statistically proven. The increase in the value of the indices took place at subsequent analysis dates, but it was clearly faster in the fruit stored in ULO and slower in the fruit stored in CA2, regardless of the year and variety. Therefore, after 12 weeks, the fruit stored in ULO was often characterized by a higher glucose and fructose contents than the fruit stored in CA2.
Apart from the sugar content, the fruit flavor is distinguished by the acid content. Hardy kiwi fruits have high TA which, after harvesting, can even exceed 1.0% citric acid. In both years of research, both cultivars were characterized by high TA after harvest (Table 6
). During fruit ripening in the cold store, a loss of acidity was observed, but it was dependent on the storage conditions and the variety. Overall, it can be concluded that storage in conditions with high CO2
contents help to keep the acidity of the fruit at a high level. Especially in ‘Ananasnaya’, despite the decrease in the index value in fruits stored in CA1 and CA2 conditions, this process was not proven in any of the years of research. On the other hand, after storage in low-oxygen conditions (DCA and ULO), the TA of the ‘Ananasnaya’ fruit decreased significantly. Similar relationships were observed in both years of research in ‘Geneva’ fruit, but a significant reduction in TA was observed regardless of the O2
content. Citric and malic acids are mainly responsible for the acidity of the fruits of minikiwi. The fruits of both varieties contained the most citric acid, whereas the content of malic acid was several times lower (Table 7
and Table 8
). The contents of acids in the fruit of both cultivars depended on the year of the research. ‘Geneva’ fruits were characterized by a higher content of citric acid in 2017 than in 2018, especially immediately after harvest. The higher content of acid in the fruit during harvest did not significantly affect its higher content after 12 weeks of storage. The content of malic acid was twice as high in 2017 than 2018 in ‘Ananasnaya’ fruit, whereas in ‘Geneva’ fruit, no difference was observed between years. The composition of the atmosphere determined the rate of reduction in both acids. The concentration of CO2
at the level of 10% contributed to maintenance of the contents of citric and malic acid in ‘Ananasnaya’ fruit at a statistically unchanged level in both years of study. Similar relationships were observed in ‘Geneva’, but not as effective at inhibiting acid loss. On the other hand, fruits stored in ULO conditions were characterized by a dynamic loss of both discussed acids during storage.
Mass loss is an important indicator of the consumer quality of fruit, describing its drying up. Data analysis showed that both cultivars of fruits were characterized by a fairly similar rate of mass loss during storage (Table 9
). However, after 12 weeks of storage, the ‘Geneva’ fruit exhibited a higher mass loss than the fruit of ‘Ananasnaya’. The discussed index was determined by the conditions in which the fruit was stored. In both years of research, it was found that high concentrations of carbon dioxide at levels of 5% and 10% inhibited fruit mass loss during storage. Fruits stored in the CA1 and CA2 conditions after 12 weeks lost 42% and 54% less weight, respectively, than the fruit stored in an ultra-low oxygen (ULO) technology environment. The rate of mass loss of fruit stored in DCA and ULO was much faster in the initial storage period; a slowdown was observed after 8 weeks of storage. Despite quite significant mass loss, reaching the value of 3–4% after 12 weeks of storage, no signs of drying such as shrinkage or wrinkling of the skin were observed on the fruit.
Minikiwi fruits selected for the tests were characterized by an intensely green color of the skin immediately after harvesting. However, during storage, the basic color of the rind changes, which was more pronounced in ‘Geneva’ fruits. Analysis of the results showed that the fruits stored in DCA and ULO technology lost their green color faster during storage (Table 10
). A similar trend in the loss of chlorophyll pigments was observed in both cultivars in both years of study. High concentrations of carbon dioxide in the cooling chamber (CA1 and CA2 conditions) effectively inhibited the progressive decomposition of chlorophyll; unfortunately, it had an adverse effect on color saturation. Chroma index describing color saturation was much lower in fruit stored in technologies with a high content of carbon dioxide, which caused the skin to become dull (Table 11
). The value of peel color saturation decreased with increasing carbon dioxide concentration in the chamber. Similar relationships were observed in both cultivars when analyzing the brightness (index L) of the fruit peel (Table 12
). Additionally, in this case, the increasing concentration of carbon dioxide caused a decrease in the value of the L index, which substantiates the darkening of the skin color.
In recent years, the commercial cultivation of fruit species previously considered as amateur ones has been developing dynamically in Europe. This is due to the “Eco” trend among consumers who are looking for new fruit with an interesting taste, but with functional food features, i.e., increased pro-health values, such as antioxidants or fiber. Minikiwi fits perfectly into this trend because its fruits contain very large amounts of C vitamin [12
] and carotenoids [34
], and they can also be used for their anti-cancer properties and in other diets [35
]. A weakness of minikiwi is its poor storage capability compared to kiwi fruit, but similar to other berries, such as highbush blueberries [11
The development of an appropriate minikiwi fruit storage procedure that allows for the long-term storage and long-distance transport of the fruit has been the goal of many research studies in recent years [11
]. The main problem is maintaining the proper FF. Some studies [14
] emphasize that minikiwi loses its firmness during 1–2 weeks of storage in low-temperature conditions. Storing the fruit in a controlled atmosphere will extend this period to a maximum of 6–8 weeks [11
]. In our own experiment, the influence of different O2
concentrations on FF during storage was assessed. It turned out that the low-oxygen conditions are not conducive to inhibiting the softening process of the minikiwi, as is the case with pome fruit. However, it turned out to be beneficial to increase the CO2
content in the environment of the fruit, because even after 12 weeks of storage in CO2
at a concentration of 10%, the berries did not soften enough to be fit for consumption. This proves a stronger influence of CO2
on ethylene synthesis, which causes the activation of exo-polygalacturonase and other enzymes, contributing to the softening of minikiwi [18
]. Some authors point out that the loss of firmness in the fruits of kiwi is not only caused by ethylene, because its softening takes place at very low levels (i.e., 0.005–0.01 mL L1) [18
]. There is also a hypothesis that low temperatures may induce softening and increase ethylene sensitivity, as is the case in kiwi fruit (Actinidia chinensis
]. The results of our own research seem to confirm these reports, because CO2
is a well-known inhibitor of ethylene production, and its high concentration effectively blocked the minikiwi softening process at low temperatures.
The process of fruit ripening is not only softening, and ethylene is responsible for other physiological aging processes of the fruit. During the growth of the minikiwi on the shrub, the total contents of sugars and starch increased in order to decrease during ripening [13
]. Starch is degraded during fruit storage. The changes take place under the influence of enzymes such as α-amylase, ß-amylase, α-glucosidase, and starch phosphorylase. As a result of these transformations, oligosaccharides, maltose, glucose, or phosphate-1-glucose are formed. The results of our own research indicate a fast growth rate of SSC, whereas the increases in glucose and fructose content were slower. In the experiment, a drastic increase in SSC was observed during the first 2 weeks of storage. Additionally, Park et al. [39
] observed an increase in extract content during the storage of A. deliciosa
fruits. The results obtained by Strik [30
] show that the SSC in A. arguta
fruits can reach values of up to 25° Brix, but neither in these studies nor in the work of Fisk et al. [14
] were high values of SSC achieved. The SSC is associated with the products of starch degradation (poli- and monosaccharaides) [40
] and organic acids, amino acids, and other soluble substances. The consumption of monosaccharaides in the respiration process and the decomposition of sucrose can be determined by the concentration of gases in the cooling chamber. It turns out that a drastic reduction in O2
to a level of 0.4% in the atmosphere slows down respiration and, at the same time, the consumption of monosaccharaides, hence the observed increase in their content. It is worth noting, however, that after 8 weeks of storage, a different trend was noted and the glucose and fructose contents in fruit stored, especially in ULO, started to decrease slightly or remained unchanged. This probably indicates a faster respiration rate of the fruit stored in this way, as a similar effect was not observed in the combinations of CA1 and CA2. Therefore, it can be concluded that the minikiwi respiration process is equally effective, if not more effective, in blocking storage in high CO2
concentrations, because the content of both monosaccharides for 12 weeks increased in the fruit stored in these combinations, regardless of the cultivar.
The literature reports to date [14
] indicate that during minikiwi maturation, the acidity decreases. This is due to the consumption of acids during the respiration process; therefore, limiting this process should inhibit the loss of acidity by the fruit. There are two basic acids in minikiwi: citric and malic. As expected, the acid content decreased during the 12 weeks of storage, but the process depended on the conditions in which the fruit was stored. Earlier reports [11
] have indicated that modifications of the gas composition in the cooling chamber (1.5%:1.5%, CO2
) effectively reduce the loss of organic acids during storage compared to normal cooling conditions. The results of the experiment showed that at high CO2
concentrations, the TA and the content of citric and malic acid remained stable, which was particularly evident in ‘Ananasnaya’. Low-oxygen conditions (DCA) turned out to be slightly less effective. By analyzing the results of SSC, TA, and the content of simple sugars and acids, it can be concluded that CO2
is a much more effective inhibitor of respiration processes, because the mentioned indicators are determined by fruit respiration.
The peel color change is caused by the degradation of chlorophyll as the fruits ripen. Both cultivars were characterized by an intensely green basic skin color, immediately after harvesting. During fruit ripening in the cold store, it was found, similarly to the previously discussed indicators, that the loss of green color progressed slightly faster under ULO and DCA conditions than CA1 and CA2. There is no information in the literature on changes in the skin color of minikiwi as a result of the interaction of variable compositions of O2
. In the research by Szpadzik [11
], it was shown that the storage of the ‘Ananasnaya’ minikiwi in CA technology more effectively reduces the degradation of dyes responsible for the green color of the peel. Fisk et al. [14
] proved that the harvest date is an important factor influencing color changes during minikiwi storage in refrigerated conditions. The results of our own research indicate that CO2
contributes to the maintenance of the green color of the fruit, but the fruit is duller and darker than the fruit stored in ULO or DCA.
An important feature of the appearance of the fruit, apart from its color, is the structure of the skin. During storage, fruit loses its mass due to transpiration and respiration. Losing mass causes the fruit to wilt, which can be manifested by wrinkling of the skin. In the experiment, no such effects were found on the peel of the fruit, despite the relatively high mass loss. According to Fisk et al. [14
] minikiwis can lose up to 3% of their weight during 8 weeks of refrigerated storage. Unfortunately, the authors did not conduct experiments in a controlled atmosphere. The results obtained in the experiment show that the storage conditions affect mass loss, which is caused by more intensive transpiration in the DCA or ULO technology, where the relative air humidity was slightly lower than in the CA1 and CA2 conditions. The difference in air humidity is related to the more frequent adjustments of the gas composition made by the CO2
absorber and nitrogen generator. Lowering the relative air humidity by a few percentage points results in higher fruit mass loss, as reported by Szpadzik et al. [11
]. The authors also emphasize that earlier fruit harvest with SSC 6.5° Brix may contribute to greater mass loss during storage, explained by a smaller wax layer covering the skin. Our own research showed significant varietal differences. ‘Geneva’, with an earlier fruit ripening time than ‘Ananasnaya’, showed almost a twofold lower mass loss than ‘Ananasnaya’, which may prove that the wax coverage of the skin is more strongly determined as a cultivar trait.