| The frozen food industry has developed for over 140 years,widely involving cryopreservation of the food-related microorganisms and production,storage and transportation of the frozen foods.Freezing technologies bring benefits for human beings but also cause troubles including low cell viability after cryopreservation,interfacial ice and frost accumulation of freezers,and freezing-adhesion problems of the food materials,which could hinder the further development of the frozen food industry.Through learning form the cold-tolerant“teachers”in nature,anti-freezing strategy based on the bioinspired natural deep eutectic solvents（NADESs）were proposed in this work.A series of passive or semi-passive anti-freezing soft-materials enabled by NADESs were fabricated,of which anti-freezing capacities as well as the corresponding mechanisms for different applications were explored.It is hoped that the current work could provide a green,safe and highly efficient bioinspired anti-freezing strategy for the frozen food industry.The summarized contents and the predominant conclusions of this work are listed as follows:（1）Basic characterization of the NADESs inspired by cold-tolerant animals.Inspired by the small-molecule anti-freezing substances found in wood frogs,earthworms and codling moths,four NADESs including Pro:Glc（5:3）,Pro:Glc（1:1）,Pro:Sor（1:1）,and Urea:Glc:Ca Cl2（3:6:1）were prepared,of which polarity(ENR),viscosity（η）,conductivity（σ）,activation energy of viscous flow（Eη）and conduction（E?）,glass transition temperature（Tg）and transverse relaxation time（T2）were investigated.Results showed that the NADESs were much polar than ethanol but less polar than water(ENR:48.57～50.10 kcal mol-1),possessing robust hydrogen-bonding networks（T2:9.90～17.79 ms）.The NADESs showed no freezing behaviour at low temperature while only glass transition phenomena were observed（Tg:-65.28～-56.88℃）.Besides,the hydrogen-bonding interaction within the NADESs could show temperature-response,among which Pro:Sor（1:1）showed the greatest temperature sensitivity.（2）Binary NADESs as green cryoprotectants for yeast cryopreservation.NADESs[Pro:Glc（5:3）,Pro:Glc（1:1）,and Pro:Sor（1:1）]as the test groups and the traditional cryoprotectants[dimethyl sulfoxide（DMSO）and glycerol]as the control groups were used for cryopreserving yeast cells.Cytotoxicity evaluation showed that the three tested NADESs could be safe and biocompatible.Besides,cytomorphology indicated that the cells cryoprotected by NADESs could show plump profiles with smooth cytoderms,while rough cytoderms or even severe cell distortion was partially observed for the ones cryopreserved by glycerol and DMSO.Living/apoptosis/death test of resuscitated cells based on flow cytometry illustrated that the NADESs-cryopreserved cells could show higher cell viability（75%～80%）with almost no apoptosis（<1.0%）.Besides,according to the results of“thermal behaviours and temperature-responsive hydrogen-bonding interaction”and“quantitative evaluation of the ice recrystallization inhibition enabled by image threshold segmentation”,mechanisms of the NADESs-supported cryopreservation were further revealed,which suggested that the NADESs could exhibit reinforced hydrogen-bonding strengths when decreasing temperature and correspondingly restrain the water molecular motion,making the NADESs effectively inhibit ice recrystallization and prevent yeast cells from ice-induced mechanical damage.（3）Water-tailoring effects and interfacial anti-freezing capacities of the NADESs.Effects of water on the hydrogen-bonding strengths within NADESs were investigated.Water-tailored NADESs were used as novel anti-freezing agents for interfacial anti-frosting,deicing,and anti-freezing-adhesion of frozen foods.A critical T2=24.60 ms forηchanges was obtained,and the relationship betweenηand T2 was determined as lnη=-1.398ln T2+10.688（R2=0.9916）,which could be used for task-specific design of NADESs.Differential scanning calorimetry（DSC）and low-field nuclear magnetic resonance（LF-NMR）analyses indicated NADESs could restrain the motion and reorientation of water molecules as temperature decreased through enhancing the hydrogen-bonding strengths,hence preventing the crystallization of water.The anti-frosting capacities showed a tendency of Pro:Sor（1:1）>Pro:Glc（5:3）≈Pro:Glc（1:1）>Urea:Glc:Ca Cl2（3:6:1）.For deicing test,NADESs-coated substrates possessed significantly smaller ice adhesion strengths（3.98～5.48 k Pa）than the control group（59.86 k Pa）,which indicated that all the prepared NADESs could effectively reduce the interfacial ice adhesion.Besides,applications in frozen chicken breast further announced the anti-freezing-adhesion capacities of the NADESs.In summary,the NADESs as food-grade temperature-responsive anti-freezing agents could show great potentials for interfacial anti-freezing in many application scenarios of the frozen food industry.（4）Fabrication and anti-freezing capacities of the binary NADESs-based eutectogels using calcium alginate（CA）as backbone（NADES@CA）.Passive anti-freezing eutectogels（PG53,PG11,and PS11）through gelling the NADESs[Pro:Glc（5:3）,Pro:Glc（1:1）,and Pro:Sor（1:1）]were fabricated,of which basic characteristics,freezing-tolerance and anti-frosting capacities were investigated.Results showed that the NADES@CA eutectogels were two-dimensional planar film-like soft-materials showing sandwich-like structure,and the eutectogels possessing plenty of hydroxy groups within systems could show more hydrophilic surfaces（47.6°～55.7°）as compared with the control group（69.1°）.Besides,PS11 showed the best anti-frosting capacity,exhibiting only 36.31%frost-covered area after 120 min under extreme conditions,probably due to its more rugged surface possessing more and spikier hill-like profiles which could achieve greater moisture capture area.For freezing-tolerance,all the eutectogels could stay flexible at-40℃and show stable mechanical strengths before and after extreme temperature fluctuation.Given this,an absorption-inhibition hypothesis was proposed and verified by gradient moisture absorption test and low temperature thermodynamic test,through which critical absorption relative humidity（CARH）and Tg were obtained,which suggested that the eutectogels could firstly absorb the ambient moisture easily（CARH:49%RH～55%RH）and subsequently inhibit the crystallization of water through immobilizing the moisture within the systems（Tg:-58.6～-50.5℃）.（5）Fabrication and anti-freezing capacities of photothermal eutectogels enabled by doping carbon nanotubes（CNTs）in NADES@CA eutectogel（CNTs/NADES@CA）.Using PS11 as matrix,normal CNTs（n CNTs）or hydroxylated CNTs（h CNTs）in different mass fractions（4%,8%,12%,and 16%）were incorporated to fabricate CNTs/NADES@CA photothermal eutectogels,of which basic characteristics,photothermal conversion capacities,moisture absorption-desorption capacities,and interfacial anti-frosting capacities were investigated.Results showed that the CNTs/NADES@CA photothermal eutectogels were semi-passive anti-freezing soft-materials possessing enhanced mechanical properties,rough and hydrophilic surfaces,and sandwich-like structure.The dopped h CNTs could participate in the formation of supramolecular hydrogen-bonding networks,endowing the materials with better mechanical properties.Although higher CNTs loads increased the breaking strengths,higher values of emissivity were detected according to the absorptance/emissivity spectra,which could hinder the maintaining of high working temperature.Besides,all the tested samples showed moisture desorption capacities under photothermal driving force,while higher CNTs loads could slow down the desorption rates.Among the fabricated CNTs/NADES@CA photothermal eutectogels,4%-h CNTs showed the greatest photothermal conversion efficiency at different working temperatures,which also exhibited excellent moisture desorption capacity and interfacial anti-frosting capacity under contact heat transfer mode. |
PDF Full Text Request