Processing to control (bio)chemical reactions influencing volatile profiles in Brassica and Allium vegetables – an integrated food chain approach

In the context of food, there is an expansion in the (individual) demands from consumers, making the production challenging. The availability and price, as well as the quality of the product are important parameters that can impact the consumption. The quality of food, including vegetables, is determined by both its health and sensorial quality. The latter comprises parameters such as color, (micro)structure and flavor (i.e., aroma (odor) (i.e., volatile characteristics) and taste characteristics). Food can often not be eaten raw and has moreover a limited shelf-life. Hence, in order to create palatability and/or a safe product, food very often needs to be processed. In that context, quality features are prone to be (largely) influenced upon processing the food (product), which can act on (bio)chemical conversions and their volatile profiles accordingly. Indeed, volatile compounds in foods are witnesses of (bio)chemical reactions, covering both enzymatic and non-enzymatic reactions. The extent to which a particular food processing step determines the preference/acceptance of the product is dependent on several considerations. Due to the large number of processing conditions that can affect the quality (differently), and the large pool of (product) parameters that can be affected upon processing, it is clear that there are many factors that need to be taken into account and considered carefully in developing a (vegetable/food) product, while keeping in mind the consumers preferences. This makes the production challenging and research of paramount interest. In addition, there is a need to design, implement, align and optimize (new) processing technologies and/or conditions that can reduce the negative impact of processing on quality, while maintaining the required properties. Hence, in this doctoral research, the impact of various well-considered and industrially relevant (sequences of) processing steps on the volatile profiles of vegetables was investigated. Specific conditions were selected based on industrial relevance. However, most of the steps were implemented on a lab-scale level to allow standardization and accurate control of process parameters. These results can be considered as a scientific knowledge base to build on to apply on and investigate the further scaling up to a large-scale level. Vegetables from two different plant families, containing distinctive substrate-enzyme systems, were considered, namely Brussels sprouts (Brassica oleracea var. gemmifera) and in a particular case broccoli (Brassica oleracea convar. botrytis var. italica) and leek (Allium ampeloprasum var. porrum), which are for Flanders commercially important horticultural crops. Volatile profiles arisen throughout the processing steps were measured using instrumental analyses, more specifically, via headspace-solid phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) and data were analyzed using multivariate data analyses (MVDA). To ameliorate and underpin the interpretation of the volatile profiles attained, enzyme activities were determined of samples of relevance. Pretreatments were selected in order to facilitate biochemical conversions or to minimize them, realized by a tissue disruptive step, or a heat treatment, respectively. A particular focus was on a pulsed electric field (PEF) pretreatment at low field strength, expected to impact the tissue (i.e., membrane) integrity. Also, preservation, preparation and reheating steps were implemented, during which the implementation of heat was necessary. It was observed that pretreatments and other heat treatments such as pasteurization, steaming, reheating via convection and microwave reheating (largely) affected the volatile profiles of Brussels sprouts and leek. Mixing and heating could majorly result in tissue disintegration leading to biochemical reactivities and thermally-induced reactivities, respectively, causing changes in the volatile profiles. Surprisingly, notwithstanding the expectation, for Brussels sprouts, pulsed electric field (PEF) treatment at 1.01 kV/cm and 2.7 kJ/kg prior to heating was assumed to not have caused significant membrane permeabilization since similar volatiles were observed in the case of only heating, as opposed to mixing, which was different for leek. Additional experiments on Brussels sprouts showed that applying more intense PEF conditions did affect the volatile profile substantially. Due to the promising application of PEF at low electrical field strength in the current context, a more detailed PEF set-up was applied to broccoli stalks. The results showed that volatile profiles of this matrix could also be steered by applying different PEF conditions resulting from different levels of cell membrane integrities obtained. The finding that (pre)treatment is a determinative step through the food processing chain was verified multiple times throughout the experimental approach, and is of great interest to keep in mind in selecting processing conditions. In contrast, frozen and cooled storage and subjecting a vegetable system to a holding time over a 3 h period at 80 °C, were seen to impact the volatile profiles to a lesser extent. This shows the potential to create a desired volatile profile by pretreatment which can be retained (to a particular extent) during processing and storage, for which preservation conditions and a storage period are initially designated. Additionally, an important conclusion in this context is that the kind of pretreatment greatly influences the effect upon the subsequent applied processing steps such as preservation, storage, preparation and reheating. Since preparation and reheating steps just before consumption are often neglected in research that have already been conducted, it was deemed relevant to also consider these steps. A single-heat-preparation and a double-heating-preparation line (i.e., preparation and reheating) were implemented as these are regularly applied in industrial kitchens. The results showed that steaming had a particular effect on the volatile profiles of (intact) industrially blanched Brussels sprouts, unblanched leek dices, Brussels sprouts puree of industrially blanched Brussels sprouts and leek puree made from unblanched leek, being more pronounced for the purees (and for the leek systems in comparison to Brussels sprouts systems). The impact of reheating did not noticeably influence the volatile profiles of the Brussels sprouts systems. The abovementioned minor effects observed for blanched Brussels sprouts systems were not in line with the results for unblanched leek. In this PhD, the potential in designing processing steps/conditions to attain a particular volatile profile was shown (reverse engineering approach). Distinctive plants were considered from which it could be elucidated that particular steps were seen to largely influence one matrix more compared to the other. Due to the latter, it can be stated that the intracellular properties (e.g., enzyme-substrate systems, different microstructural properties) of the vegetable are important aspects to consider in designing processing conditions to attain a specific volatile profile (for example in the context of acceptance/preference). In other words, inter-plant family variability in the effect upon processing is existing. In addition, it can be hypothesized that the effect upon processing on the volatile profiles of one of the studied vegetables will be similar for other vegetables from the same plant family, as intracellular properties, greatly determining the volatile profiles/effect upon processing, show great similarities in vegetables from one specific plant family. This can be relevant to discover in future research.

Publication year:2024

Accessibility:Embargoed