Biological activity and nutritional properties of processed onion products

Abstract

Nowadays, there is a constant and increasing social concern about the food we daily eat and particularly about vegetables. There is a great demand of minimally processed food or food ingredients with functional properties as a consequence of the current changing healthy and environmental sustainable living. Onion (Allium cepa L.) is an important vegetable traditionally used as a food ingredient in the Mediterranean diet that has a high production, domestic, and foreign trade worldwide. It is consumed raw, cooked or processed into different onion products in the daily diet of many subjects. This Allium vegetable has been proven to have interesting technological properties and beneficial health effects including antioxidant, anticarcinogenic, antimicrobial, prebiotic, hypolipidemic, and antithrombotic properties. Onion biological properties have been related with its bioactive compounds such as flavonols (quercetin and its glucosides), dietary fibre (fructans and fructooligosaccharides (FOS)), and organosulfur compounds (OSCs). The main aim of this PhD Thesis was to study the biological activity and nutritional properties of processed onion products in order to evaluate a possible development of novel and innovative functional onion ingredients that could substitute available synthetic ingredients according to the current consumers demand and global concern towards a healthier diet and living. To achieve this objective, in vitro and in vivo studies have been carried out with onion products of three Spanish onion cultivars: Allium cepa L. var. cepa, ‘Recas’, Allium cepa L. var. cepa, ‘Figueres’, and Allium cepa L. var cepa, ‘Grano de Oro’.

The first part of the PhD Thesis aimed to evaluate in vitro effects of food processing and preservation technologies on onion nutritional and technological properties. The first in vitro study analyzed ‘Figueres’ and ‘Recas’ onion by-products (juices, pastes and bagasses) stabilized by sterilization, pasteurization, and freezing technologies. Results demonstrated that processing ‘Recas’ onion wastes to obtain onion pastes and the subsequent stabilization with pasteurization trigger to safe onion by-products with a good bioactive compound content and excellent antibrowning properties. In parallel, a second in vitro study showed that processing fresh ‘Grano de Oro’ onion with a nonthermal technology that combines high hydrostatic pressure with low temperature enhances flavonol (quercetin and its glucosides), total phenol extractabilities and maintains the antioxidant activity compared with the unprocessed onion. The second key topic of the investigation was the in vivo evaluation of the biological activity and nutritional properties of ‘Recas’ onion by-products. For that purpose, we studied the pasteurized ‘Recas’ onion paste chosen in the previous in vitro study, and its two derived onion fractions, an onion extract (rich in fructans and FOS) and an onion residue. The biological responses shown in healthy rats fed ‘Recas’ onion by-products revealed that these by-products are not genotoxic and that they exert interesting antioxidant and prebiotic properties. Moreover, they showed antithrombotic effects exerting a potential usefulness of these by-products in a cardiovascular disease (CVD) preventive diet. Onion OSCs and FOS seem to be the bioactive compounds responsible of some of the effects shown. Furthermore, the dimethyl sulfone was identified as a dietary biomarker for onion intake in a nutri-metabonomic study. Being able to detect and quantify specific onion intake biomarkers is highly beneficial in control of nutritionally enhanced functional foods and in human intervention studies.

To go deep inside the biological activity evaluation of processed onion products, a human intervention study with overweighed subjects was also conducted using a ‘Recas’ onion product which was added to two four precooked dishes as a food ingredient. Currently, the main health effects are being evaluated. In the future, in vitro and in vivo studies would be required including different onion products processed by high hydrostatic pressure or other nonthermal technologies in order to study in depth the potential improvement of onion bioactive compounds bioaccessibility that could promote preventive or protective effects on CVD and obesity, two of the more prevalent diseases nowadays. In conclusion, the current PhD Thesis shows that pasteurizating onion pastes obtained from onion wastes and high-pressure processing fresh onion are two valuable processing food technologies for the food industry nowadays. These processed onion products studied have good technological and nutritional quality as added value. Therefore, there is a new challenge for the food onion industry using processed onion products as novel and innovative ingredients for the potential functional food design and development.