Evaluation of the effect of high-pressure treatment conditions on pumpkin cubes: a multidisciplinary study

ABSTRACT With the world’s increasing population, people mainly focus on nutritious, convenient, and sustainable foods that have “fresh-like’’ characteristics. Sustainable food processing necessitates the efficient use of natural resources to reduce energy consumption. In recent years, various innovative technologies have emerged that have the potential to replace traditional thermal methods employed in the food processing industry and reduce energy consumption. On the other hand, the food industry has embraced various efficient non-thermal methods including pulsed electric field (PEF), ultrasonication (US), cold plasma, high-pressure processing (HPP), and ultraviolet radiation. These methods have demonstrated their effectiveness in terms of energy efficiency and their ability to preserve the quality of food and mitigate nutrient loss compared to traditional systems. High-pressure processing (HPP) is considered a substitute for thermal methods due to its ability to enhance food safety, extend shelf life, maintain the sensory, physiochemical, and nutritional properties of food, eliminate bacterial cells, yeasts, and molds without the use of heat. Another positive feature of HPP technology is considered to be the sustainability of the process in terms of environmental sustainability. This technology has the capability to achieve a form of "cold pasteurization" and has proven effectiveness in the preservation of fruits and vegetable juices, smoothies, as well as paste-based products such as guacamole, and jams. However, the effect of the application of HPP technologies depends on (i) the type of plant treated, (ii) the tissue subjected to treatment, (iii) the treatment times, and (iv) the temperature used. To better understand the effect of HPP technology on vegetables, it is advisable to study the best treatment conditions for each treated vegetable to obtain the best results. The present Ph.D. thesis deals with the application of high-pressure processing (HPP) on fresh pumpkin cubes, and this work is carried out in two phases. Various approaches were explored to achieve the following goals: i. Understanding the impact of high-pressure treatment on fresh pumpkin cubes ii. Evaluation of the effect of different pressure-time combinations during high-pressure processing on fresh pumpkin cubes In this work, pumpkins (Cucurbita moschata Duchesne ex.Poir. cv. Violina) have been chosen as a “model food” for a comprehensive multidisciplinary study on processing innovation. Pumpkins have a long shelf life and are good sources to investigate the process. Pumpkin is commonly consumed either in boiled or baked form or processed into products including puree, soup, jams, and baked goods. Pumpkin flesh is abundant in bioactive substances, particularly total phenolic, total flavonoid, total carotenoid, and mineral contents, which have various beneficial activities on human health. To better understand the effect of HPP on pumpkin cubes, a research program was set up aimed at evaluating the effect of the intensity of pressure and the treatment time. During the three years of study, the research involved various steps, the results of which are represented by various scientific publications. At first, we did an in-depth literature review analysis in order to obtain a scientific state of the art. From the literature overview, we reviewed the structural modifications of pumpkins after the high-pressure processing (paper 1). The focus of the review was to describe all the structural and histochemical changes that the different authors had described as a result of their experiments. This comprehensive review of the literature enabled us to pinpoint the objective or goal of the thesis: to understand the impact of high-pressure processing and the effect of time and pressure combination on several qualitative and quantitative parameters of fresh pumpkin cubes that could be useful for process optimization by maximizing positive attributes and by minimizing negative ones. The first part of the doctoral activity involved the evaluation of the effect of high pressures on pumpkin cubes. In this case, the aim of the study was to understand the effect of different pressure intensities (from 100 MPa to 600 MPa) on the physico-chemical, microstructural and microbiological aspects of pumpkin cubes (paper 2). Pumpkin microstructure was studied in terms of change of cell arrangement, integrity of cells, cell elongation, cell perimetral segment, status of cell wall, and cell wall thickness. It was evident that the greatest microstructural changes in vegetable cells were found at higher pressure, greater than 300 MPa. The results also showed that high pressures play a key role in the cell-cell interaction process, in fact as the intensity of the treatment increases, a greater presence of calcium inclusions is observed. Calcium is responsible for binding between cells as it allows the interaction between pectin molecules of adjacent cells. An increase in calcium inclusions, after HPP treatment, indicates a breakdown of the pectin-calcium bond and, therefore, cell separation, and, consequently, loss of structure. The greatest loss of structure was observed on samples treated with pressures above 500 MPa. Also, the colorimetric and textural parameters decreased by increased pressure: a color and texture change in the treated samples was observed compared to untreated ones whereas the highest antioxidant availability was observed in high-pressurized samples. A longer shelf life was expected from HPP treatment at 400 to 600 MPa, indicating that much higher pressure is recommended to ensure microbial inactivation. The second section of the PhD research evaluated the effects of the HPP process at different pressures on the chemical composition of samples (Paper 3). In particular, we evaluated the modifications on health-promoting constituents of this vegetable, namely the polyphenols, carotenoids, volatiles and sugar. The results showed that the levels of bioactive compounds decreased especially at higher pressures whereas moderate pressures ranging from 200 to 400 MPa, increased and maintained the amount of availability of bioactive compounds. Based on our assessment, HPP at moderate pressure levels seems to be suitable for retaining stability and concentration of all bioactive components and sugar molecules. The third section of the research assessed the effect of selective pressure and time combinations on selected qualitative parameters of pumpkins (paper 4: submitted). High-pressure treatment negatively influenced the structure of pumpkins and showed slightly lower colorimetric values than untreated (UNTR) samples. A significant effect was observed for pressure and for interaction between pressure and time. Time alone didn’t result significantly in textural modifications. The combined effect of pressure and time demonstrated a significant effect on the alteration of pumpkin samples’ microstructure and cell morphology and increased antioxidant activity by releasing active compounds from damaged tissues. Based on our assessment, HPP at moderate and high pressure-time levels we obtained varied outcomes and it appeared that certain pressure-time (400 and 600 MPa/1,3 and 5 min) levels seemed to be suitable for retaining stability, and concentration of all bioactive, volatiles and sugar components than untreated samples. After carrying out this two-phase study we obtained two different results in each phase, these results could be useful for process optimization by enhancing positive traits and eliminating negative ones. In conclusion, the results obtained have allowed the understanding of the changes in plant tissues subjected to high-pressure treatments. It was also possible to evaluate the effect of the duration of the HPP treatment on the pumpkin cubes. The research therefore made it possible to understand the variations not only at the tissue level but also from the cellular and chemical composition point of view. Therefore, the results of this research could encourage and be useful to other researchers, specifically, this study offers the opportunity to strengthen knowledge related to this topic.