New food processing technologies based on high hydrostatic pressure (HP) in combination with high temperature (HT) and pulsed electric field (PEF) treatment of raw materials and food produced thereby have gained increasing attention during recent years. This has also resulted in an integrated EU-supported project “Novel Q” (www.novelq.org), comprising the analysis of potential chemical reactions in the food materials treated with HP/HT and/or PEF techniques.
Background
Processing of fruits and vegetables used for human consumption comprises different technologies used for various purposes. Several technologies are based on heat transfer operations such as microwave and thermal processing for sterilization or pasteurization to reduce problems caused by microorganisms, or post-harvest autolyses such as non-enzymatic and enzyme-catalyzed reactions. The major emphasis of food processing is thus preservation or shelf-life extension. Included herein is the prevention of undesirable changes in the concentration of bioactive components affecting the wholesomeness of the food, its nutritive value, its health-promoting properties and its sensory qualities.
It is, however, difficult to avoid the unwanted chemical reactions affecting food quality parameters when only HT processing is used. Therefore, recently, increasing focus has been devoted to new processing technologies, such as HP, HP/HT and PEF treatment of raw materials and food.[1–5] Introduction of new processing technologies also requires analytical procedures for the determination of which biomolecules are affecting food quality. Such methods or procedures are considered to be important tools for following the chemical reactions and changes in native plant components as a function of processing conditions, and for final documentation of product quality,[6] particularly the identification of biomarkers for specific chemical reactions.
The present work focused on increased knowledge of chemical reactions resulting from the HP/HT and PEF processing of fruits and vegetables. Plant materials of four different genera and families were selected for the studies: apple, carrot, tomato and broccoli.
The overall chemistry of the plant material, aside from its origin and chemo-taxonomical composition,[7] is also defined by the plant part and influenced by several factors, such as growing conditions, ripeness, harvest conditions and post-harvest treatments.[8] Identification of biomarkers requires specific procedures for precise quantification of their concentration and possible fate during chemical reactions.[9–12]
The aim is to analyze compounds with structures that allow chemical reactions to occur as a function of processing conditions. Two approaches were used. Studies to identify specific biomolecules and their transformation products were performed. Since it is likely that not all chemical reactions are measurable by such an approach and some reactions may simultaneously affect the concentrations of several compounds (thereby masking the effects on a single biomarker),[10] analytical fingerprinting was also performed to identify unspecific changes in the concentration of low molecular weight (LMW) compounds present in the tested materials.[13,14] The goal was to identify patterns that may be used to indicate both known and unknown chemical reactions as well as synergistic effects on the LMW compounds that result from both enzymatic and non-enzymatic catalysis.
Results and Discussion Sampling and Pretreatment of Samples
Among the key problems related to evaluation of chemical reactions resulting from HP/HT and PEF treatment of fruits and vegetables (solids and fluids) are the use of proper sampling techniques. This is caused by variations in individual types and concentrations of native LMW compounds and enzyme activities that often can be appreciable, even among fruit and vegetables of the same plant variety and among different parts of the plants.[6,7]
Processing Conditions
Mild PEF treatments (4 kV/cm) increase cell permeability up to 50% in apples, leading to a decrease in total polyphenolics due to release of external liquid.[15] At 2 kV/cm, the higher permeability caused a three-fold increase in extraction of flavonols from apple skin.[16] Food processing is performed with high-PEF treatment (20–80 kV/cm and short pulses; 1–10 µs) to inactivate microorganisms.[17] The right combination of high electric-field strength and total treatment time will ensure enzymatic inactivation without any over-processing.[18,19] For this investigation, several treatment conditions were applied to carrots and tomatoes. Selected combinations of voltage (0–20 kV) and temperature (20–40 °C) were applied.