Change in water content
The water content of jujube fruits at different development stages is presented in Fig. 2. The water content of all jujube fruit cultivars at different development stages differed significantly. In general, water content was found to decrease with respect to stage of development; the highest moisture levels, in the range of 86.52% (SZ) to 89.31% (DBL), were found at the young fruit stage (S1), and the lowest water content, in the range of 65.09% (HZ) to 76.60% (JY), at the red maturity stage (S5). These results are similar to the trend reported by Wu and Song 23,24, but the values for water content obtained in this study were significantly lower than the data reported in these two studies. The reason behind this discrepancy is probably that the climate (long periods of sunshine and a large temperature difference between day and night) during the growth of jujube fruit in Xinjiang results in the accumulation of solids in the fruit.
Change in titratable acid (TA)
TA was evaluated at different development stages of the jujube fruit and the results are shown in Fig. 3. TA at different development stages of all jujube fruit cultivars showed significant differences. The TA content of DBL was found to increase as development progressed, and the TA content of FCM was found to be higher in the early developmental stages (up to S3) and stabilized from S3 to S5, while the TA of other cultivars was found to increase during the early stage and then decrease as development progressed. These results are consistent with studies reported previously by Wu24. Among all cultivars, SZ showed the highest concentration of TA, which was observed to almost double at the final stage of development. Increasing TA concentrations during development may be due to increasing ethylene production, which is at its peak at the early stages of development but declines progressively thereafter17.
Change in ascorbic acid
AA is a well-known compound in fruits that acts as an antioxidant and helps to scavenge free radicals by inhibiting radical chain reactions13. AA, present in abundance in jujube fruits, acts as a reducing and chelating agent25. Jujube fruit provides a rich source of AA, meeting the adult daily requirement with the consumption of about 20 g of the fruit26. The AA concentration at different development stages (Fig. 4) showed significant differences (P<0.05) in all cultivars. During development the highest concentrations of AA were found in the HZ and FS cultivars, at about 8.51g/Kg FW and 11.20 g/Kg FW, respectively, at the S1 stage, which gradually decreased thereafter. The highest rate of decrease of AA was found up to the S3 stage, whereas the rate of AA decline slowed thereafter. Wu24 reported similar findings in the case of the ripening stages of ‘pear-jujube’ (Zizyphus jujuba Mill.). Up to the S5 stage, the AA content of all cultivars decreased, ranging between 0.79 g/Kg FW (JZ) and 4.86 g/Kg FW (FS). Generally, FS, FCM, and DBL cultivars are considered to be table fruit, whereas JZ, HZ, ZH, and JY are used for commercial processing. This discrimination has been made on the basis of AA concentrations, where fruit containing higher concentrations are consumed as table fruit and those with lower concentrations used commercially. JY might be considered to be a potential commercial cultivar due to its having the highest amount of AA (4.48 g/Kg FW) compared to other commercial cultivars at the fully-matured stage (S5).
Change in fructose, glucose and sucrose
The sugar compounds in fruit increase the sweetness of its taste and aroma, which helps to maximize consumer acceptance17,20. In the present investigation fructose, glucose, and sucrose were the main sugars found in jujube fruit, which coincides with the result reported by Wu25 and Song24. As shown in Fig. 5, fructose and glucose were the main sugars at the early fruit stage (S1) and their levels increased during the early development stages then decreased as development progressed (Fig. 5A and 5B) because they can turn into polysaccharides and participate in various physical and chemical reactions25. In addition, fruit weight increased by between 54% and 112% from stages S3 to S5 (Table 1), indicating that the concentration of fructose and glucose may be diluted as development progresses.
Meanwhile, sucrose was initially undetected in early development stages (S1 and S2) but started to appear thereafter and then levels substantially increased and sucrose became the dominant sugar in most of the jujube cultivars (Fig. 5C). Generally, the red maturity stage presented the highest amount of total sugar content, ranging from 615.33 g/Kg DW (SZ) to 813.26 g/Kg DW (HZ). These results corresponded with the fact that the sweetness of jujube fruits rapidly increases after the green ripe stage (S3), confirming that the accumulation of sugars in jujube fruits occurs mainly in the later stages of development17.
Change in cAMP and cGMP
As a second messenger in living organisms, cAMP metabolism is related to cell growth and differentiation and it exhibits anti-inflammatory effects 7. In the present study a significant difference in cAMP and cGMP content was observed in jujube fruit (Fig. 6). cAMP and cGMP were initially undetected at the initial development stages (S1 and S2) but they started to occur at increasing concentrations at stages S3 to S5. Among all the cultivars evaluated, JY was found to contain the highest amount of cAMP, i.e. 480.92 mg/Kg DW at the fully ripened stage (S5), followed by JZ, FCM, HZ, DBL, ZH, FS, and SZ. SZ had the lowest level cAMP (15.51 mg/Kg DW at the S5 stage). These findings suggested that cultivar is the major factor affecting cAMP accumulation in jujube fruit. Also, cAMP levels were almost twice those of cGMP in all cultivars at development stage S5, which means that cAMP might be more important than cGMP in the case of jujube fruit. The cAMP content of jujube fruit has been reported to be 30–160 mg/Kg, which is the highest value observed in more than 180 plants 7. Previous studies have reported that cGMP is also found at high concentrations in jujube fruit5,27. Variation in compound concentrations may depend on the environment where the plant is cultivated, soil, and plant species.
Change in total phenolic content (TPC)
The jujube fruit is a promising source of antioxidants due to the considerable amount of phenolic compounds it contains. As shown in Fig. 7A, significant differences (P < 0.05) were found in the TPC of jujube fruit at various stages of development. Generally, jujube fruit from the eight cultivars exhibited similar total phenolic patterns and a decreasing trend in TPC was observed with respect to development progression. The highest values were recorded at the young fruit stage (S1), ranging from 26.19 g GAE/Kg DW (HZ) to 35.06 g GAE/Kg DW (SZ), and the lowest at the red maturity stage (S5), ranging from 5.92 g GAE/Kg DW (DBL) to 9.68 g GAE/Kg DW (SZ).Remarkably, the decreasing trend was most pronounced from S1 to S3, but less pronounced thereafter until S5. The TPC of jujube fruit decreased with development stage, which was in agreement with those reported previously for jujube fruit28,29. However, the values of TPC in jujube fruit (at S5) obtained from this study were higher than previously published results28,29. These higher levels of TPC may partially be attributed to the effect of cultivar, tissue, harvesting time, and/or climatic conditions, as well as the high altitude of the study area30.
Change in total flavonoid content (TFC)
Flavonoids are currently considered to be essential components for various applications, such as nutraceuticals, pharmaceuticals, medicines and cosmetics. It has also been reported that flavonoids determine anti-oxidative, anti-inflammatory, anti-mutagenic, and anti-carcinogenic properties 31. Jujube fruit contains considerable amounts of flavonoids, including rutin, and TFC is mainly determined using ethanol or methanol extracts of jujube fruit. As shown in Fig. 7B, jujube fruit TFC was found to differ significantly, at P < 0.05, at all development stages. TFC was found to exhibit a downward trend with respect to development progression, similar to that of TPC. The highest values were recorded at the young fruit stage (S1), ranging from 11.66 g RE/Kg DW (FS) to 19.30 g RE/Kg DW (DBL), and the lowest at the red maturity stage (S5), ranging from 0.29 g RE/Kg DW (ZH) to 5.99 g RE/Kg DW (DBL). The DBL cultivar of jujube exhibited a very much higher amount of TPC than that of other cultivars at all development stages. Wang29 reported a similar trend and observed that rutin is the dominant flavonoid present in jujube fruit. Wang28 also noted similar changes in TFC concentration during jujube fruit ripening. These results clearly indicate that the TPC of jujube fruit is dependent on developmental stage and cultivar.
Phenolics are important compounds present in fruit, which are beneficial to consumers because of their antioxidant activity. In the present study fifteen phenolic compounds, comprising hydroxybenzoic acid (p-hydroxybenzoic acid), hydroxycinnamic acids (caffeic acid, p-coumaric acid, and ferulic acid), other phenolic acids (chlorogenic acid), proanthocyanidins (catechin, epicatechin, and proanthocyanidins), and flavonoids (rutin, quercetin, phloridzin, quercetin-3-glucoside, quercetin-3-rhamnoside, quercetin 3-xylosyl-glucoside, and quercetin 3-rutinoside-7-pentos), were well separated and quantified by HPLC-DAD. As shown in Fig. 8, proanthocyanidins (Fig. 8D), catechin (Fig. 8A), epicatechin (Fig. 8B), and rutin (Fig. 8C) were found to be the predominant phenolic compounds of jujube fruit, and significant differences were noted between total and individual phenolic compounds among all cultivars at various development stages. The concentration of individual phenolic compounds was found to exhibit a downward trend with respect to development progression, which was similar to that of TPC and TFC. The highest phenolic compound content varied from 20.41 g/Kg DW (JY) to 41.00 g/Kg DW (SZ) at the start of development (S1) while the lowest content was found to range from 0.28 g/Kg DW (ZH) to 6.49 g/Kg DW (FS) at the fully mature stage (S5). This change is consistent with the results reported by Wang32. Phenolic compounds, such as procyanidins, catechin, and epicatechin, were found to be predominant at the young fruit stage (S1), which is directly linked to sensory attributes such as bitterness and astringency33, these unpleasant flavour characteristics prevent their consumption. However, these unpleasant taste characteristics were reduced in association with decreasing phenolic concentration correlated with respect to ripening, helping to develop a pleasant and sweet flavour in jujube fruit. Therefore, ripe and fully ripe stages are considered better for consumption and processing.
Phenolic compounds act as antioxidants primarily by scavenging radicals via electron transfer mechanisms and by chelation with transition metals that are involved in generating free radicals 4. The results obtained for the antioxidant activity of jujube fruit in terms of its free radical scavenging capacity (DPPH and ABTS) are shown in Fig. 9. The antioxidant activity in jujube fruit was found to differ significantly (P < 0.05) at different developmental stages. Radical scavenging activity, as determined by DPPH and ABTS assay, demonstrated a similar trend in the present investigation. The highest values were observed at the young fruit stage, and varied between 89.92 and 135.64 mmol TE/Kg DW (DPPH) and 119.03 and 345.87 mmol TE/Kg DW (ABTS), respectively, among all cultivars. The values of antioxidant activity were found to decrease with respect to developmental progression. Similar results, of decline in antioxidant activities along with increasing development stage, have also been reported by others29,34. These results clearly suggest that antioxidant activity in jujube fruit is dependent on stage of development, and the highest levels of radical scavenging activity were found in the greener stages of ripening. Xie35 reported that the antioxidant capacity of fruit could vary due to cultivar, soil conditions, postharvest practices, and the ripeness of the fruit. They also reported that the concentration of antioxidant compounds may vary according to cultivar.
Principal composition analysis (PCA)
Principal component analysis (PCA) is a well-known statistical technique that was used to understand the relationships among the quality of the different jujube cultivars from Xinjiang. All the chemical parameters detected in this study were used as variables in a PCA performed to visualize resemblances and differences by reducing the dimensionality of numerical datasets36. Generally, the principal components (PCs) have more than 85% cumulated reliability of the original dataset, and then these PCs can be used to replace the original one36.
In this study, 96% of the variability in the datasets could be explained by two PCs, where the first two PCs accounted for 92% and 4% of the total variance, respectively. As can be seen from the correlation loadings plot (Fig. 10), complete differentiation of these indices and jujube samples was not possible. However, the samples and indices can be appropriately divided into three groups. On the right plane, moisture, AA, TPC, TFC, phenolic compounds, and antioxidant activities obviously gathered together, forming the first group. All samples in the second group centred in the middle, and TA, fructose, glucose, sucrose, cAMP, and cGMP were clearly scattered on the left plane and formed the third group. The correlation loadings plot demonstrated that group one had high levels of these indices during the early development stage, while group three had high levels of these indices in the final developmental stage of all jujube cultivars. Based on the size and distance between the samples and indices, AA, TPC, TFC, phenolic compounds, and antioxidant activities are characteristic components of unripe jujube fruit, while fructose, glucose, sucrose, cAMP, and cGMP are characteristic components of mature jujube fruit.