Introduction

Goji berry (Lycium barbarum L.), also known as Lycium fruit or wolfberry and by a number of other vernacular names has been recognized as latest ‘super food or fruit’ and sometimes also refer as ‘berry of youth’ due to its anti-aging and several others health benefits (Yao et al. 2011). It is a deciduous woody and thorny shrub of family Solanaceae, can tolerate the extreme temperatures ranging from −15 °C to +40 °C, usually 3 to 6 feet tall in commercial field conditions due to pruning (Demchak 2014) while in wild conditions can reach 8–12 feet tall. It is originally belonging to China (particularly western provinces) where it is commercially cultivating and exporting worldwide. Its excellent nutritive and medicinal properties attracted and promoted its cultivation in European (Dzhugalov et al. 2015) as well as North American (Demchak 2014) agro-climatic conditions in recent years (Amagase and Farnsworth 2011). The fruits are 1–2 cm bright orange-red ellipsoid berries look like tomato with a sweet and tangy flavor containing 20–40 tiny seeds. The seeds are heterozygous in nature and hence propagation on large scale done usually by vegetative means. Goji fruits usually consumed in dried form, or pressed to make goji juice or as a powder form for medicinal purposes (Zhu 1998). Fruit can be also eaten fresh, but surprisingly, there are no or very less published studies available in the scientific literature about its postharvest behavior and improving its storage life with high fruit quality.

Goji berries possessing several bioactive compounds such as polysaccharides, minerals, carotenoids and polyphenolics which are associated with the health benefits of weight loss, anti-aging, diabetic, vision, diabetic, kidney and liver disorders as well as act as an anti-cancer agent. (Luo et al. 2004). Generally, goji berry has 10.34% moisture, 4.11% crude oil, vitamins 48.94 mg/100 g, 3.44 mg GAE/100 ml polyphenols, 487.29 g/100 ml total sugar, β‑carotene 7 mg/100 g, polysaccharides 5–8% of total dry weight, and antioxidant activity value as 20.78% (Endes et al. 2015). The amount and percentage of nutritive and biochemical constituents vary depending upon the genotype (i. e. seedlings grown or cultivar), type (i. e. fresh or dried) and the way they preserved.

The application of postharvest technology requires a thorough understanding of the structure, composition, biochemistry and physiology of any horticultural crop specially berries fruits. Berry fruits need diverse range of storage environmental conditions to prolong its storage life. The handling or controlling of the storage environmental conditions is a prime factor in developing a successful postharvest protocol of any fruit crop. The important environmental factors relate with controlling the temperature and relative humidity during storage. The ideal postharvest temperature is just above the freezing point where metabolism is slowest. The storage conditions of 0 °C and 90–95% relative humidity) is the optimum storage conditions for many berry species in which strawberries can be stored for 7–10 days, blueberries for 2–4 weeks, cranberries for 2–4 months, and raspberries and blackberries for 2–5 days. Whereas, the cranberries being sensitive to chilling injuries should be stored above freezing point like 3 °C. Generally, the storage life of any fruit species is greatly reliant on the management of berry fruits in both pre-harvest as well as postharvest process. Whereas, strawberries and blackberries can tolerate the maximum freezing temperature of −0.8 °C, cranberries (−0.9 °C), raspberries (−1.1 °C) and blueberries (−1.3 °C) (Lidster et al. 1988; Mitcham et al. 2006, 2007).

Various studies have been carried out on its biochemical, allergic, antioxidant and medicinal properties, but mostly performed on dried goji berries and even in some cases with fresh, but the fruits were acquired from some known cultivars. The work on its postharvest behavior of seedling grown goji berries is unique and completely a new approach employed in current study to optimize a suitable storage temperature to improve its storage life as well as maintaining its post-storage fruit quality.

Materials and Methods

The fruit samples were collected from the seedling grown goji berry plants from research garden of the Dept. of Pomology, Faculty of Agriculture, University of Zagreb, Croatia (Fig. 1) in August 2015. The fruits were harvested with stalks and taken in plastic boxes and brought immediately to the lab conditions. The fruit samples were subjected to test different storage temperatures i. e. 0 °C, −2 °C, 10 °C and 20 °C (room temperature or control) with 95% relative humidity to optimize the suitable storage temperatures for fresh goji berries. The fruits were weighed in plastic boxes and the measurements regarding fruit color, weight loss% and rest of the biochemical attributes after storage of 12 days.

Fig. 1
figure 1

Seedling grown Goji berries at Dept. of Pomology garden, University of Zagreb, Croatia

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Sample Preparation

The extracts of the goji berries prepared using the modified conventional extraction method of Komes et al. (2016). The 5 g of homogenized squashed fraction was poured with 50 ml of distilled hot (80 °C) water and then hold in hot water at 80 °C with continuous stirring for 15 min. After the extraction, the extracts were sieved, cooled and centrifuged at 1800 rpm for five min. The supernatants were filtered and filled up to 50 ml of distilled water. The concentration of obtained extracts was thus 100 g/l that used further for phytochemical analyses.

Measurements and Chemical Analysis

The weight loss% was obtained using a simple formula by subtracting the initial weight from the final weight and then by dividing with final weight and multiplying with 100. The soluble solid contents (SSC) were measured using hand digital refractometer (Atago, PAL-2, Tokyo, Japan). Whereas, the titratable acidity (TA) was measured by titrating 2 mL of goji fruit juice with NaOH 0.1 N and the malic acid was calculated with the conversion factor of malic acid:

$$\text{Malicacid}=\frac{\text{TA}\left (\text{NaOH}\right )\text{amount}\,\times\,\text{milliquivalent factor}\,\times\,10}{\text{amount of juice}}$$

The SSC/TA ratio was calculated by dividing the respective values of SSC and TA as malic acid.

The fruit color variables (lightness, a‑value, b‑value, hue and chroma) were measured using ColorTec-PCM Plus 30 mm Benchtop Colorimeter (ColorTec Associates, Inc. Clinton, New Jersey, SAD) according to CIE LAB system.

The total polyphenols were determined according to modified Folin Ciocalteu’s method of Singleton et al. (1999). The goji fruits extract at 0.1 ml with 7.9 ml of distilled water and 0.5 ml of Folin Ciocalteu’s reagent (diluted with distilled water in 1:2 ratio) was taken in test tube. Afterwards, 1.5 ml of 20% sodium carbonate was added and vortexed and left for two hours to begin the reaction and then the reading was taken at 765 nm (Ough and Amerine 1988). The results were expressed as gallic acid (GAE) mg/100 g of fresh weight (Fw).

For the measurement of anthocyanin, modified method by Proctor (1974) was used. Two discs (top cut) from fresh goji berries were taken (approx. 1 mm of thickness). Area of discs was calculated with an area of ellipse formula \(A=\pi ab\). Then, goji fruit discs were shaken 3 ml of acidified methanolic solution (10 ml HCl/l) until submerged and treated for three hours at room temperature under dark conditions. The absorbance levels were measured at 532 nm and 653 nm and the obtained values for the optical density of anthocyanins were calculated according to the formula reported by Wells (1995):

$$\text{Anthocyanin}=\text{Absorption}_{532nm}-0.25\left (\text{Absorption}_{653nm}\right )$$

Afterwards, the molar concentrations of anthocyanins/cm2 were obtained by dividing the optical density values by the molecular extinction coefficient of cyanidin (\(2.45\,\times\,10^{4}\)), then again dividing by the area of the leaf discs (Siegelman and Hendricks 1957). However, the results are expressed in mg of cyanidin per cm2.

The 2,2-Diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) antioxidant activities were recorded as per procedures reported by Brand-Williams et al. (1995) and Re et al. (1999) respectively and the results were expressed as Trolox equivalents average µmol TE/100 g of Fw.

The β‑carotene level was measured according to the procedure reported by Barros et al. (2008). The 10 ml of acetone-hexane mixture (4:6) taken with the methanolic extract (100 mg of fresh goji fruits) and shaken well or vortexed for one min and then filtered through Whatman No. 4. The volume was set to 10 ml. The absorption readings were measured at 453, 505 and 663 nm and the results were expressed as β‑carotene µg/g of Fw. whereas, the content of β‑carotene was calculated using the equation as follows:

$$\beta \text{-carotene}\left (\frac{mg}{100\,\text{mL}}\right )=\begin{aligned}0.216\,\times\,A_{663}-0.304\\ \,\times\, A_{505}+0.452\,\times\,A_{453}\end{aligned}$$

Sensory Evaluation

The sensory evaluation of treated goji fruits done using Miller et al. (2005) procedure and divided into taste properties (Firmness, texture, juiciness, sugar-acid ratio, aroma, taste fullness and general impression) and external properties (shape, size and color). Evaluation was based on a score ranked from 1–5 hedonic scale comprising excellent (5), very good to excellent (4.5), very good (4), good to very good (3.5), good (3), average (2.5), acceptable (2), unsatisfied to acceptable (1.5) and unsatisfied (1). The goji fruit samples were distributed to trained panel members related to pomological science for sensory evaluation.

Statistical analysis

Effects of treatments were analyzed by ANOVA, and the significance of differences among means were obtained using LSD test at p < 0.05 and the standard deviation calculated by using SAS statistical software ver. 9.4 (SAS Institute, NC).

Results and Discussion

Weight Loss

Due to the berries highly perishable nature, the significant reduction in weight losses during storage is common as in case of goji as well, but the rate of weight loss percentages was significantly different among the different storage temperatures. Because, the storage temperatures have an important role in determining the rate of weight loss or moisture loss in stored fruits and especially the berries if they stored at very low humidity conditions. Such as, the 0 °C and −2 °C appeared with lowest weight loss (13.08% and 14.95%, respectively) that was significantly different as compared to weight loss observed at 10 °C (18.29%) (Fig. 2) in goji fruits. The findings obtained by Bounous et al. (1997) though belong to highbush berries but are in agreement with our results. They found that the weight losses in highbush berries (cvs. ’Coville’, ’Darrow’ and ’Dixi’) stored at 1 °C for 2 weeks varied from cultivar to cultivar where cv. ’Darrow’ appeared with highest weight loss (17.5%). Similarly, 7.91% (at 2 °C) and 14.83% (at 20 °C) weight losses have been reported in Brazilian blackberries cultivars when stored for 12 days (Antunes et al. 2003). Cited reports are in accordance to the results obtained in our study although they are obtained with different berry species as there is not much literature available on goji postharvest aspect for comparison. As per findings of Nunes et al. (1998) the weight losses and decaying in strawberries increased with increasing temperature and storage duration from 1 or 10 °C for 8 days and 20 °C for 4 days of storage in wrapped strawberries (2–11%) and the losses were high in case of un-wrapped berries (11–40%).

Fig. 2
figure 2

Weight loss%, SSC, TA and SSC/TA ratio of goji fruits stored under three different storage temperatures. (Note: values marked with the same letter are not significantly different according to the LSD test)

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SSC, TA and SSC/TA Ratio

There was minor decrease noticed in goji fruits during storage in terms of SSC content but statistically the fruits stored under 0 °C (18.5) were significantly different from −2 °C and 10 °C. Similarly, there were only bit variation found in terms of TA in the fruits stored under −2 °C (0.93) and 0 °C (0.90) but were significantly different from 10 °C (0.84). However, the SSC/TA ratio was appeared little bit higher in 10 °C (20.59) and 0 °C (20.48) in comparison with the fruits stored under −2 °C (18.79) (Fig. 2). The results are in harmony with the findings obtained by Ban et al. (2015) as they found decrease in total soluble content from 21.68% (initial) to 17.32% (after storage) in control samples of Chinese wolfberry fruits stored under 2 °C ± 0.5. According to Robbins et al. (1989) the loss in water most probably was the reason behind the decrement in TA and increment of SSC contents in red raspberry fruits stored at 0 °C. On contrary, Forney et al. (2008) found decrease in both SCC and TA in blueberries (cv. ‘Burlington’) during storage. Sjulin and Robbins (1987) found that such changes between SSC and TA during storage greatly affected the sensory attributes of red raspberries.

Storage Disorders

As expected, the fruits kept under room temperature of 20 °C as a control treatment deteriorated with fungal infestation (Fig. 3a) within a day or two showing its highest perishable nature and hence data not shown here. Moreover, the fruits stored under 0 °C showed minor deterioration and had fresh and healthy appearance (Fig. 3b). However, the fruits stored at 10 °C showed highest deterioration with softening, shriveling, cracking and peel disorder (Fig. 3c). Whereas, the fruits stored at −2 °C showed moderate deterioration with minor disorders of shriveling (Fig. 3d) and peel disorders.

Fig. 3
figure 3

Goji fruits after storage showing; a deterioration with fungal decay at 20 °C; b fresh and healthy appearance at 0 °C; c cracking, softening and peel disorders at 10 °C; d shriveling storage disorder at −2 °C

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Color Variables

There were no any significant differences in terms of all fruit color parameters (lightness, a‑value, b‑value, chroma and hue) among all treatments in goji fruits and hence data is not shown here. In general, fruit color greatly influenced by storage temperatures (Varseveld and Richardson 1980) and also dependent of the type of berry cultivars (Sjulin and Robbins 1987) after storage.

Determination of Biochemical Contents Using Spectrophotometric Analysis

There were significant differences among storage temperatures in terms of all biochemical parameters excluding total anthocyanins (Fig. 4).

Fig. 4
figure 4

Different Phytochemical contents in goji fruits stored under three different storage temperatures. (Note: values marked with the same letter are not significantly different according to the LSD test)

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Total Polyphenols

The highest total polyphenol concentration was found in fruits stored at 10 °C (175.36 mg GAE/100 g of Fw) followed by −2 °C (166.42 mg GAE/100 g of Fw) and 0 °C (158.92 mg GAE/100 g of Fw) (Fig. 4). The differences among all three storage temperatures were significantly different. According to Forney et al. (2008) there was slightly decline of phenolic contents in cvs. ‘Duke’, ‘Bluecrop’ and ‘Brigitta’ of blueberries when stored in CA storage (0–15 kPa CO2 in combination with 1–16 kPa O2) for 9 weeks at 0 °C. Whereas, as per findings obtained by Song et al. (2003) the amount of total phenolic compounds were stable and not changed in highbush blueberries (cv. ‘Coville’) when stored in CA (10 kPa CO2 and 15 kPa O2) at 0 °C for 4 weeks and in air at 10 °C for 1 week (Song et al. 2003). Türkben et al. (2010) reported that there was decline in total polyphenolic compounds but varying from cultivar to cultivar in raspberries due to freezing.

Total Anthocyanins

Whereas, there were no significant differences in terms of total anthocyanins (expressed as cyanidin) in goji fruits among all storage temperatures (Fig. 4). However, the fruits stored at 0 °C appeared with highest anthocyanins level (0.11 mg/cm2). The anthocyanins are the natural pigments providing excellent red, purple, blue and black colors in the berry fruits. The biosynthesis of anthocyanins in fruits endures even after the harvest process and greatly dependent on storage temperatures (Moor et al. 2012) and increases usually during short storage. Similarly, there was increase of anthocyanin contents in strawberries (cvs. ‘Sonata’, ‘Polka’ and ‘Honeoye’) stored for 5 days at 3 ± 1 °C, and the raspberries (cvs. ‘Resa’, ‘Rumiloba’, ‘Schönemann’ and ‘Tulameen’) stored for single day at 20 °C or 3 days at 2–4 °C followed by one day at 20 °C found with highest anthocyanins levels as compared to the fresh raspberries) (Krüger et al. 2011).

DPPH & ABTS Antioxidant Activity

The DPPH antioxidant activity was significantly lower (227.53 µmol TE/100 g of Fw) in the fruits stored at 0 °C than in those stored at 10 °C (283.33 µmol TE/100 g of Fw) or −2 °C (263.25 µmol TE/100 g of Fw) (Fig. 4). However, there was no significant differences between −2 °C (1165.5 µmol TE/100 g of Fw) and 0 °C (1173.95 µmol TE/100 g of Fw) but both were significantly different and lowest from 10 °C (1438.66 µmol TE/100 g of Fw) in terms of ABTS antioxidant activity. There are several reports available mentioning the high antioxidant activity levels in berries varying from specie to specie and their maturity stages (Kähkönen et al. 2001). Wang and Lin (2000) conducted experiments to compare the antioxidant activity levels in different berry species and found it highest in strawberries followed by black raspberries (Rubus occidentalis L.), blackberries (Rubus sp.), and red raspberries (Rubus idaeus L.). on the other hand, Mölder et al. (2011) found increased in the antioxidant activity of raspberries (cv. ‘Glen Ample’) during 4 days of storage that was not dependent on the applied storage temperatures (1 or 4 °C) or the packaging material used.

β-Carotene

The levels of β‑carotene were non-significant between −2 °C (101.09 µg/g of Fw) and 10 °C (115.56 µg/g of Fw) but both were lowest and significantly different from 0 °C (163.86 µg/g of Fw) (Fig. 4). β‑carotene is one of the common carotenoids contributing towards the color of fruits and optimizing the levels of this compound after storage has vital importance as the color of fruits is an influencing factor to attract the consumers. The β‑carotene levels up to 46 µg/g of dry fruit weight in various berry species (such as, blueberry, black currant, black chokeberry, lingonberry and raspberry) have been reported (Prior et al. 2001).

Sensory Evaluation

The sensory attributes were significantly different and prominent in the goji fruits stored under 0 °C in comparison with −2 °C and 10 °C. However, at −2 °C the scores were closely similar to 0 °C in terms of shape and size, but fruits stored under 10 °C appeared with the lowest scores due to the extremely high storage disorders ratio during 12 days of storage. Though, firmness, juiciness and aroma exhibited marked decline during storage period of 12 days but fruits stored at 0 °C were of quite acceptable quality (Fig. 5). However, as per findings of Ban et al. (2015) who tested different heat different treatments in combination with chitosan coating on Chinese wolfberry the chitosan coated fruits were slightly better in appearances and juiciness, but there were not significant differences in terms of sweetness and tart between the control and heat treated fruits.

Fig. 5
figure 5

Sensory analysis of goji fruits stored under three different temperatures

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Conclusion

The storage temperatures or conditions have a key role in prolonging the storage life as well as in determining the post-storage quality of any fruit crop. The fruits stored under 0 °C appeared with lowest weight loss%, highest fruit quality in terms of phytochemical and sensory attributes with healthy and fresh appearance after 12 days of storage in comparison with 10 and −2 °C storage temperatures which appeared with higher weight losses in addition of some storage disorders. The findings obtained ensures the highest fruit quality of goji berries in terms of biochemical properties with minimum weight losses after 12 days of storage and will enable its possible marketing and consumption as fresh fruits worldwide.