Valorisation of cold plasma-treated onion skin waste in chocolate production

Onion skin is generated in high amounts as food waste that is rich in bioactive components. Thus, the valorisation of onion skin waste in food production following suitable methods of treatment can be regarded as a sustainable strategy in terms of nutrition, economy, and environment. Skin waste from onions (Allium cepa) was ground and exposed to dielectric barrier discharge cold plasma (DBDCP) at 40 kV for either 0 or 30 min. Then the cold plasma-treated onion skin powder (OSP) was added to chocolate at a concentration of 0%, 1%, or 2% (w/w). The DBDCP-induced changes in the microbial load, colour, water content, total phenolic content (TPC), and antioxidant activity of OSP were assessed. The TPC, antioxidant activity, and sensory properties of the chocolate samples added with OSP were also monitored. Cold plasma treatment increased the TPC and antioxidant activity of OSP, but it did not affect the water content, colour, and microbial load. The antioxidant activity and TPC of the chocolate samples incorporated with DBDCP-treated OSP at a level of 2% were higher than the samples with or without untreated OSP. However, the samples with 2% DBDCP-treated OSP demonstrated lower overall acceptability scores compared to samples without OSP. It can be said that cold plasma can be utilised in the treatment of onion skin waste and valorisation of cold plasma-treated onion skin waste in the production of chocolate to improve its antioxidant properties. Future studies can focus on the enhancement of the sensory properties of onion skin-added chocolate products.