3.1. Nutritional Composition
Results regarding the nutrient content and energy value of the analyzed goji berries are presented in
Table 1.
All goji berry fruits were characterized by high moisture content. Black goji fruits had significantly higher moisture content than the other samples (
p < 0.05), consequently resulting in the lowest energy value. The energy value of black goji berries was 38% and 30% lower compared to the red and yellow goji berries, respectively. In addition to the influence on energy density, the high moisture content limits the shelf-life of fresh goji berry fruits. Therefore, the majority (about 90%) of fresh goji berries are processed into dried goji berries, juice, wine, tea, tinctures, powders, and tablets [
2,
8].
The fat content of the red goji was significantly higher than that of the other varieties (
p < 0.05). Proteins and ash contents were not significantly different among goji berry varieties (
p > 0.05), although slightly higher protein contents were observed in the yellow goji berry fruits. In general, the TDF content in red goji and yellow goji was similar and significantly higher than that of the black goji. These results are in line with previous studies pointing out that dietary fibers are the second macronutrient in goji berries [
7,
38]. As red and yellow goji berries had more than 3 g TDF per 100 g, they meet the requirement for the “source of fiber” claim [
39].
The fatty acid compositions of the studied goji berries are presented in
Table 2. The most abundant fatty acids were linoleic (C18:2n-6), oleic (C18:1n-9), palmitic (C16:0), and stearic (C18:0) acid, accounting for about 95% of total fatty acids. Similar results were reported for red goji berry cultivated in Italy [
40,
41], Greece [
42], Turkey [
43], North Macedonia [
44] and for Chinese black goji berries [
24,
45].
To the best of our knowledge, there are no previous data regarding the fatty acid composition of yellow goji berry. Despite the similar fatty acid profiles, there were significant differences in the relative percentages of individual fatty acids among the studied goji berries, resulting in different SFA, MUFA, and PUFA contents. The black goji berry had the highest SFA content, followed by red and yellow goji berries. The most abundant SFA in all goji berries was palmitic acid (11.8–20.4%), followed by stearic acid (3.0–6.9%), while the presence of heptadecanoic acid was only detected in red and yellow goji berries. Oleic acid content, a predominant MUFA, varied between 17.1% and 23.6% in black and red goji berry, respectively. All studied goji berries contained more than 50% of linoleic acid. The highest amount of linoleic acid in yellow goji berry (59.38%) corresponded with the highest PUFA content of this variety compared to others.
Considering the fatty acids’ potential health effects, the PUFA/SFA should be more than 0.45 [
46]. In this study, the PUFA/SFA ratio for red, yellow, and black goji berry was 2.52, 3.59, and 1.86, respectively, indicating that all goji berries’ lipids could be beneficial for human health. In addition to the highest PUFA/SFA ratio, yellow goji berry was characterized by the lowest value of the atherogenic index (AI). In terms of oxidative stability, the lowest Cox value was determined for black, followed by red and yellow goji berry oils.
The content of macro- and microelements for the studied goji berries is shown in
Table 3. Mineral concentrations varied significantly across the different goji berry samples. The decreasing order of minerals in both
L. barbarum varieties was: K > P > Na > S > Mg > Ca > Fe > Zn > Cu > Mn > Cr > B > Se. The red goji berries had about two times more Ca and Fe than yellow goji berries. On the other hand, the higher concentrations of K and Cu and much lower Na were found in yellow goji than red goji berries.
Considering that both goji berry samples were collected from the same plantation and share the same harvesting and post-harvesting handling procedure for mineral analysis, observed variations in mineral content could be mostly attributed to different goji varieties. The decreasing order of minerals in black goji berries showed some differences, in comparison to other goji berry samples: K > P > Na > Ca > Mg > S > Fe > Zn > B > Mn > Cu > Cr > Se. A similar mineral profile of black goji berries was evidenced in a study conducted by Liu et al. [
7]. In this study, except for the Na and Ca, all mineral content in black goji berries was lower than in red and yellow goji berries.
Considering the content of minerals expressed as a percentage of the Recommended Daily Allowance (RDA) values (
Table 4), there was evidence that all goji berry samples contained K, P, and Cu levels which were more than 15% of the RDA. The red goji berries contained 15.7% of the RDA for iron and could be recommended as a source of this valuable microelement. In this study, the iron content in red goji berry was about 5-fold higher than found in fresh red goji berries of Italian origin [
38,
41].
3.2. Physicochemical and Bioactive Compounds Analysis
The TSS, TA, their ratio (TSS/TA), and pH values, as important indicators of sensory quality of goji berries, are shown in
Table 5. The Brix value, which reflects the TSS content in fruits, ranged from 9.43 to 16.73% with significant differences among analyzed goji berry varieties. The red goji had the highest TSS content, followed by the yellow goji, while the black goji had the lowest TSS level. The TA of analyzed goji berries ranged from 0.70 to 0.89% and the average pH values were 4.56–4.71, with the highest acidity observed for the black goji berry. Our results are comparable to those of Zhang at el. [
47], who also found the lowest TSS and the highest TA value in
Lycium ruthenicum compared to the other genotypes. In this study, the TSS/TA ratios were, on average, two times higher in red and yellow goji than black goji, indicating higher sweetness for
L. barbarum berries.
The
Lycium fruits are considered valuable sources of micronutrients, including vitamins, such as thiamin, riboflavin, and vitamin C [
9]. Donno et al. [
48] showed that average vitamin C content in fresh red goji berries was 48.94 mg/100 g, which makes up approximately 60% of the RDA [
20]. Most of the other studies have reported that vitamin C of
L. barbarum berries ranged from 30 to 60 mg/100 g FW [
16,
38,
41,
49,
50], depending on the goji berry cultivars and their growing regions. However, some authors found a ten times lower vitamin C content in some goji berry varieties [
51,
52]. In addition to genotypic differences, pre-harvest climatic conditions and cultural practices, harvesting and post-harvesting handling procedures, including experimental conditions, could also affect the vitamin C content [
53]. In addition to L-ascorbic acid, the fruit of
Lycium barbarum L. contained AA-2βG, documented as pro-vitamin C [
54,
55] with unique antioxidant activity [
56]. AA-2βG is estimated to account for 0.5% of dried goji berry, comparable to the fresh lemons’ ascorbic acid content [
54]. Kosińska-Cagnazzo et al. [
57] found a range from 0.35 to 2.79 mg/g DW AA-2βG in six goji cultivars from Switzerland, with the highest content of AA-2βG in a cultivar with the highest sugar content. In our study, the AA-2βG content of red goji berry (0.61 mg/g FW; corresponding 2.5 mg/g DW) was higher than in the yellow goji berry (0.48 mg/g FW, corresponding 2.2 mg/g DW), while no AA-2βG was detected in black goji berry. Chromatograms are shown in
Figure 1. The obtained results of AA-2βG correspond to the L-ascorbic acid content of 33.4 mg/100 g FW and 24.2 mg/100 g FW for red and yellow goji berry, respectively.
Regarding total carotenoids, red goji berry contained significantly higher amounts compared with yellow goji berry and no TCC content was detected in black goji berry. These results are in agreement with those obtained by Liu et al. [
58]. They found that red goji accumulated high carotenoids content (primarily zeaxanthin), while the TCC was undetectable in black goji fruits at the ripe stage. The significant variability in TCC among different
Lycium species was also previously reported by Zhang et al. [
47] and Peng et al. [
59].
The black goji berry presented the highest TPC content, which was significantly different from those of the other goji berries. The highest TFC was found in yellow goji, followed by red goji and much lower content in black goji berries.
The tannin content of black goji berry samples was two times higher than that of the red and yellow goji berries. In contrast to black goji berry, anthocyanins were not detected in red and yellow goji berries. Overall, these results indicate that the phenolic content differs significantly among different goji berry varieties, probably reflecting their specific health-promoting effects [
60]. In addition, the observed differences in the polyphenol contents compared to the literature data, including previously published data on red goji berry fruit cultivated in north Serbia [
16], could be explained by different climate and soil factors that affected the plant development [
61,
62], as well as extraction procedures that were applied [
63].
3.3. Antioxidant Activities
The results of the antioxidant activities of methanol goji berry extracts carried out by five different assays are presented in
Table 6.
In particular, black goji showed the best results in metal-reducing activity (FRAP, CUPRAC) and radical scavenging activities (DPPH, ABTS). The extracts from red and yellow goji berries had a higher capacity to inhibit lipid peroxidation by β-carotene bleaching assay. Similarly, in the study by Islam et al. [
5], black goji berry extracts showed higher antioxidant activity for the FRAP and radical scavenging assays (DPPH, ABTS), compared to the red goji berries. Xin et al. [
64] also reported that black goji samples have higher FRAP and DPPH activities than the red goji berries. There is evidence that the main bioactive compounds, phenolics, flavonoids, carotenoids, and polysaccharides, contribute differently to the antioxidant activities of goji berries [
47]. Recently, Liu et al. [
7] showed that the major contributors for antioxidant activity (DPPH, ABTS, FRAP, and ORAC assays) of black goji berries are phenolic compounds and polysaccharides. Overall, the methanol extract from black goji berry showed the highest ACI values (94.45), indicating that this fruit has nearly 1.7 times higher antioxidant potential than red and yellow goji berries.
3.4. Antimicrobial Activities
The investigated goji berries extracts showed mild antimicrobial activity against Gram-positive, Gram-negative bacteria and yeast (
Table 7).
The best activity was obtained with yellow goji berry extract, which inhibited the growth of three Gram-negative strains (
K. pneumoniae,
S. abony and
P. aeruginosa) and yeast
C. albicans at 2 mg/mL. Similarly, previous research showed that hydromethanolic extract of goji berries inhibited the growth of Gram-positive (MIC values 2.5–5 mg/mL) and Gram-negative bacteria (MIC values 2.5–20 mg/mL) [
65]. The higher content of bioactive compounds, such as flavonoids, in the yellow goji berry extract, is probably associated with its prominent antibacterial potential compared to other analysed extracts [
66]. In addition, Pedro et al. [
67] showed that organic goji berry oils and extracts showed higher antimicrobial activity in comparison with samples of conventional fruits, which can be explained by the highest contents of fatty acids and carotenoids. So, the observed antibacterial potential of yellow goji berry could be attributed to the highest UFA and linoleic acid content. The black goji berry extract with high anthocyanin content did not inhibit microorganisms’ growth at the investigated concentrations (0.125–2 mg/mL). Previous research showed that anthocyanins from black goji berries were not digested, but interacted with intestinal microbiota and stimulated fermentation and short-chain fatty acid production [
68]. Furthermore, such modulation of the intestinal microbiota, due to a diet rich in anthocyanins, could lead to an anti-inflammatory effect in the gastrointestinal tract [
69].