Open Access

In this study, the impact of a pulsed electric field (PEF) treatment on the final quality of freeze-dried apples was investigated. The PEF treatment has been performed at an electric field intensity equal to 1.07 kV/cm and a specific energy input of 0.5, 1 and 5 kJ/kg. The samples were freeze-dried (without a separate pre-freezing step) at varying temperatures (set on 40 °C and 60 °C) and pressures (0.1, 0.25 and 1 mbar). The quality of dried material was evaluated by the analysis of residual moisture content, macro- and microscopic properties, colour, the total content of phenolic compounds and the antioxidant activity as well as texture and acoustic properties. It was found that the residual moisture content of PEF treated samples was reduced by up to ∼82% in comparison to the intact tissue. For electroporated samples, a good preservation of macro-shape, an inhibition of shrinkage and the development of large pores were observed. The PEF treated material exhibited a higher total phenolic content, but a smaller antioxidant activity. Mechanical and acoustic analysis showed a higher crunchiness and brittleness for PEF-treated tissue, whereas untreated tissue was characterised by a harder and rather crackly texture.

Pulsed Electric Field (PEF) processing has been investigated in the past for gentle preservation and for the disintegration of biological tissue. The principle is based on polarization of the cell membrane by an external electric field, which results in electroporation. This increases the permeability of the membrane, which positively affects processes in which mass transfer is important. Extraction, such as sugar from sugar beet or juice from fruit, aims to separate ingredients from a matrix. By disintegrating the cells by PEF, a facilitated extraction of sugar or a higher juice yield can be achieved. Beneficial effects were also observed during water removal, as the drying time or the drying temperature can be reduced using PEF as a pre-treatment. Furthermore, a softening of PEF-treated tissue was observed, which can be highly beneficial for the vegetable industry such as in processing French fries or potato chips. The cutting force is reduced and smoother surface with less breakage can be achieved. The chapter gives an overview on the effects achieved in the PEF treatment of solid material.

The effect of the addition of Spirulina platensis flour and of extrusion parameters on texture, cooking yield, expressible moisture, total phenolic content (TPC), total flavonoid content (TFC), Trolox equivalent antioxidant activity (TEAC), in vitro protein digestibility (IVPD) and conformational changes of proteins using Fourier-transform infrared spectroscopy (FTIR) of lupin protein based meat analogues was studied. High moisture extrusion (HME) cooking was used to produce the meat analogues. The Spirulina concentration (15, 30 and 50%), extruder barrel temperature (145 °C, 160 °C and 170 °C), water feed (50, 55 and 60%), and screw speed (500, 800 and 1200 rpm) were varied. The Spirulina concentration and extrusion parameters significantly affected physical properties, such as texture, cooking yield and expressible moisture of the extrudates. The addition of Spirulina generally increased the TPC, TFC and TEAC values of the extrudates. Increased temperature and screw speed as well as decreased water feed slightly improved the content of TPC, TFC and TEAC, respectively. The addition of Spirulina at a level of 30% decreased the IVPD of the extrudates from 82 to 75.6%. However, increased water feed and screw speed partly counterbalanced this effect. Protein conformational analyses of the extrudates by FTIR showed that β-sheets were decreased, whereas α-helix, β-turn and antiparallel β-sheets were increased compared to the raw extrusion mixtures. On the whole, the HME process improved the values of TPC, TFC, TEAC and IVPD in the extrudates compared to the raw extrusion mixtures. The addition of Spirulina along with controlled extrusion parameters can deliver meat analogues with improved physico-chemical and nutritional properties.

The study investigated the potential of pulsed electric fields (PEF) technology application for the improvement of cell disintegration and subsequent extraction/fractionation of cyanobacteria Arthrospira maxima into valuable components (phycocyanin, proteins, oils and carbohydrates). Pulsed electric fields were applied to fresh A. maxima suspension, dried powder and dried sticks in combination with water as extraction solvent and freeze-thawing. Pulsed electric fields application for cell disruption reached approximately 90% increase of C-phycocyanin extraction compared to bead milling. Obtained fractions of phycocyanin and bulk proteins were also of higher purity and had twice lower environmental impact than similar fractions obtained without Pulsed electric fields treatment. Extracts with improved purity can be directly applied in pharma and food industry without any further processing and purification steps.