| Patulin is a mycotoxin which has detrimental effects on the health of humans and animals.The mycotoxin can contaminate various fruits,especially in apple and apple products worldwide.Due to its special physical and chemical properties,it is impossible to completely remove patulin and degrade in the process of production.Patulin residue beyond the standard has become a barrier in international trading.Attempts have been made to develop methods either to remove patulin from contaminated foods and feeds or to degrade the present toxin into less toxic compounds.However,these traditional approaches are expensive and impractical for some regulatory limits,and can diminish the quality of fruit juices.As a promising approach,using inactivated lactic acid bacteria(LAB)strains to adsorb patulin shows some advantages including low cost,high efficiency and abundant resources.In this study,the mechanism of patulin adsorption by heat-inactivated LAB strains was investigated by many research approachs in order to provide dataand technology supports for the contaminationcontrol of patulin in the acidic liquid food.The main contents and results are as following:1.The bacterial cells were characterized by Scanning Electron Microscopy coupled with Energy Dispersive X-Ray Spectroscopy(SEM-EDS),Transmission Electron Microscopy(TEM)and Brunauer-Emmett-Teller(BET)technique.The patulin-exposed bacterial cells and patulin-unexposed bacterial cells were characterized by Fourier Transform Infrared Spectroscopy(FTIR),Zeta Potential and Contact Angle Method.The studies indicated thatthe patulin adsorption was related tothe specific surface area,cell wall volume,nitrogen-to-carbon ratio(N/C),hydrophobicity,group electronegativities,and cell wall structurewhich benefited the diffusion of patulin.The main functional groups involved in adsorbing patulin might be C-O,OH and/or NH groups,especiallythe nitrogen-containing groups.Above all,the adsorption capacity of heat-inactivated LAB cellshad close relationships with physical and chemical properties ofbacterial cells.2.The effects of various physical,chemical,and enzymatic pre-treatments,simultaneous treatments,and post-treatments on the patulin adsorption performances of six LAB strains were evaluated.The pre-treated cells were characterized by scanning electron microscopy(SEM).Results showed that the adsorption abilities were widely increased by NaOH and esterification pre-treatments,and reduced by trypsin,lipase,iodate,and periodate pre-treatments.Additionally,the adsorption abilities were almost maintained at pH 2.2–4.0,and enhanced significantly at pH 4.0–6.0.A lower proportion of patulin was released from the strain with higher adsorption ability.Analyses revealed that the macrostructure integrity of the cell wall was not a principal factor.Aromatic amino acids,acidic amino acids,andvicinal OH and carboxyl groups were not notably involved in patulin adsorption,while alkaline amino acids,thiol and ester compounds were important for patulin adsorption.Additionally,besides hydrophobic interaction,electrostatic interaction also participated in patulin adsorption,which was enhanced with the increase in pH(4.0–6.0).Physical adsorption and chemical adsorption occurred at the same time in the adsorption process of patulin.3.To identify the main property and the dynamic adsorption process of patulin onto heat-inactivated LAB cells,the effects of various experimental parameters and the kinetic,isotherm,thermodynamic,mass transfer characteristics were studied.The thermodynamic parameters and other related results indicated the adsorption process,which was endothermic and incompletely reversible,was mainly attributed to the chemical adsorption.Additionally,the adsorption process fitted well to the second-order adsorption model and the Freundlich model,which revealed a complicated multilayer adsorption process of patulin onto different nature sites of heterogeneous surface of heat-inactivated LAB cells.The intra-particle diffusion was the main rate controlling step when the system was well stirred.The results also showed that the effects of the initial adsorbent concentration were dependent on the amount of available sites and the increase of patulin concentration in solution for the rehydration of lyophilized cells;the effects of the temperature were dependent on the thermal diffusion ability of patulin and the activation energy of adsorbent;the effects of the initial patulin concentration were dependent on the actuation performance of concentration gradient and the amount of available adsorption sites on the adsorbent.4.To verify the chemical adsorption mechanism and explore high-efficiency adsorption material,the adsorption abilities and the physicochemical properties of related components were compared.Under the condition of acid,results showed that reduced glutathione(GSH),arginine,lysine,histidine,and L-cysteine were capable of removing almost all patulin.The removal efficiencies of patulin by Chitin(90.09%),chitosan(70.51%),and peptidoglycan(46.73%)were also remarkable.Further analysis found that the adsorption ability of amide bond was stronger than amino,and chitin was a promising adsorbent material in controlling patulin.Under the condition of acid,the adsorption ability of ?-amino at amino acids or polypeptides was affected by the ?-carboxyl.Groups located on the side chains of amino acids or polypeptides,which have lone pairs of electrons in nitrogen or sulfur atoms,were the main functional groups.Both positively charged and negatively charged organic materials could adsorb patulin.The author thinks that the chemical adsorption of patulin could be divided into two cases: under neutral or alkaline medium,for nitrogen,sulfur or oxygen atoms which have lone pairs of electrons,the adsorption mechanism was dominated by complexation or chelation;under acid medium,patulin could be adsorbed by the protonated functional groups mainly through ion exchange or electrostatic attraction.Meanwhile,the complicated ionization mode of biological adsorbent will lead to more complicated adsorption process. |
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