Study Of Exclusion And Inactivation Of Alicyclobacillus Acidoterrestris In Apple Juice By Microfiltration Membrane

Alicyclobacillus acidoterrestris(A.acidoterrestris)is thermophilic,acidophilic bacilli species.It can not only survive during the pasteurization process,but also grow and reproduce in acidic apple juice which brings severe challenges to the apple juice industry.In the present study,we explored the application of polyvinylidene fluoride(PVDF)microfiltration(MF)membranes to remove A.acidoterrestris in apple juice.In addition,the PVDF MF membrane was modified by polydopamine coating and nisin grafting.The modification can further improve the sterilization performance of the membrane.The main conclusions are as follows:1.The PVDF MF membranes with different pore size were tested at different transmembrane pressures(TMP)to filter apple juice with a colony number?2.3×10~8 CFU/m L.The log retention values(LRV)under each operating condition were detected.The results showed that:0.10?m membrane was high-efficiency bacteriostatic membrane(LRV>6)under three TMP.The removal rate of A.acidoterrestris could reach 100%by 0.10?m membrane at 0.1 bar and 0.2 bar.The 0.22?m membrane was also a high-efficiency bacteriostatic membrane under the TMP of 0.1 bar and 0.2 bar.But when the pressure was increased to 0.3 bar,the rejection rate of A.acidoterrestris decreased significantly(LRV<4).The 0.45?m membrane was low-efficiency bacteriostatic membranes under all of TMP tested.2.A facile and efficient membrane modification method was developed.The polydopamine(PDA)coating followed by nisin grafting was adopted to modify 0.22?m PVDF MF membrane.The contact angle and water flux of the modified membrane were measured.According to the results of SEM,AFM and inhibition zone,the maximum water flux of PDA modified membrane was reached at a coating time of 14 hours by dopamine solution with a concentration of 1 g/L.The nisin grafting was performed on the PVDF membrane coated by PDA.The optimal grafting condition was identified as the nisin concentration of 1 g/L and the grafting time of 12 hours.3.The modified membrane was used to filter apple juice with a colony number?2.3×10~8 CFU/m L.The real-time flux of apple juice during MF and the LRV value of the membrane were measured.The modified membrane had a slow decreasing rate of flux during the process of apple juice indicating an improved anti-fouling performance.The LRV value of MF membrane at 0.3 bar could be increased from 3.89 to 5.75 by the modification.The retention rate of A.acidoterrestris was significantly improved.The results of SEM and fluorescence microscopy confirmed that nisin-modified PVDF MF membrane could achieve the retention and inactivation of A.acidoterrestris simultaneously during the MF of apple juice.4.Simulated water cleaning and chemical cleaning(acid,alkaline and SDS washing)were carried out on the stability test of nisin grafted membrane.The LRV value of the membrane after cleaning was determined by the inhibition zone test.All of four cleaning methods caused the loss of the anti-bacterial activity of the membrane.However,the LRV value of membrane after four cleaning processes was still all higher than 4,indicating a highly efficient bacteriostatic performance.The modified membrane showed a good stability under membrane cleaning.Our research explored the application of MF technology as a non-thermal sterilization technology to remove A.acidoterrestris in apple juice.At the same time,MF process could achieve simultaneous exclusion and inactivation of A.acidoterrestris via nisin grafting modification.This study provides a new method for the removal of A.acidoterrestris from apple juice,and provides a new insight for the application of membrane based non-thermal sterilization in food processing.