Dairy and Food Engineering

Lesson 29. Changes Undergone By Fruit Components During Extraction, Filtration and Membrane Processing

29.2 CHANGES IN FRUIT JUICE COMPOENTS DURING PROCESSING

29.2.1 EXTRACTION PROCESSE OF FRUIT JIUCES

Extraction process involves extraction of nutritionally important components from fruit without much change in its chemical and physical state. Once the fruit is delivered for processing, the critical operation of juice extraction begins. In general, juice extraction should be done as rapidly as possible so as to minimize oxidation of the juice by naturally present enzymes. The oxygen absorbed in the juice during extraction operations leads to oxidation of ascorbic acid and many oxidation induced changes in extracted fruit juice like aroma or flavour changes. Pectic substances released during fruit juices extraction causes a considerable increase in their viscosity, thereby impeding filtration and subsequent concentration processes.

29.2.2 CONVENTIONAL FILTRATION PROCESS OF FRUIT JUICES

Raw fruit juice is turbid, viscous, and dark in colour. Apart from lower molecular weight components like sugar, acid, salt, flavour, and aroma compounds, it contains significant amounts of macromolecules (100–1000 ppm) such as polysaccharides (pectin, cellulose, hemicellulose, and starch), haze-forming components (suspended solids (SSs), colloidal particles, proteins, and polyphenol), etc. Hence, the juice needs to be clarified before its commercial use. Clarification involves the removal of such macromolecules. An enzyme treatment of raw juice is usually carried out with enzymes (pectinase and amylase) to reduce the pectic substances and starch content. This enzyme treatment reduces the cloudiness and makes the filtration pro­cess easier by lowering its viscosity. The enzyme treatment is followed by the addi­tion of fining agents such as gelatin, bentonite, etc., which are used to enhance the settling of formed flocks. Then, suspended solids, colloidal particles, proteins, etc., are removed by conventional filtration. These conventional methods to clarify fruit juice are labour intensive, time-consuming, and batch operated. The use of additives (fining agents and filter aids) may leave a bitter taste in the juice. Moreover, the solids obtained after filtration, which contain enzymes, filter aids, and fining agents, cannot be reused and cause pollution problems due to their disposal.

29.2.3 MEMBRANE PROCESSING OF FRUIT JIUCES

In this regard, microfiltration (MF) and ultrafiltration (UF) are considered as energy-efficient and valid alternative membrane processes for the clarification of additive-free high-quality fruit juices with natural fresh taste. The objective of UF/MF of fruit juice is to retain high molecular weight pectin and its derivative, proteins, colloids, etc., and to allow low molecular weight solute-like sugars, acid, salt, etc., to permeate through the membrane together with water. These processes are non-thermal and involve no phase change or addition of chemicals. The production of concentrated fruit juices is of interest at the industrial level since it reduces the storage volume, thus reducing the package, transportation, and storage cost, and also facilitates the preservation. The concentration of fruit juices is usually carried out in a multistage vacuum evaporator. However, high-energy consumption, off-flavour formation, colour changes, and reduction of nutritional value due to thermal effects are the main disadvantages of traditional evapora­tion processes. Alternative techniques to thermal evaporation to obtain stable concentrated juice with its original colour, aroma, nutritional value, and structural characteristics involve freeze concentration (cryoconcentration) and membrane processes. Reverse osmosis (RO) seems to overcome some of the drawbacks associated with thermal evaporation with less energy consumption; however, it is generally used as a pre-concentration technique allowing concentration values of about 25–30 °Bx due to high osmotic pressure limitations. Recently, direct osmosis concentration (DOC), osmotic distillation (OD), and membrane distillation (MD) have been proposed as attractive membrane processes allowing very high concentrations (above 65 °Bx) to be reached under atmospheric pressure and ambient temperatures. Considerable research has been carried out for the clarification and concentration of a variety of fruit juices including apple, mosambi, pineapple, grapefruit, kiwi, passion fruit, etc. There are three primary areas where membranes can be applied in processing of fruit juices:

• Clarification: for example, in the production of sparkling clear beverages using microfiltration or ultrafiltration

• Concentration: for example, using reverse osmosis to produce fruit juice concentrates of greater than 42° Brix

• Deacidification: for example, electrodialysis or nanofiltration to reduce the acidity in citrus juices.

Membrane technology is a novel and emerging separation technology. Membrane-based separation processes have the following advantages:

• Continuous separation processes

• Energy consumption is generally low

• Easily combined with other separation processes

• Operated at room temperature

• Up-scaling is easy

• No additives are required

• No physical or chemical changes required to feed streams

However the following disadvantages are worth noting:

• Concentration polarization

• membrane fouling

• Low membrane life time

• Low selectivity

The ranges of membrane processes can be used based on size of fruit juice components. RO is mainly used to concentrate the fruit juices by dewatering. MF and UF are used for clarification of juices by removing large suspended particles and colloids. Nanofiltration (NF) can be used to clarify and concentrate juice as well as to decolorize the juices. All processes remove the microorganisms to a great extent.

Many changes take place during the membrane processing of fruit juice, but most of them are comparable with respect to the changes during traditional filtration process. Many changes like browning of juices, colour changes, aroma and nutritional losses of fruit juices can be avoided by membrane filtration. Further, membrane process produces the clear juice without haziness along with retention of enzymes like pectinase and polyphenoloxidases (PPO) which otherwise leads to browning of fruit juice during processing.

29.2.3.1 MICROFILTRATION AND ULTRAFILTRATION

Among all membrane filtration processes, microfiltration is the closest to an ordinary filter. Microfiltration (MF) can be used as pre-filtration process to remove the bacteria from fruit juice. The most suitable application of MF and UF in fruit juice processing is removal of pectin from juice with the help of pectic enzyme to avoid clouding of fruit juice. The removal of large suspended particles and colloids are considered as key changes taking place during MF and UF of fruit juice. Advantages of MF and UF over traditional processing include production of clear juice with retention of depectinized enzymes which can be recycled for further use in fruit juice processing.

 29.2.3.2 NANOFILTRATION

The mechanism of separation for Nanofiltration (NF) is not fully understood, but possibly works both on size exclusion like MF and UF, and on solution diffusion like RO. NF can be used to clarify and concentrate juice as well as to decolorize juices. Another major change during NF of fruit juice is loss or rejection of polyvalent ions to a much greater extent than monovalent ions. 

29.2.3.3 REVERSE OSMOSIS

The main function of RO in fruit juice processing is to removal of water without phase change and thereby energy is used more efficiently. The beneficial effect of using RO for fruit juice processing is lying with minimum heat damage to color, aroma and viscosity characteristics of juice. Further the retention of volatile flavor compounds, better preservation of organoleptic and nutritive properties are main advantages of RO for fruit juice processing.

29.3 CONCLUSION

Membrane processing of fruit juice is considerably increasing field of interest. Membrane processing preserves the fruit juice components with minimal changes in their inherent quality. Furthermore, thermal labile nature of many food components clearly suggests that non thermal membrane technologies would be much better alternative for the food/ agricultural engineers.

References:

1. Fellows, P. J. (2000). Food Processing Technology, Principle and Practices, CRC Press.

2. Rahman, M. Shafiur (2007). Hand Book of Food Preservation, CRC Press.

3. Girard B. and Fukumoto L. R. (2000). Membrane Processing of Fruit Juices and Beverages: A Review, Critical Reviews in Food Science and Nutrition, 40(2):91–157.

4. Liu, Sen X. (2005). Membrane technology for postharvest processing of fruits and vegetables. Stewart Postharvest Review, 2:1.

 Suggested Readings:

1. Desrosier, N.W. and Desrosier, J.N. (1987). The Technology of Food Preservation. CBS Publishers and Distributors, New Delhi.

2. Farid, Mohammed M. (2010). Mathematical Modeling of Food Processing, CRC Press.