Phenolic composition in selected dietary spices and medicinal herbs and their effects on probiotic and pathogenic bacteria

Abstract

Phenolic antioxidants, which can prevent oxidative stress-related diseases, and probiotic bacteria, which have beneficial health effects on the human body, especially positive impacts on the gastrointestinal tract, are important functional ingredients in the food industry. Since phenolic antioxidant and probiotic bacteria provide different health benefits, it would be interesting to develop new functional food products containing both phenolics and probiotic bacteria to provide extra health benefits. However, the effects between phenolic compounds and probiotic bacteria in the same food system have scarcely been studied. Thus, the interactions between natural plant antioxidants and probiotic bacteria were chosen for investigation in this study. Six common dietary spices and medicinal herbs, i.e. Padang cassia, oregano, clove, Chinese cassia, Japanese knotweed and pomegranate peel, were selected in this study. All the selected edible plants were shown to have a high phenolic content and antioxidant capacity. By HPLC-UV analysis, various phenolic compounds, such as trans-cinnamaldehyde, (+)-catechin, (-)-epicatechin, carvacrol, resveratrol and eugenol, were identified. Although the phenolic content and the variety of phenolic composition in soluble extracts of the selected edible plants were generally higher, some of the phenolic compounds were more abundant in the bound extracts. Besides, phenolic compounds were shown to be responsible for the high antioxidant capacity in plant extracts of the dietary spices and medicinal herbs. Among the six selected edible plants, pomegranate peel was found to have the highest level of phenolic content and strongest antioxidant capacity. After determining the phenolic content, phenolic composition and antioxidant capacity of the plant extracts, their antimicrobial effects on ten different bacteria were examined. It was found that phenolic extracts from the plant samples selectively inhibited the food-borne pathogenic bacteria, but did not inhibit the lactic acid bacteria, which are common probiotics. Phenolic compounds, including trans-cinnamaldehyde, carvacrol, eugenol and p-coumaric acid, were found to be mainly responsible for the antibacterial activity. Pomegranate peel extract exhibited the strongest antibacterial effect among all the selected plant samples. Pomegranate peel extract was added to milk for further investigation. It was found that phenolic extract of pomegranate peel in milk culture (2% w/v) did not affect the growth and survival of Lactobacillus acidophilus during fermentation. Meanwhile, there was a significant increase in phenolic content and antioxidant capacity after fermentation. The phenolic compositions in polyphenol-enriched milk were analysed with LC-MS/MS, and 13 phenolic compounds were identified. Although there was no observable change in phenolic composition, some of the identified polyphenols, e.g. punicalagin and pedunculagin II, increased in concentration after fermentation. Overall, dietary spices and medicinal herbs can be excellent natural sources of antioxidant polyphenols. With the antibacterial properties, it is possible for phenolic extracts to inhibit food-borne pathogenic bacteria and promote food safety. In addition, it is promising to develop function-enhanced food products containing both antioxidant polyphenols and probiotic bacteria since there were positive interactions between the phenolic extracts and lactic acid bacteria in the same food system.