After being concentrated, the volume of the juice decreases while the concentrations of sugar, acid and flavor components increase. The flavor of the product enhances, making it easier to store and transport, and facilitating the improvement of the stability of juice storage. In industry, the multi-stage vacuum evaporation method is commonly used to increase the concentration of juice. However, due to the influence of high temperature in the evaporation process, a large amount of aroma components in the juice are lost, the color decomposes, "cooked flavor" is generated, the quality deteriorates and the energy consumption is high. 

The reverse osmosis concentrated juice is processed at low temperatures without phase change. It has the advantages of preserving the flavor and nutritional components of the juice well, reducing energy consumption, and being simple to operate. 

Common process route:

Low-concentration fruit juices are usually concentrated under high pressure using reverse osmosis membranes or high-precision nanofiltration membranes, with the juice concentration ranging from 20 to 30 °Bx. 

Juice extraction usually requires pre-treatment steps such as enzymatic hydrolysis, primary filtration, ceramic membrane or hollow membrane cross-flow filtration for clarification, etc., to further release the juice components and remove pectin, proteins, cellulose and residual pulp, etc., to obtain clear and low-concentration juice. Then, this juice is concentrated through reverse osmosis membranes to obtain concentrated juice. 

Studies have shown that the material of the membrane affects the retention of flavor substances in the juice. Polyamide material has a better effect. Not only does it have a high flux, but it can also retain most of the apple juice flavor substances and the sugar, acid, and volatile components in orange juice. The total retention rate is 93%, with the sugar retention rate (98%) being higher than the acid retention rate (85%). The increase in the sugar-acid ratio improves the taste and enhances the quality of the juice. 

Membrane filtration parameters, such as pressure, temperature, and feed flow rate, all have an impact on the filtration flux of concentrated juice and the retention rate of juice components. It is necessary to optimize the membrane filtration process based on the variety and quality of the juice to achieve the best quality of concentrated juice.