Enzyme infusion prior to thermal/high pressure processing of strawberries: Mechanistic insight into firmness evolution Ilse Fraeye, Griet Knockaert, Sandy Van Buggenhout, Thomas Duvetter, Marc Hendrickx, Ann Van Loey ⁎ Laboratory of Food Technology and Leuven Food Science and Nutrition Research Centre (LFoRCe), Department of Microbial and Molecular Systems (M 2 S), Katholieke Universiteit Leuven, Kasteelpark Arenberg 22, B-3001 Leuven, Belgium abstract article info Article history: Received 12 November 2008 Accepted 30 June 2009 Editor Proof Receive Date 27 July 2009 Keywords: Strawberries Thermal processing High pressure processing Pectin Pectinmethylesterase Infusion Microscopy Firmness Fractionation Strawberries were infused with fungal pectinmethylesterase (PME) and calcium chloride, followed by a thermal (70 °C–0.1 MPa), a high pressure (25 °C–550 MPa) or a combined thermal-high pressure (70 °C– 550 MPa) process. Macroscopic (firmness) and microscopic characteristics were assessed to evaluate the texture of the fruits. In order to interpret the texture changes, the chemical structure of pectin was investigated. Processing of strawberries caused a decrease in firmness, which was limited by infusion of PME and calcium chloride, although the extent of beneficial effects depended on the type of processing. PME was able to decrease the degree of methoxylation of pectin, which was accompanied by an increased crosslinking of the chains. During high pressure or combined thermal-high pressure processing, the degree of methoxylation of pectin in infused strawberries was even further decreased, probably due to a higher activity of the fungal PME under high pressure. In case of the high pressure process, this was reflected in a very firm texture. However, the combined thermal-high pressure process caused more severe tissue damage, in spite of the advantageous pectin properties. Industrial relevance: During high pressure processing of strawberries many nutritional and sensorial characteristics are quite well preserved. Unfortunately, texture of strawberries deteriorates during such processes. This paper provides mechanistic insight into how infusion of fungal pectinmethylesterase and calcium ions in strawberries can preserve the firmness of these fruits during high pressure processing. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Fresh strawberries have precious nutritional and sensorial char- acteristics, which makes them highly appreciated by consumers. Thermal processing of strawberries for preservation purposes causes a significant deterioration of flavor, color, texture and nutritional value. Hence, thermally processed strawberries in e.g. jams, yoghurts or pies do not reflect the quality aspects of fresh fruits. Therefore, food technologists focus on novel processing techniques which have a less detrimental effect on quality of fruits and vegetables. High hydrostatic pressure is an innovative food processing technique which allows inactivation of microorganisms and spoilage enzymes while quality attributes are quite well preserved (Ludikhuyze, Van Loey, Indrawati, & Hendrickx, 2001). Several studies on strawberries and strawberry based products indicate that color, flavor, aroma and vitamin C content were only minimally affected by high pressure treatments (Kimura, Ida, Yosida, Ohki, Fukumoto, & Sakui, 1994; Lambert, Demazeau, Largeteau, & Bouvier, 1999; Sancho, Lambert, Demazeau, Largeteau, Bouvier, & Narbonne, 1999; Zabetakis, Leclerc, & Kajda, 2000; Gimenez, Kajda, Margomenou, Piggott, & Zabetakis, 2001). However, like most processing techniques, high pressure processing of fruits can cause textural damage (Basak & Ramaswamy, 1998). Texture of fruit and vegetable based products relates to micro- structural quality and can be attributed mainly to the structural integrity of the cell wall and middle lamella, as well as to turgor pressure (Jackman & Stanley, 1995). The cell wall is mainly composed of cellulose microfibrils, embedded in a matrix of pectin, hemicellu- lose, proteins and phenolic compounds (Brett & Waldron, 1996). Changes in texture often relate to transformations in structure and composition of pectin because of its abundance, solubility and sensitivity to chemical reactions (Jackman & Stanley, 1995; Sila et al., 2008). Pectinmethylesterase (PME, EC3.1.1.11) is an enzyme which catalyses the hydrolysis of methoxyl esters on pectin, resulting in the release of methanol and pectin with free carboxyl groups. In presence Innovative Food Science and Emerging Technologies 11 (2010) 23–31 Abbreviations: AIR, alcohol insoluble residue; CSP, chelator soluble pectin; DM, degree of methoxylation; GalA, galacturonic acid; NSP, sodium carbonate soluble pectin; PG, polygalacturonase; PL, pectate lyase; PME, pectinmethylesterase; WSP, water soluble pectin; NI, non-infused; INF, infused with PME and calcium chloride; T, thermal processing (70 °C–0.1 MPa); HP, high pressure processing (25 °C–550 MPa); THP, combined thermal-high pressure processing (70 °C–550 MPa). ⁎ Corresponding author. Tel.: +32 16 32 15 67; fax: +32 16 32 19 60. E-mail address: Ann.vanloey@biw.kuleuven.be (A. Van Loey). 1466-8564/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2009.06.007 Contents lists available at ScienceDirect Innovative Food Science and Emerging Technologies journal homepage: www.elsevier.com/locate/ifset