Effect of non-thermal plasma treatment on antioxidative and prebiotic properties of aqueous herbal extracts

To provide food with a longer shelf life, synthetic antioxidants are frequently used in the food industry. Common synthetic antioxidants includebutylated hydroxyanisole(BHA), butylated hydroxytoluene (BHT), propyl gallate (PG), and tert-butylated hydroquinone (TBHQ). Since those compounds are suspected to cause some health problems, such as liver damage, there is a strong initiative to develop and utilizeantioxidantsfrom natural origins.Medicinal plants could be a good alternative source of these compounds. Besides antioxidative properties, natural compounds derived from medicinal plants often have various biological activities, like anti-inflammatory, antimicrobial, analgesic, etc.These compounds can contribute to food's functional and technological properties and interact with food microbiota. In this study, aqueous extracts of tea production residues of common nettle (Urtica dioica L.), St. John’s wort (Hypericum perforatum L.), and hoary willowherb(Epilobium parviflorum L.) were treated with non-thermal plasma. The effect of different durations of plasma treatment on the antioxidative capacity of these extracts was monitored. The fermentation of these extracts solely and extracts supplemented with MRS broth by Ligilactobacillus salivarius was examined. Antioxidative properties of plasma-treated extracts were analyzed using DPPH and ABTS methods. The total phenol content of these extracts was determined by the Folin-Ciocalteu reagent. Generally, hoary willowherb extracts were rich in total phenol content, two or three orders of magnitude richer than the other two plants. They also showed significantly higher antioxidative activity. This activity slightly decreased with prolonged plasma treatment. However, that didn’t affect bacterial growth in these extracts. It was shown that L. salivarius growth was enhanced in the pure plasma-activated extracts of hoary willowherbin comparison to extracts supplemented with MRS broth. Though, the observed effects are substrate-dependent and vary from plant to plant. The antioxidative capacity of common nettle extracts remained intact during the plasma treatment. With the addition of MRS broth (25% v/v), bacterial growth was completely unhindered, providing approximately 109 CFU/ml during the exponential phase. Moreover, the 10-minute-long plasma treatment was effective in the inactivation of microorganisms already present in herbal biomass, preventing the spoilage of treated extracts. This could be an important step up in extraction by hurdle technology, especially in the production of symbiotic preparations possessing both antioxidative and probiotic properties