Discolouring of grape juice concentrate : causes and possible ways of inhibition

Loedolff, Matthys Johannes (2004-12)

Thesis (MScEng) -- University of Stellenbosch, 2004.

Thesis

ENGLISH ABSTRACT: The grape juice concentrate (GJC) plant of the KWV at Robertson spent significant amounts of money on the decolourisation of grape juice concentrate. A chemically activated powdered activated carbon (PAC) purchased from Norit, namely CA1, was used as decolourisation product. Apart from the expenses involved, it contributed largely to the solid waste produced at this plant. A way was sought to minimise or prevent GJC discolourisation (and possibly solid waste) without increasing operating expenses. Browning reactions in GJC are as old as the product itself. Numerous researchers have studied the origins of these reactions, the reactants and products involved, as well as the reaction kinetics of these reactions. From the work of these researchers four possible browning reaction pathways were identified, namely: • enzymatic oxidative browning, • non-enzymatic oxidative browning, • non-enzymatic browning (the Maillard reaction), and • caramelisation. It was also identified that 5-hydroxymethylfurfural (HMF) are indicative of the browning potential of GJC. A method to analyse for HMF (quantitative and qualitative) was develop for the purposes of this study, namely positive electron-spray ionisation preceded by high-pressure liquid chromatography (HPLC) and followed by dual mass spectrometry. This method showed good repeatability and was used to analyse all samples generated during this study. It was confirmed that the manufacturing process at this plant favours nonenzymatic browning reactions, since mild heat treatment deactivates enzymes. Further investigation indicated that the overruling browning reaction on this plant was non-enzymatic oxidative browning. It was shown that neither the presence, nor the absence of protein had any effect on the rate of formation of HMF. It was, however, confirmed that HMF formation could be attributed to high temperatures and prolonged exposure to these temperatures. Other adsorption products were evaluated against the then current PAC (CA1), namely a steam activated PAC supplied by Norit, SA4, and a polymeric adsorbent, Polyclar V (polyvinylpolypyrrolidone/PVPP). Both SA4 and PVPP indicated superior HMF adsorption capacities. Replacing CA1 with SA4 could result in operating expenses savings and possible solid waste reduction. However, PVPP were too expensive to be considered an economically viable replacement for CA1. Improved concentration technologies such as reverse osmosis (RO) membrane concentration followed by centrifugal evaporation (CE) or twostage CE should be considered as possible replacement for the existing concentration technology (multi-stage falling film evaporator). This should decrease heat treatment/exposure by more than 90% and thus reduce browning significantly. An added advantage could be the reduction of solid waste, since less (if not no) decolourisation will be required. Alternatively, juice should be stored with added sulphur dioxide (SO2), since it was shown that this juice contained much lower HMF concentrations than diluted concentrate (stored for the same time). This should reduce heat exposure by up to 50% and thus minimise browning reactions.

AFRIKAANSE OPSOMMING: Die druiwesapkonsentraat (DSK) aanleg van die KWV in Robertson het jaarliks aansienlike bedrae geld spandeer tydens die ontkleuringsproses van DSK. ‘n Chemies geaktiveerde verpoeierde koolstof (GVK) verkrygbaar van Norit, naamlik CA1, is gebruik as ontkleuringsproduk. Buiten die kostes verbonde aan hierdie produk het dit ook grootliks bygedra tot soliede afval by hierdie aanleg. Oplossings is gesoek om die verbruining/ontkleuring van DSK (en dalk ook soliede afval) te verminder (of selfs te voorkom) sonder om bedryfskostes te verhoog. Verbruiningsreaksies in DSK bestaan al so lank soos DSK self. Verskeie navorsers het die oorsake, reaktante, produkte en reaksiekinetika van hierdie reaksies oor die jare heen bestudeer. Uit die werk van sommige van hierdie navorsers kon vier moontlike verbruiningsreaksieroetes geïdentifiseer word, naamlik: • ensiematiese oksidatiewe verbruining, • nie-ensiematiese oksidatiewe verbruining, • nie-ensiematiese verbruining (die Maillard-reaksie), en • karamelisering. Daar was verder geïdentifiseer dat 5-hidroksiemetielfurfuraal (HMF) aanduidend is van die verbruiningspotensiaal van DSK. ‘n Analitiese metode (kwalitatief en kwantitatief) om vir HMF te analiseer is vir die doel van hierdie studie ontwikkel, naamlik positiewe elektronsproei ionisasie, voorafgegaan deur hoëdruk vloeistof chromatografie en gevolg deur dubbele massa spektrometrie. Hierdie analitiese metode het goeie herhaalbaarheid getoon en was deurgaans gebruik om monsters te analiseer gedurende hierdie studie. Dit was bevestig dat die vervaardigingsproses by hierdie aanleg nieensiematiese verbruiningsreaksies begunstig, aangesien geredelike hittebehandeling ensieme deaktiveer. Verdere navorsing het getoon dat die oorheersende verbruiningsreaksies by hierdie aanleg nie-ensiematiese oksidatief van aard is. Resultate het getoon dat proteinstabiliteit geen invloed op die vormingstempo van HMF het nie. Dit was egter bevestig dat vorming van HMF direk verband hou met hoë temperature en lang blootstellingsperiodes aan hierdie temperature. Ander adsorpsieprodukte was vergelyk met die huidige GVK (CA1), naamlik ‘n stoom geaktiveerde verpoeierde koolstof (Norit se SA4) en ‘n polimeriese adsorbant, Polyclar V (polivinielpolipirrolidoon/PVPP). Beide SA4 en PVPP het CA1 oortref wat betref HMF adsorpsie. Moontlike bedryfskostebesparings (en soliede afval verminderings) potensiaal bestaan indien CA1 vervang word met SA4. Die teenoorgestelde is egter waar vir PVPP wat bedryfskoste aangaan. Instede van die huidige verdampinstegnologie, naamlik vallendefilmverdamping, hoort verbeterde konsentrasietegnologieë soos tru-osmose membraankonsentrasie gevolg deur sentrifugale verdamping, of, alternatiewelik, twee-stadium sentrifugale verdamping, orrweeg te word. Op hierdie wyse behoort hittebehandeling (en dus verbruining) met sowat 90% verminder te word. ‘n Moonlike addisionele voordeel is die vermindering van soliede afval aangesien minder ontkleuring nodig sal wees. Indien die verbeterde tegnologieë te duur is moet daar gekyk word daarna om die ongekonsentreerde sap met addisionele swaweldioksied (SO2) te stoor, aangesien veel laer HMF konsentrasies in sulke sap waargeneem is as in verdunde direkte konsentraat wat vir dieselfde typerk gestoor is. Hittebehandeling sal op hierdie wyse met tot 50% verminder word (en dus verbruining ook).

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