Application of Spiral Ultrafiltration Membrane in Soy Sauce Clarification and Filtration

1. Introduction

Soy sauce fermentation is a complex biochemical reaction process. After the end of fermentation, the crude sauce liquid is mixed with unhydrolyzed macromolecular colloid, residual mycelium, protein floc and fine solid particles. If these substances are not removed, the finished soy sauce is prone to layered precipitation after being placed for a period of time, which affects the appearance quality and market acceptance. For a long time, most soy sauce manufacturers adopt diatomite pre-coating filtration. Although this method can achieve clarification, diatomite will adsorb part of amino acids, pigments and flavor substances in soy sauce, resulting in the loss of flavor and color value. In addition, the waste diatomite belongs to solid waste, which brings pressure to environmental protection treatment. Some manufacturers use high-temperature sterilization combined with precipitation, but high temperature is easy to damage heat-sensitive flavor substances and change the original taste of soy sauce. With the upgrading of food processing standards, low-temperature physical separation technology has gradually been applied in the condiment industry. Spiral wound ultrafiltration membrane, with compact structure, large membrane area and good anti-flow channeling performance, is very suitable for the clarification of medium-viscosity fermented liquid such as soy sauce. It can intercept macromolecular colloidal impurities and allow small molecular flavor components, amino acids and reducing sugars to pass through the membrane, so as to balance clarity, flavor retention and production continuity.

2. Working Principle of Spiral Wound Ultrafiltration Membrane

The spiral ultrafiltration membrane element is formed by winding flat membrane sheets, flow guide mesh and permeate collection mesh around the central perforated collecting pipe. Under the driving of cross-flow pressure, the crude soy sauce flows along the surface of the membrane sheet. Small molecular substances and water penetrate the membrane layer, enter the permeate side, flow into the central collecting tube and become clarified permeate liquid. Macromolecular colloids, suspended particles and bacterial bodies are trapped on the feed side and discharged out of the system with the concentrated liquid of cross-flow circulation. Different from dead-end filtration, cross-flow operation can continuously wash the membrane surface by the tangential flow of material liquid, slow down the accumulation of intercepted pollutants and extend the stable operation time of the membrane system. By selecting the ultrafiltration membrane with proper molecular weight cut-off (MWCO), enterprises can control the interception range according to product positioning, meet the clarification requirements of light soy sauce, dark soy sauce and high-grade brewed soy sauce respectively.

3. Complete Process of Soy Sauce Clarification by Spiral Ultrafiltration Membrane

The standard membrane clarification production line is usually matched with pre-filtration protection to reduce the impact of large particles on the membrane element. The whole process is as follows: Crude fermented soy sauce → pre-filter (bag filter / self-cleaning filter, 50~100 mesh) → raw material tank → feed pump → security filter (5~10μm cartridge filter) → spiral ultrafiltration membrane module → permeate tank (clarified soy sauce, entering subsequent sterilization and filling) → concentrate tank (recycled to fermentation workshop or treated separately) → CIP cleaning system. Pre-filtration is a necessary front-end protection. Large mycelium and coarse particles are removed in advance to avoid scratching the membrane surface and blocking the flow channel of the spiral element. The system adopts circulating cross-flow mode. When the turbidity of permeate rises or the transmembrane pressure difference increases to the set threshold, the system automatically switches to the cleaning procedure.

4. Core Advantages in Soy Sauce Clarification

First of all, the whole process runs at normal temperature, no heating, no filter aid added. It effectively retains the natural flavor, amino nitrogen and reddish-brown pigment of brewed soy sauce, avoiding flavor attenuation caused by diatomite adsorption. Secondly, the clarification effect is stable. The finished soy sauce after ultrafiltration has low turbidity, not easy to precipitate during shelf life, and the product consistency is better than traditional filtration batches. Thirdly, the automatic degree of the membrane system is high. It can realize automatic feeding, circulating concentration, liquid level interlock and automatic CIP cleaning, reduce manual intervention and reduce the labor cost of filtering posts. Fourthly, the solid waste output is greatly reduced. There is no waste diatomite, only a small amount of concentrated colloid liquid, which can be returned to the production process for recycling, and the environmental protection cost is reduced.

5. Membrane Fouling Characteristics and Cleaning Control

Soy sauce contains protein, polysaccharide and pigment polyphenol substances, which are easy to form organic pollution on the membrane surface and pores during long-term operation, manifested as the rise of TMP and the decline of permeate flux. Regular online CIP cleaning is the key to guarantee the service life of spiral ultrafiltration membrane. The conventional cleaning sequence is: water flushing → alkaline cleaning (low concentration sodium hydroxide, matched with food-grade surfactant, remove protein and organic colloid) → water flushing to neutral → acid cleaning (citric acid, remove inorganic salt scaling) → pure water flushing. After cleaning, the flux can be restored to more than 90% of the initial state. In daily operation, it is necessary to strictly control the feed temperature and cross-flow velocity. Too low flow velocity will accelerate the accumulation of pollutants, while too high flow will increase energy consumption and shear damage to flavor molecules.

6. Application Summary and Prospect

Spiral wound ultrafiltration membrane clarification is an environmentally friendly upgrading scheme for traditional soy sauce filtration technology. It is especially suitable for medium and large brewed condiment factories that pursue stable product quality and clean production. At present, this technology has been widely promoted in soy sauce, vinegar, fermented sauce and other condiment liquid clarification. In the future, combined with precise molecular weight grading membrane elements and online turbidity monitoring system, the membrane process can further realize graded separation of soy sauce components, develop differentiated high-end brewed condiment products, and provide more diversified technical options for the upgrading of the condiment industry.