Abstract:

Objective With the growing fierce competition in the beer market and the increasing demand for high beer quality, the breweries try to reduce the cost and improve the quality of beer via optimizing their manufacturing process and raw materials, among which the filtration of beer is one of the main important steps. However, the flux instability of beer often occurs in the membrane filtration process. Therefore, it is of practical significance to develop a membrane material which can reduce the turbidity of beer while ensuring high throughput.

Method In this research, polyvinyl alcohol-ethylene copolymer (PVA-co-PE) nanofibers were prepared by melt blending phase separation method. Nanofibrous membranes with gradient porous structure were further fabricated by coating nanofibers with different diameters in different coverage density sequentially, crosslinked by glutaraldehyde (GA). The gradient bactericidal membrane was characterized by scanning electron microscopy, aperture distribution and flux analyzer. The filtration performances including permeability, turbidity and bacteria removal ability, antifouling and flux stability of the as-prepared membranes were investigated compared with that of Pall^® commercial nylon microporous membrane.

Results The experimental results showed that the morphology of sterilized membranes were similar to that of the nascent one without GA crosslinking modification, indicating an effective strategy for the preparation of high-performance sterile membrane. After modification and high temperature sterilization, the average pore size of the membranes were slightly reduced because of the increased the fiber diameter and partial filling of the pores associated with GA crosslinking. The decreased pore size of crosslinked membrane should be beneficial to the sterilizing filtration application. During the long-term water flow test, GSM-3 with additional 6 g/m² and 750 nm nanofiber layer always showed the highest pure water flux within 60 min even after it became stable. The cyclic filtration flux chart of membrane materials towards BSA solution was presented, and GSM-3, in comparison to other sterilization membranes, exhibited a remarkably high flux recovery rate, reaching 80% of its initial flux. Furthermore, sterile filtration performance tests revealed that all sterilization membranes effectively block the passage of Serratia bacteria which completely covered the membrane surface, obstructing the membrane channels and significantly reducing filtration efficiency. However, when filtering bacterial suspensions using the gradient sterilization membrane GSM-3, filtration proceeded smoothly with lower resistance and higher efficiency. Additionally, GSM-3 gradient sterilization membrane maintained a stable flux of approximately 750 L/(h·m²), with a 95% reduction in turbidity of the filtrate. These results indicate that the GSM-3 with a three-layer gradient structure possesses the most excellent filtration performance.The outer 750 nm fiber layer presented a loose structure primarily responsible for trapping larger particles and preventing them from clogging the inner pores, while the inner 450 nm fiber layer featuring dense pores increased the effective surface area of the membrane, resulting in improved membrane permeability and better control of membrane fouling.

Conclusion In this study, GA in situ cross-linked EVOH nanofibers was used to improve the sterile filtration properties of beer, where a nanofibrous membrane with fine nanofibers (average diameter of 450 nm) as the substrate membrane, and it was further coated with a coarse nanofiber layer (average diameter of 750 nm) on its surface. It was found that the nanofibrous membrane presented transmembrane gradient pore structure, which is beneficial to the improvement of water permeability with a high interception of particular contaminants especially bacterial in beer brewing. The superiority of the structure is attributed to the combination of inner fiber layer consisted of nanofibers with diameter of 450 nm and skin fiber layer made up of nanofibers with diameter of 750 nm fibers. The skin fiber layer has a loose structure, which not only intercepts larger particles to prevent them from clogging the smaller pores inside, but also provides the channels for interflow. The inner fiber layer provides a dense pore structure, the smaller pore size provides a prerequisite for intercepting bacteria, ensuring the bacterium removal performance of the membrane. They jointly contribute to the stepwise interception.

Key words: nanofiber, gradient porous membrane, sterile filtration, antifouling performance, beer stability