Title：Effects of lactic acid bacteria on antioxidant activity in vitro and aroma component of Eucommia ulmoides tea

Journal: Journal of Food Science and Technology

IF: 3.3

Original link：https://doi.org/10.1007/s13197-023-05833-w

Reporter：Xinyun Zhang-25-master

Eucommia ulmoides tea is a popular functional health drink in Asian countries, but its unique herbal aroma is difficult for consumers to accept. The effects of four lactic acid bacteria strains (Lactobacillus plantarium, Lactobacillus bulgaricus, Lactobacillus acidophilus and Streptococcus thermophilus) fermentation on the physicochemical property, antioxidant activity in vitro and aroma component of E. ulmoides leaves were studied. Within the four strains, the sample by L. bulgaricus fermentation showed the higher concentrations of chlorogenic acid, geniposidic acid and stronger antioxidant activity in vitro. Moreover, the sample by L. bulgaricus fermentation produced a stronger fruity and f loral flavor. These results suggested that L. bulgaricus was the best strain for fermentation E. ulmoides tea. The differences between different strains should be considered when selecting lactic acid bacteria for raw material fermentation of fruits and vegetables.

The bark, leaf, male flower and seed of E. ulmoides all have beneficial health care. E. ulmoides leaves are used for medicinal purposes as well as food. It is rich in iridoid terpenes, flavonoids and phenolic compounds, and has the effects of antioxidation, lowering blood pressure, and nourishing the liver and kidney. Among these, chlorogenic acid and geniposidic acid are the most important bioactive components of E. ulmoides, and chlorogenic acid is a natural antioxidant and geniposidic acid affects lowering blood pressure.

At present, the problem of E. ulmoides tea is its unique herbal aroma, which is difficult for consumers to accept. Currently, LAB are extensively utilized in the fermentation process of fruits and vegetables to enhance the sensory quality and flavor of plant extracts while facilitating the conversion of bioactive constituents. Concurrently, it can also augment the antioxidant and antibacterial properties of plant extracts.

It can be seen that different strains of fermentation may have different effects on fruit and vegetable. However, few studies have been conducted to compare the differences between different strains. Therefore, the effects of LAB fer mentation on the physicochemical properties, antioxidant activity in vitro and aroma components of extracts from E. ulmoides leaves were studied, which will provide technical support for improving E. ulmoides tea, and allow us to understand the differences in the fermentation of different LAB species.

1. Physicochemical characterization analysis

The physicochemical characterization of the fermentation broth of E. ulmoides leaves was shown in Table 1. Compared with the control, it was observed that the pH decreased, but the TA increased after 48 h fermentation. The reason was that during the fermentation process, LAB produced many organic acids, which reduced the pH and increased TA in the fermentation broth of E. ulmoides leaves.

2.  Color analysis

According to the color parameter, the L* value representing the lightness decreased after 48 h. The L* value of the L. bulgaricus fermentation sample was the highest (46.30), and the L. plantarium fermentation sample was the lowest (31.67), which were lower than those of the controls (47.21). The reason was that the growth of LAB increased the turbidity, resulting in the decrease of L* value. Meanwhile, the a* and b*value decreased, and only L. bulgaricus and S. thermophilus treated samples had similar values as the control after 48 h.

3.  Bioactive compound analysis

The quantification of the five compounds of four LAB strain fer mentation samples was shown in Fig. 1A. The contents of chlorogenic acid and geniposidic acid of E. ulmoides leaves were higher, but the geniposide and quercetin contents were lower. No matter which LAB was used for fermentation, the chlorogenic acid content of fermentation broth was higher than the control (p < 0.05). The highest chlorogenic acid and geniposidic acid contents were observed in L. bulgaricus fermented samples with 0.24 and 0.19 mg/mL, respectively. Thus, fermentation could improve the number of phenolic compounds, which were the result of microbial hydrolysis reaction.

4. Antioxidant capacity analysis

The scavenging rates of hydroxyl, DPPH, and ABTS radicals of fermentation broth were presented in Fig. 1B. Compared with the antioxidant activity of E. ulmoides leaves, the antioxidant activity of E. ulmoides leaves after fermentation by LAB was improved. LAB fermentation had positive effects on free radical scavenging components in E. ulmoides leaves. LAB fermentation increased the antioxidant activity of mulberry juice. The increase in the antioxidant activity during fermentation may be due to some compounds, such as flavonoids, polyphenols and so on. According to Fig. 2A, the chlorogenic acid con tent in E. ulmoides fermentation broth was higher, which also confirmed the high free radical scavenging ability of fermentation broth.

5. Aroma component analysis

The volatile flavor compounds of different LAB fermentation samples were shown in Fig. 2. The results showed that there were 6 key aroma components in the control. There were 7 key aroma compo nents in the fermentation broth of L. bulgaricus.

There were 7 key aroma compo nents in the fermentation broth of L. bulgaricus.7 odor types were confirmed on the basis of their odor descriptors, including woody, floral, fruity, green, roasted, chemical and fat (Table 2).

As shown in Fig. 3, the volatiles in E. ulmoides leaves mainly exhibited floral, woody, green, fruity odors followedby roasted, chemical and fat odors.

Based on these results, suitable selection of LAB strain may improve the flavor of E. ulmoides leaves and lead to a more appealing product with high acceptance, and L. bulgaricus was the best strain of E. ulmoides fermentation.

In this study, the fermentation effects of four LAB (L. plantarium, L. bulgaricus, L. acidophilus and S. thermophilus) on water extracts from E. ulmoides leaves were compared, including physical and chemical properties, antioxidant activity in vitro and aroma components. The results showed that proper fermentation could improve the content of active ingredients in the water extract of E. ulmoides leaves as well as its antioxidant and aroma properties. Among them, the content of chlorogenic acid and flavonoids increased, the antioxidant capacity was slightly improved, and the flavor was more intense. However, there were differences among different strains, and L. bulgaricus had the best fermentation effect on E. ulmoides leaves, which significantly enhanced the contents of chlorogenic acid, genipin acid, rutin and other substances as well as its antioxidant capacity. At the same time, the samples obtained after fermentation by L. bulgaricus had strong fruity and floral characteristics. In general, the differences between different strains should be considered when selecting LAB for raw material fermentation of fruits and vegetables.