Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Self-assembly in a near-frictionless granular material: Conformational structures and transitions in uniaxial cyclic compression of hydrogel spheres
    Walker, D.M. ; Tordesillas, A. ; Brodu, N. ; Dijksman, J.A. ; Behringer, R.P. ; Froyland, G. - \ 2015
    Soft Matter 11 (2015). - ISSN 1744-683X - p. 2157 - 2173.
    force chains - packing - systems - shear - continuum - networks - matter
    We use a Markov transition matrix-based analysis to explore the structures and structural transitions in a three-dimensional assembly of hydrogel spheres under cyclic uniaxial compression. We apply these methods on experimental data obtained from a packing of nearly frictionless hydrogel balls. This allows an exploration of the emergence and evolution of mesoscale internal structures — a key micromechanical property that governs self-assembly and self-organization in dense granular media. To probe the mesoscopic force network structure, we consider two structural state spaces: (i) a particle and its contacting neighbours, and (ii) a particle's local minimal cycle topology summarized by a cycle vector. In both spaces, our analysis of the transition dynamics reveals which structures and which sets of structures are most prevalent and most likely to transform into each other during the compression/decompression of the material. In compressed states, structures rich in 3-cycle or triangle topologies form in abundance. In contrast, in uncompressed states, transitions comprising poorly connected structures are dominant. An almost-invariant transition set within the cycle vector space is discovered that identifies an intermediate set of structures crucial to the material's transition from weakly jammed to strongly jammed, and vice versa. Preferred transition pathways are also highlighted and discussed with respect to thermo-micro-mechanical constitutive formulations.
    Substitutional impurity-induced vitrification in microgel crystals
    Higler, R. ; Appel, J. ; Sprakel, J.H.B. - \ 2013
    Soft Matter 9 (2013). - ISSN 1744-683X - p. 5372 - 5379.
    medium-range order - metallic glasses - liquids - spheres - scattering - colloids - packing
    We study the effect of larger substitutional impurities on the structure of soft microgel crystals. At the size ratio we employ, rsmall/rlarge = 0.67, we observe the unexpected co-crystallisation of the large impurities with the base crystal, at low fractions of impurity particles. A single impurity takes the place of 4 tetrahedrally coordinated small particles within the lattice. However, as this is accompanied by local deformations of the particles, this distortion-minimizing structure transforms into a random surrounding of the impurity particle at higher fractions of large substitutional impurities. The distortions in the lattice become longer ranged through this transformation, and ultimately result in vitrification of the sample
    Verpakkingen voor vers vlees : verwachte ontwikkelingen tussen 2011-2030 en het energiebesparingspotentieel
    Thoden van Velzen, E.U. - \ 2011
    Wageningen : Wageningen UR - Food & Biobased Research (Rapport / Wageningen UR Food & Biobased Research 1287) - ISBN 9789461730824 - 36
    vlees - intelligente verpakkingen - verpakken - houdbaarheid (kwaliteit) - duurzaamheid (durability) - energiebesparing - voedselverspilling - meat - smart packaging - packing - keeping quality - durability - energy saving - food wastage
    In dit rapport worden achtereenvolgens de volgende onderwerpen behandeld: 1) De huidige stand van zaken ten aanzien van het gebruik van verpakkingstechnologieën voor vers vlees, vleeswaar, verse maaltijden, gemarineerd vlees, bewerkt vlees en snacks in Nederland en Europa. 2) Verbeterperspectieven voor 2015, 2020 en 2030 voor bestaande en nieuwe verpakkingstechnologieën, inclusief de kritieke punten, mogelijke barrières en verwachte toepasbaarheid. 3) De energiebesparingsmogelijkheden die innovatie in de productieketens van de vleesverwerkende industrie bieden.
    Maak draagtassen van papier, stof of bioplastic
    Helmink, I. ; Bolck, C.H. - \ 2010
    Supermarkt Actueel 20 (2010)April. - ISSN 1386-3010 - p. 26 - 27.
    supermarkten - houding van consumenten - duurzaamheid (sustainability) - verpakken - kunststoffen - milieubeheer - bioplastics - biobased economy - supermarkets - consumer attitudes - sustainability - packing - plastics - environmental management - bioplastics - biobased economy
    Jaarlijks worden er in Nederland miljarden plastic tasjes gebruikt. Veelal worden deze gratis in winkels weggegeven en na eenmalig gebruik weggegooid. Supermarkten verkopen per jaar naar schatting 280 tot 560 miljoen plastic tassen. De bewuste klant neemt zijn eigen tas mee, voor de iets minder bewuste klant zijn opties genoeg.
    Haalbaarheid bulkbewaring leliebollen boven nul in MAP
    Gude, H. ; Dijkema, M.H.G.E. - \ 2009
    Lisse : PPO Bloembollen en Bomen - 23
    verpakken - bloembollen - opslag - kwaliteit - packing - ornamental bulbs - storage - quality
    Lange bewaring van leliebollen bij temperaturen boven nul in transitbags volgens het Modified Atmosphere-principe, gecombineerd met een droog vulmiddel, leidt tot een slechtere kwaliteit bollen dan de standaard-bewaring bij temperaturen onder nul in plastic zakken met gaten en een vochtig vulmiddel. In Modified Atmosphere-verpakkingen (MAP, Modified Atmosphere Packaging) kunnen bollen en planten in principe langer bewaard worden. MAP-folies zijn beperkt doorlaatbaar voor zuurstof en ondoorlaatbaar voor water. De combinatie van beperkte doorlaatbaarheid voor zuurstof en de ademhaling van de verpakte producten veroorzaakt een verlaagd zuurstofniveau in de verpakking. Hierdoor wordt de ademhaling en daarmee de ontwikkeling van verpakte producten geremd. Door de ondoordringbaarheid voor water kunnen producten in MAP-folies (de types zonder lasergaatjes) niet uitdrogen. Dat laatste maakt het gebruik van een veel droger vulmiddel mogelijk.
    'Modified Atmosphere' - verpakking en vulmiddel voor bloembollen en vaste planten
    Gude, H. ; Dijkema, M.H.G.E. ; Dalfsen, P. van; Leeuwen, P.J. van - \ 2007
    Lisse : PPO Bloembollen en Bomen - 85
    verpakken - gemodificeerde atmosfeer opslag - bloembollen - overblijvende planten - packing - modified atmosphere storage - ornamental bulbs - perennials
    Sommige soorten vaste planten en bollen hebben tijdens de bewaring bij de handels-of exportbedrijven of bij de retailer sterk de neiging tot uitlopen en/of uitdroging. Het uitlopen en uitdrogen kan voorkomen of geremd worden door ze te verpakken in een MAP-folie. Deze folie heeft een beperkte doorlaatbaarheid voor zuurstof waardoor, ten gevolge van de ademhaling van het product, het zuurstofgehalte in de verpakking daalt en de groei, en dus uitloop, geremd worden. Het zuurstofgehalte dat ontstaat in de verpakking wordt bepaald door de doorlaatbaarheid van de verpakking én door de ademhalingsactiviteit (afhankelijk van de temperatuur) van het product. Doordat een MAP-folie volledige ondoorlaatbaar is voor water wordt uitdroging voorkomen. Om condensvorming en dus ziekte te voorkomen, moet in combinatie met een MAP-folie een vulmiddel toegepast worden dat het overtollige vocht in de verpakking in voldoende mate kan opnemen en afgeven. Het doel van het onderzoek was om voor verschillende soorten bloembollen en vaste planten die gevoelig zijn voor uitdroging en/of uitlopen, een consumentenverpakking (t.b.v. de retail) en/of ‘bulk’-verpakking (t.b.v. handels- en exportbedrijven) van MAP-folie te ontwikkelen en tevens een vulmiddel te vinden, waarbij het product geen (of een aanvaardbare) spruitgroei vertoont en vochtproblemen (uitdroging of condensvorming) voorkómen worden. Het onderzoek bestond uit 3 onderdelen: I. De bepaling van de ademhalingsactiviteit tijdens een ketensimulatie. Dit was nodig om een MAP-folie met de juiste zuurstofdoorlaatbaarheid te kunnen ontwikkelen. II. M.b.v. de verkregen ademhalingsgegevens zijn door een foliefabrikant MAP-folies ontwikkeld voor de consumentenverpakking (t.b.v. retailer) en de ‘bulk’-verpakking (bewaring bij handels-of exportbedrijf). Een aantal soorten planten/bollen is verpakt in MAP-verpakkingen met verschillende type vulmiddelen en verschillende vochtgehaltes. De planten/bollen zijn gedurende een aantal weken/maanden op het laboratorium bewaard onder ketenomstandigheden. De kwaliteit van planten/bollen uit de MAP- verpakkingen is vergeleken met die van planten/bollen uit praktijkverpakkingen. III. Planten en/of bollen zijn bij bedrijven verpakt in consumenten- of in ‘bulk’-verpakkingen van MAP-folie in combinatie met de vulmiddelen die uit het onderzoek als meest perspectiefvol naar voren kwamen. Na (langdurige) bewaring is de kwaliteit van planten/bollen uit de MAP-verpakkingen vergeleken met die van planten/bollen uit praktijkverpakkingen.
    Self-association of transmembrane alpha-helices in model membranes: Importance of helix orientation and role of hydrophobic mismatch
    Sparr, E. ; Ash, W.L. ; Nazarov, P.V. ; Rijkers, D.T.S. ; Hemminga, M.A. ; Tieleman, D.P. ; Kilian, J.A. - \ 2005
    Journal of Biological Chemistry 280 (2005)47. - ISSN 0021-9258 - p. 39324 - 39331.
    lipid-bilayers - molecular-organization - proteins - peptides - residues - packing - environment - simulation - stability - mechanism
    Interactions between transmembrane helices play a key role in almost all cellular processes involving membrane proteins. We have investigated helix-helix interactions in lipid bilayers with synthetic tryptophan-flanked peptides that mimic the membrane spanning parts of membrane proteins. The peptides were functionalized with pyrene to allow the self-association of the helices to be monitored by pyrene fluorescence and Trp-pyrene fluorescence resonance energy transfer (FRET). Specific labeling of peptides at either their N or C terminus has shown that helix-helix association occurs almost exclusively between antiparallel helices. Furthermore, computer modeling suggested that antiparallel association arises primarily from the electrostatic interactions between ¿-helix backbone atoms. We propose that such interactions may provide a force for the preferentially antiparallel association of helices in polytopic membrane proteins. Helix-helix association was also found to depend on the lipid environment. In bilayers of dioleoylphosphatidylcholine, in which the hydrophobic length of the peptides approximately matched the bilayer thickness, association between the helices was found to require peptide/lipid ratios exceeding 1/25. Self-association of the helices was promoted by either increasing or decreasing the bilayer thickness, and by adding cholesterol. These results indicate that helix-helix association in membrane proteins can be promoted by unfavorable protein-lipid interactions.
    Nieuwe verpakking houdt vaste plant vitaal
    Gude, H. ; Dalfsen, P. van - \ 2005
    De Boomkwekerij 18 (2005)35. - ISSN 0923-2443 - p. 14 - 15.
    tuinbouw - overblijvende planten - potplanten - sierplanten - bloembollen - tuinplanten - verpakkingen - verpakkingsmaterialen - verpakken - folie - behandeling na de oogst - systemen na de oogst - achteruitgang (deterioration) - kwaliteit - marketingkanalen - kwaliteitszorg - horticulture - perennials - pot plants - ornamental plants - ornamental bulbs - bedding plants - wrappings - packaging materials - packing - foil - postharvest treatment - postharvest systems - deterioration - quality - marketing channels - quality management
    Modified Atmosphere Packaging (MAP) is al bekend van de zakjes groente in de supermarkt. Uit onderzoek blijkt dat ook vaste planten gebaat zijn bij dit type verpakking. PPO Bollen en Bomen heeft de afgelopen twee jaar gewerkt aan het ontwikkelen van MAP-verpakkingen voor vaste planten en bloembollen, opdat de planten nog in goede staat zijn als zij bij de consument arriveren
    Positieve resultaten met verpakken van Conference: bewaring
    Geijn, F. van de - \ 2005
    De Fruitteelt 95 (2005)30/31. - ISSN 0016-2302 - p. 12 - 13.
    peren - kratten - voedselverpakking - houdbaarheid (kwaliteit) - fruit - verpakken - pears - crates - food packaging - keeping quality - packing
    Effect van 3 verschillende verpakkingsvormen tijdens de bewaring: 1. bodemvel (op kistbodem) plus gesloten afdekfolie 2. bodemvel (op kistbodem) plus randfolie en gesloten afdekfolie 3. geperforeerde zak in de kist plus afdekfolie (TFM-concept)
    Alternatieven voor tuinturf bij droogverkoop van bollen en vasteplanten
    Groen, N.P.A. ; Gude, H. - \ 2004
    Lisse : PPO Bloembollen - 29
    bloembollen - verpakken - uitdrogingstolerantie - turf - handel - landbouwkundig onderzoek - nederland - ornamental bulbs - packing - desiccation tolerance - peat - trade - agricultural research - netherlands
    Een aantal bloembollen en vaste planten drogen zonder goede beschering tijdens de verkoopfase te veel uit waardoor de groei kan stagneren. Ook beschadiging moet worden voorkomen, evenals condensvorming. Op dit moment wordt voornamelijk tuinturf of houtmot in een plastic verpakking gebruikt. De beschikbaarheid van goede tuinturf wordt steeds minder. Een uitgebreide literatuurstudie is uitgevoerd naar mogelijke vervangers van tuinturf of houtmot als verpakkinsmateriaal bij de droogverkoop van bloembollen. Het doel van dit literatuuroverzicht is handvatten aan te reiken ter vervanging van tuinturf of houtmot en ter verbetering van de kwaliteit van bollen of vaste planten in de verkoopfase.
    Vaste planten: slechte hergroei van gespoelde landplanten; doe er wat aan!
    Dalfsen, P. van; Gude, H. - \ 2004
    De Boomkwekerij 17 (2004)47. - ISSN 0923-2443 - p. 16 - 17.
    overblijvende planten - gewasbescherming - schoonmaken - wassen (activiteit) - verpakken - transport - hergroei - onderzoek - perennials - plant protection - cleaning - washing - packing - regrowth - research
    Voor de export naar de VS worden vaste planten gespoeld om het zand tussen de wortels te verwijderen. Eenmaal op de plek van bestemming, blijken verschillende soorten niet meer in staat de groei te hervatten. PPO deed onderzoek naar de oorzake
    Nieuwe spelregels voor verpakkingen : ATO voorspelt kleine revolutie
    Reuvers, M. - \ 2003
    Pakblad 25 (2003)3. - ISSN 1389-7381 - p. 6 - 6.
    voedselbewaring - innovaties - verpakkingen - verpakken - voedingsmiddelen - food preservation - innovations - wrappings - packing - foods
    ATO verwacht binnen een jaar veelbelovende doorbraken op het gebied van voedselconservering. Deze innovatieve technologieën stellen echter andere eisen aan het verpakken dan de conventionele conserveringsprocessen
    Automatische aardappelsorteerder : optimaliseren van de opbrengst
    Noordam, J.C. ; Pauli, A. ; Medema, W.J. - \ 2001
    Landbouwmechanisatie 52 (2001)10. - ISSN 0023-7795 - p. 14 - 15.
    aardappelen - sorteermachines - sorteerders - computertechnieken - beeldverwerking - verpakken - rendement - sorteren - kwaliteit - potatoes - sorters - graders - computer techniques - image processing - packing - returns - sorting - quality
    Pro-Pak International en ATO ontwikkelden een automatische sorteerder voor gewassen aardappels die werkt via beeldverwerking. Een inpakbedrijf kan hiermee een partij aardappelen in diverse uniforme assortimenten sorteren waar meer geld voor gevraagd kan worden
    Tafelaardappel krijgt meer aandacht door nieuwe verpakking
    Boerrigter, H. - \ 2000
    Aardappelwereld 54 (2000)9. - ISSN 0169-653X - p. 25 - 27.
    aardappelen - solanum tuberosum - verpakken - verpakkingsmaterialen - grote zakken - verkoopbevordering - houdbaarheid (kwaliteit) - opslag met klimaatbeheersing - potatoes - packing - packaging materials - sacks - sales promotion - keeping quality - controlled atmosphere stores
    Nieuwe kwaliteitsbeschermende verpakking voor tafelaardappelen in Nederland
    Zilverschurft bij tafelaardappelen: een keteninventarisatie
    Hoof, M. van - \ 2000
    Aardappelwereld 54 (2000)2. - ISSN 0169-653X - p. 25 - 27.
    solanum tuberosum - aardappelen - plantenziekteverwekkende schimmels - helminthosporium solani - plantenplagen - plantenziekten - epidemiologie - distributie - verliezen na de oogst - opslagverliezen - verpakken - verpakkingsmaterialen - grote zakken - potatoes - plant pathogenic fungi - plant pests - plant diseases - epidemiology - distribution - postharvest losses - storage losses - packing - packaging materials - sacks
    Onderzoek naar de uitgroei van en besmetting met zilverschurft van consumptieaardappelen. Gegevens in bijgaande figuren: 1) het verloop van zilverschurft in de keten; 2) resultaat van een verpakkingsexperiment
    Verpak witlof in een ander jasje
    Broek, R.C.F.M. van den; Putter, H. de - \ 1997
    PAV-bulletin. Vollegrondsgroenteteelt / Praktijkonderzoek voor de Akkerbouw en de Vollegrondsgroenteteelt 1997 (1997)4. - ISSN 1385-5298 - p. 6 - 8.
    cichorium intybus - cichorei - verpakken - rassen (planten) - cultivars - rassen (taxonomisch) - houdbaarheid (kwaliteit) - cichorium intybus - chicory - packing - varieties - cultivars - races - keeping quality
    Perspectieven vleesveredelingsbedrijven
    Vlieger, J.J. de - \ 1997
    Den Haag : LEI-DLO (Mededeling / Landbouw-Economisch Instituut (LEI-DLO), Afdeling Structuuronderzoek 579) - ISBN 9789052423807 - 66
    landbouw - rentabiliteit - macro-economie - vlees - behandeling - verwerking - behoud - slachthuizen - verpakken - nederland - agriculture - profitability - macroeconomics - meat - treatment - processing - preservation - abattoirs - packing - netherlands
    Beschrijving van de functies, de werkgelegenheid, het aan- en verkooppatroon en van toekomstige veranderingen daarin voor vleesveredelingsbedrijven, hun concurrenten (slachterijen) en hun klanten (slagers, grootwinkelbedrijven, horeca-groothandel, cateringbedrijven en vleesvoorverpakkers. De onderzoeksvraag was na te gaan welke invloed de sanering van de runder- en varkensslachterijen had op de marktpositie van vleesveredelings- en vleeshandelsbedrijven. Daarbij diende tevens rekening te worden gehouden met de omschakeling op IKB-vlees en de introductie van samenwerking in ketens.
    The development of a corrugated board container for seed potatoes
    Sman, R.G.M. van der - \ 1997
    Aardappelwereld (1997)8. - ISSN 0169-653X - p. 30 - 33.
    Solanum tuberosum - aardappelen - verpakken - ruwe grondstoffen - vezels - karton - bordpapier - pulp- en papierwarenindustrie - potatoes - packing - raw materials - fibres - paperboard - cardboard - pulp and paper industry
    Biopreservation in modified atmosphere packaged vegetables
    Bennik, M.H.J. - \ 1997
    Agricultural University. Promotor(en): F.M. Rombouts; L.G.M. Gorris. - S.l. : Bennik - ISBN 9789054858089 - 96
    uitrusting - verpakking - verpakkingsmaterialen - verpakken - planten - dieren - behoud - machines - equipment - packaging - packaging materials - packing - plants - animals - preservation - machines

    Recent trends in food preservation are the use of mild preservation techniques, such as modified atmosphere (MA) packaging and refrigeration, to prolong the shelflife of foods without affecting the fresh character of the product. This has resulted in the development of a new generation of chill stored, minimally processed foods, such as vegetables that are packaged under reduced oxygen (0 2 ) and elevated carbon dioxide (C0 2 ) concentrations. These gas atmospheres in combination with refrigerated storage conditions reduce the respiration rates of these products, and can limit the growth of endogenous spoilage bacteria, thereby rendering prolonged shelflives as compared with storage under ambient conditions. Although MA packaged vegetables have a fair safety record, they may incidently harbor foodborne pathogens. Especially the outgrowth of psychrotrophic (cold tolerant) pathogens can be considered a safety concern, but quantitative data on their growth under MA conditions that are suitable for produce are essentially lacking. Detailed information on the outgrowth of both pathogens and vegetable-associated spoilage bacteria under these conditions may give better insight into possible antagonistic action of competing microflora on the growth of pathogens, and the development of adequate countermeasures toward their growth to hazardous levels.

    One of the objectives of the studies that are described in this thesis was to obtain information on the impact of refrigerated MA storage conditions on the growth of microorganisms on minimally processed, MA packaged produce. Since our initial studies substantiated the possible hazard which can be posed by psychrotrophic pathogens, the use of biopreservation for adequate control of these microorganisms was investigated. In this respect, we have focused on lactic acid bacteria (LAB) because they occur naturally on fresh and minimally processed vegetables, and are able to produce a variety of antimicrobial substances, amongst which bacteriocins.

    Chapter 1 gives a short introduction on the technology of MA packaging of minimally processed vegetables and their current microbiological safety and quality status. Furthermore, the possible role of LAB as biopreservation agents in foods is addressed with special emphasis on those strains that are able to produce bacteriocins. The characteristics and mode of action of different bacteriocins that are produced by LAB are described, as well as aspects which are relevant to the application of these antimicrobial compounds or their producing organisms as food preservatives.

    Detailed information on the impact of refrigerated MA storage on the population dynamics of the endogenous microflora on vegetables is limited. Therefore, the prevailing epiphytic microorganisms on model produce ( i.e . chicory endive, and mungbean sprouts) were identified before and after MA storage. Subsequently, the growth characteristics of the predominant spoilage bacteria were determined under MA conditions ( Chapter 2 ). It was found that the major strains on both products were Enterobacteriaceae and Pseudomonas species. There were changes in the prevalent species on chicory endive before and after MA storage, but this was not observed for mungbean sprouts. To quantify the influence of C0 2 and 0 2 on the growth of the individual spoilage bacteria, a model agar system was developed that mimicked the in situ growth of bacteria on the surface of vegetables. The analysis of the growth of the predominant strains under controlled 0 2 and C0 2 concentrations at 8°C indicated that, in general, lag times were not present under the various conditions tested, and that the maximum population sizes were not affected. However, the maximum specific growth rates generally decreased with increasing C0 2 concentrations, which was independent of the 0 2 concentrations applied. This effect was more pronounced for Pseudomonas species than for Enterobacteriaceae. Representatives of the former subpopulation already showed significantly reduced maximum specific growth rates at C0 2 concentrations that are suitable for MA packaged vegetables.

    In Chapter 3 , the effects of different 0 2 and C0 2 concentrations on the growth of Aeromonas hydrophila, Yersinia enterocolitica, L. monocytogenes, and a cold tolerant strain of Bacillus cereus were quantified in the above mentioned model agar system at 8°C. Again, the maximum specific growth rates of the various microorganisms decreased significantly with increasing C0 2 concentration, while the maximum population densities were not affected and lag times were not observed. Extrapolation of the results from the model system to MA packaged vegetables suggested that growth of the above pathogens may occur at 8°C to the same maximum population densities as compared to ambient conditions, but at 10 to 20% lower maximum specific growth rates. Because we have shown that MA packaging can only moderately retard the growth of psychrotrophic pathogens, these bacteria can be considered a safety hazard to which countermeasures need to be developed.

    L. monocytogenes is a relevant psychrotrophic pathogen on minimally processed vegetables that requires adequate control measures. This Gram-positive bacterium can be inhibited by a number of bacteriocins that are produced by LAB. Thus, biopreservation using bacteriocin producing LAB strains may effectively prevent the growth of this bacterium, and possibly other psychrotrophic pathogens. With respect to practical application, an effort was made to obtain bacteriocinogenic LAB from minimally processed vegetables since these strains might be best adapted to the specific conditions under which their antimicrobial activity is required. Out of a total of 890 LAB isolates from mungbean sprouts and chicory endive, only nine strains were found to produce bacteriocins ( Chapter 4 ). Three of these strains exhibited antimicrobial activity towards a wide variety of Gram- positive bacteria, including the foodborne pathogens L.monocytogenes, and nonproteolytic Clostridium botulinum. Two of these bacteriocinogenic isolates were identified as Pediococcus parvulus on the basis of fermentation patterns and ribotyping. Recombinant DNA experiments revealed the presence of the pediocin PA-1 gene in both LAB strains, and further biochemical analysis confirmed the production of pediocin PA-1 in absence of other bacteriocins.

    The third broad spectrum bacteriocin-producing strain was identified as Enterococcus mundtii ( Chapter 5 ). Its bacteriocin was purified to homogeneity and characterized. Elucidation of the complete primary amino acid sequence revealed a novel bacteriocin of 43 amino acids (M r 4287.2), designated as mundticin, that belongs to the class IIa bacteriocins of LAB. Other LAB that produce this class of antimicrobial peptides have previously been isolated from a wide variety of meat and dairy products, and their apparent wide-spread occurrence in foods may allow for the selection of strains that are ecologically adapted to specific food environments.

    As further described in Chapter 5 , mundticin exhibited bactericidal activity towards L.monocytogenes, and was shown to dissipate the membrane potential of whole cells, and to deplete their intracellular ATP pools. Mundticin was selected as a prototype of class IIa bacteriocins to investigate the biophysical properties of this peptide by molecular computer analysis. It was found that mundticin and related bacteriocins, contain a central α-helical region that was predicted to insert at an angle of 30 to 50° in a simulated hydrophobic/hydrophilic interface. This oblique insertion is proposed to cause a destabilisation of the phospholipid bilayer and facilitate the insertion and/or aggregation of monomers into functional pores in the cytoplasmic membrane. This concept is of interest for the rational design of bacteriocins with, for example, better biopreservation properties.

    For the effective application of bacteriocins as biopreservation agents, it is essential to gain more insight into the variation in the susceptibility of Gram-positive bacteriocins towards bacteriocins. The comprehensive study that is described in Chapter 6 addresses several factors which are associated with the natural variability in the bacteriocin-sensitivity of closely related non- bacteriocinogenic LAB. Two sets of strains for which the minimal inhibitory concentrations for nisin and pediocin PA-1 differed 100 to 1000-fold were compared. Our data provide evidence that the association of bacteriocins with the cell membrane and their subsequent insertion into the membrane take place in a similar way for cells that have a high or a low natural bacteriocin tolerance. For insensitive strains, the overall constitution of the membrane, rather than the mere membrane fluidity, may preclude the formation of pores with sufficient diameters and lifetimes to ultimately cause cell death. Further analysis of this phenomena may require more attention for the structural role of proteins in biological membranes.

    The last part of this dissertation comprises a study on the potential of the vegetable- associated bacteriocin-producing bacteria and mundticin to act as biopreservation agents on refrigerated, MA-stored vegetables ( Chapter 7 ). Both P. parvulus strains were found to be unsuitable for this purpose, due to the lack of bacteriocin production at 4 and 8°C. By contrast, E. mundtii displayed a number of characteristics that favor an application as a biopreservative agent, such as limited acid production, production of a bacteriocin with anti-listeria activity at low temperatures, and high growth rates under MA conditions. Despite these properties, this strain failed to effectively inhibit the growth of L. monocytogenes on fresh mungbean sprouts. This is probably not due to a lack of in situ bacteriocin production, but rather the result of proteolytic degradation of mundticin or the growth of L. monocytogenes cells that are resistant to mundticin. Experiments with mundticin in a washing step and as a component of a coating were successful and warrant further research on the potential of this compound as a natural preservative agent for minimally processed vegetables.

    In conclusion, this dissertation has contributed to a better understanding of the microbiological characteristics of minimally processed vegetables that are stored under MA conditions. A general concern for the possible outgrowth of psychrotrophic pathogens on MA stored vegetables was substantiated by our investigations, which stresses that hygienic cultivation and processing conditions of this type of products remain indispensable. Although bacteriocins cannot be used as the sole preservatives to enhance the safety of refrigerated MA-stored minimally processed vegetables, they can contribute to tackle safety problems that may arise from certain cold tolerant, Grampositive pathogens.

    The use of gas exchange characteristics to optimize CA storage and MA packaging of fruits and vegetables
    Peppelenbos, H.W. - \ 1996
    Agricultural University. Promotor(en): R. Rabbinge. - S.l. : Peppelenbos - ISBN 9789054856061 - 157
    voedingsmiddelen - voedselbewaring - groenten - fruit - ademhaling - katabolisme - verpakken - groenteproducten - opslag - plantaardige producten - aërodynamica - mechanica - gassen - foods - food preservation - vegetables - fruit - respiration - catabolism - packing - vegetable products - storage - plant products - aerodynamics - mechanics - gases
    Oxygen consumption as influenced by carbon dioxide
    High carbon dioxide (CO 2 ) concentrations can reduce the oxygen (O 2 ) consumption rate of a number of fruits and vegetables. This reduction can be modelled by incorporating an inhibition term in an Michaelis-Menten type of model, describing the overall respiration process as a single enzyme reaction. Four types of inhibition can be distinguished: 1. the competitive type, 2. the uncompetitive type, 3. a combination of both previous types and 4. the non-competitive type (after Chang, 1981). Using the inhibition terms a good estimation of O 2 consumption could be obtained. This supports the use of Michaelis- Menten kinetics for modelling O 2 consumption.

    Depending on the product the statistical analysis gave good results for the competitive and the uncompetitive type of inhibition. Based on gas exchange data only, no distinction between the competitive and uncompetitive type of inhibition could be made. The data suggest the simultaneous existence of both types of inhibition. However, for reasons of simplicity the non-competitive type of inhibition is preferred, giving good results for all the products tested. This non-competitive inhibition term, evaluated in chapter 2, is used in other chapters whenever an influence of CO 2 on respiration was found.

    Oxidative and fermentative carbon dioxide production
    Because the main metabolic sources for CO 2 emission by higher plants are respiration and fermentation, both processes have to be incorporated in a model describing total CO 2 production. For this purpose one existing model (Peppelenbos et al., 1993) was adjusted and, based on different theories, two models were developed. The adjusted model used O 2 as an inhibitor of fermentative CO 2 production, whereas the two new models used the ATP production rate, representing ATP concentration. The difference between the latter two models is that in the first one ATP production is calculated by using only oxidative processes, while in the second one ATP production is calculated by a combination of oxidative and fermentative processes. All models allow for increased CO 2 production at low O 2 concentrations, as is often found for several products. The best performance was found for the adjusted model and the new one which used oxidative ATP. The results do not clarify whether increased fermentation rates can be attributed to decreased O 2 levels or decreased energy fluxes. The approach used, however, enables the calculation of CO 2 production rates of different types of commodities stored under various gas conditions. This facilitates a better prediction of CO 2 conditions inside storage rooms and MA packages.

    Alcoholic fermentation as influenced by carbon dioxide
    Not only respiration can be influenced by high CO 2 concentrations. In several products this influence is also found on fermentation. This influence was incorporated in the CO 2 production model based on the inhibition of alcoholic fermentation by O 2 (the'adjusted' model). Gas exchange rates of mungbean sprouts under various O 2 and CO 2 concentrations were used to validate the model. With the modification applied, CO 2 production rates were described better. Although CO 2 production at low O 2 concentrations was reduced by high CO 2 concentrations, the data showed no influence on ethanol and acetaldehyde levels.

    The data obtained indicate large differences between gas exchange rates of different batches of mungbean sprouts. It is suggested that microbial metabolism attributes substantially to total CO 2 production rates found, and might explain these differences.

    The simultaneous measurement of gas exchange and diffusion resistance
    A method was developed to measure metabolic gas exchange rates and gas diffusion resistance of apples simultaneously, under various gas conditions. For this purpose the trace gas neon was selected. After closing a flask containing an apple already kept at a specific gas condition, the neon partial pressure was brought to 110 Pa. Changes in oxygen and carbondioxide concentration in the flask were used to calculate gas exchange, and the decrease in neon concentration was used to calculate gas diffusion resistance. The calculated resistance values were compared with data obtained from literature, and estimations of O 2 and CO 2 resistance values were made. The method worked well on apples, but this will not necessarily be the case when products are measured with small internal gas volumes.

    Functioning of gas exchange models using internal and external concentrations
    Based on gas exchange rates and diffusion resistance, internal gas concentrations of apple cultivars Golden Delicious, Elstar and Cox's Orange Pippin were calculated. Internal O 2 concentrations were 2.3 kPa lower at an external O 2 concentration of 20.7 kPa for Golden Delicious apples, and about 4.5 kPa lower at 20.1 and 20.4 external O 2 for Elstar and Cox's apples respectively. Internal CO 2 concentrations substantially exceeded normal external concentrations of 50 Pa. The Km values found for the three apple cultivars remained significantly different when internal instead of external concentrations were used. This indicates that the apple cultivars measured do not only show biophysical differences (resistance, porosity), but also differences at the biochemical level.

    For Golden Delicious apples no difference in model functioning was found when internal or external concentrations were used. In contrast, for Elstar and Cox's Orange Pippin apples the O 2 uptake and CO 2 production models showed better results (expressed as R 2) when fitted on external concentrations. It is argued that this might be explained by the experimental setup. For instance the internal O 2 concentration of Cox's Orange Pippin calculated at the optimal external O 2 concentration (1.2%) reached 0.01 %. A small change of 0. 1 % in an external O 2 concentration close to 1 % therefore can change the internal atmosphere from hypoxia to anoxia, which cannot be regarded as an equilibrium situation. The conclusion to be drawn is that also for experimental setups some precalculations using gas exchange rates and diffusion resistances will help to optimize the methods.

    Gas exchange characteristics and prediction of optimal gas conditions for CA storage
    The applicability of respiratory characteristics to determine optimal O 2 concentrations for the storage of apples was tested. A comparison was made between gas exchange rates of apples directly after harvest and after a period of storage. Optimal O 2 concentrations were based on gas exchange data and gas exchange models fitted on the data, using the Anaerobic Compensation Point (ACP) and the Respiratory Quotient Breakpoint (RQB). A third to establish optimal gas concentrations way was comparing total ATP production with estimated maintenance energy requirements, revealing the Maintenance Oxygen Concentration (MOC). ATP production was calculated using gas exchange models. MOC was defined as the oxygen concentration with the minimal ATP production rate necessary for maintaining cell viability. The optimal O 2 concentrations as established by ACP, RQB and MOC differed considerably. Because ACP values differed from normally advised values, the ACP was unsuitable for a quick determination of the optimal O 2 concentration of the apples used. The RQB, however, might be suitable, but than the limit used to establish the RQB should be more than 0.5 units higher than the RQ measured in ambient air. The ACP and the RQB were decreased to lower O 2 concentrations after storage, suggesting that the optimal concentrations decrease during storage. In contrast the MOC was increased after storage, which was in agreement with data as found in the literature. Model calculations indicated the lowest optimal O 2 concentration for the second (optimal) harvest using the ACP, the RQB and the MOC. It is suggested that research on the relationship between Maintenance Energy Requirements and cell injury will clarify an important part of the changes in optimal O 2 concentrations (or the tolerance to low O 2 concentrations) during ageing or maturation of harvested plant tissues.

    Fermentation at high oxygen concentrations
    Apples were stored at various O 2 concentrations, ranging between normoxia and anoxia. Gas exchange rates and the production of acetaldehyde and ethanol was measured. A gas exchange model, which distinguishes oxidative from fermentative CO 2 production, was fitted to the data. The results indicate alcoholic fermentation to be active at all the O 2 concentrations used, and increasing in importance when O 2 concentrations are lowered. After calculating the amount of metabolites in the apple tissue from the data measured in air, a close relationship was found between model predictions of alcoholic fermentation rates and measured metabolite production in normoxia and anoxia. In hypoxia, however, the model predicted higher CO 2 production rates in comparison to the metabolites actually found. Because the model was fitted to CO 2 production data, this indicates another source or CO 2 in hypoxia than alcoholic fermentation.

    Conclusions
    The influence of CO 2 on O 2 uptake was investigated, and the known Michaelis-Menten equation given by Chevillotte (1973) was extended with the type of inhibition adequately describing this influence. Models describing fermentative CO 2 production were developed and combined with oxidative CO 2 production, enabling the calculation of CO 2 production of various products under a range of combinations of O 2 and CO 2 . Although gas exchange of mungbean and microbial growth on it could not be distinguished, a model was developed describing the total gas exchange of mungbean and microbial growth, enabling the calculation of mungbean gas exchange in MA packages. A method was derived enabling the simultaneous measurement of metabolic gas exchange and the resistance to gas diffusion. Results of these measurements showed limitations to experimental setups using headspace techniques, and indicated that optimal O 2 concentrations are very likely limited to a specific temperature. Measurements on acetaldehyde and ethanol confirm the prediction of the models describing fermentative CO 2 production, and show that fermentation is not limited to low O 2 concentrations.

    Optimal gas conditions for storage of produce are not fixed values but change with temperature and, more important, also during the storage period. Using fixed gas conditions for long term storage, this could lead to problems and the loss of the stored produce. Interactive storage facilities, responding to physiology of the stored material, will help to reduce this risk. Processes that should be quantified are energy metabolism and fermentation rates. Parameters related to these processes are gas exchange rates and acetaldehyde and ethanol production. For the calculation of energy production the gas exchange models described within this thesis could be used. ATP fluxes, in combination with maintenance requirements, very likely help to understand the tolerance of plant tissues to low oxygen conditions.

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