Effect of thermal and nonthermal processes on selected physicochemical parameters of vegetables

Abstract

The increasing request of fruit and vegetables is bringing the food industry, together with the academia, to improve the processes targeted to prolong the crops shelf-life. Preservation of foods usually involves technologies that prevent microbial growth and retard enzymatic quality degradation reactions. Traditionally, thermal processing (≥90 °C) and freezing (≤−40 °C) have formed the core of food preservation. During freezing, the ice crystal formation causes damages of vegetables cells walls. Furthermore, frozen vegetables are previously blanched and consumed cooked, so they are subjected also to the detrimental effect of thermal treatment. Novel nonthermal technologies are one of the alternatives currently available for the preservation of vegetables because they accomplish inactivation of microorganisms and enzymes but minimize adverse thermal exposure. Among them, the use of high hydrostatic pressures for food processing is finding increased application within the food industry. Despite several studies have been conducted on the effect of processes on vegetables, a lack of literature is still present, because of the fragmented investigation of the effect of traditional methods and still poor data on the novel technologies. Based on these considerations, the present PhD thesis deals with the evaluation of the effect of different conventional and innovative preserving technologies on vegetables by means of selected physico-chemical properties. Different approaches were investigated to reach this goal. In the first part of this PhD thesis, the evaluation of the effect of different steps of a conventional industrial freezing process was investigated on asparagus, zucchini and green beans. Samples were examined in all the stages of production “from farm to fork” as raw/uncooked (control test), blanched, boiled from the raw samples and after industrial freezing. A deep investigation was carried out to evaluate qualitative (texture and colour), histological and nutritional (antioxidant activity and bioactive compounds) aspects. The obtained results will show how manufacturers and researchers have to join together in order to develop industrial freezing process conditions according to the matrix of vegetable, with the final aim being to offer the end-user consumer high quality frozen products. In the second part of the thesis, the evaluation of different cooking methods as preservation technique for vegetables was investigated. There is a general lack of study in literature on the effect of cooking on the main quality attributes (i.e. texture, colour) that have a great impact on the final consumer acceptance of vegetables. When taken into consideration, only a single type of heating treatment or one quality attribute was discussed. Thus, this topic needs to be more deeply investigated with the ultimate goal to offer a higher quality final cooked product to the consumers, achieving a greater retention of the original quality of the processed vegetable. Starting from this observation, in a first study the effect of three common cooking procedures (boiling, steaming and microwaving) on structure, texture and colour of carrots processed in an industrial plant was studied and the obtained results were compared with those obtained from raw carrots provided by the same manufacturer and cooked with the same procedures. Then, a relatively new little explored cooking approach, the combined air/steam cooking, was the object of another investigation. Several air/steam cooking time/temperature conditions were selected and applied on two commonly used vegetables, as pumpkin and Brussels sprouts, selected on the basis of their different nutritional values and physical characteristics. The results obtained in term of cooking (cooking values, weight loss), physical (texture and colour) and nutritional (antioxidant activity and phenol contents) parameters were compared with those obtained cooking the same vegetables with a common steaming procedure. In the third part of the thesis, the impact of a novel non thermal technology, such as high hydrostatic pressure process was studied at the Centre for nonthermal treatment of food”of the Washington State University of Pullman (WA) on beetroot, which presents a well-known high nutritional value. In particular, the effect of pressure treatment realized at 650 MPa and a temperature of 21°C was studied taking into account different times of processing evaluating physico-chemical, enzymatic and nutritional aspects. The obtained results were compared to the raw product and those treated with classical thermal treatments as blanching and canning. The preliminary findings obtained in this part of the thesis work, suggested that the high hydrostatic pressure treatment could be used as valid alternative for the preservation of beetroots, being less invasive than canning and quite comparable with blanching. In the last part of this PhD work, a novel spectroscopical approach as Raman spectroscopy was used for the evaluation of carotenoid pattern changes after different cooking treatments on carrots. Changes on carotenoid profiles measured by means of Raman spectroscopy were compared with their quantitative determinations acquired by the high-performance liquid chromatography and the colour parameters finding good statistical correlations among techniques. In conclusion, this PhD thesis revealed the strengths and weaknesses of both the traditional thermal and novel nonthermal vegetable treatments. In general, while the freezing and cooking traditional methods resulted in an extended modification of the quality parameters, the high pressure treated samples revealed a better preservation degree. The extent of the modifications observed after the application of both thermal or nonthermal processes was however related to the type of vegetable and the process conditions, therefore this finding should encourage to research and plan specific processing conditions. A big effort was given to the use of novel analytical technology, such as Raman spectroscopy, for the fast and easy way to obtain real time information on the chemical transformation of the nutrients under treatment.