*Priyanka, Gajanan Deshmukh, P. S, Minz & Chitranayak Sinha
Wine and juice products are a very important part of the processed food industry. As fruits are perishable food products, it is difficult to store the fruits for longer time so as to consume them in the original form. In order to preserve fruits in the original condition, temperature and relative humidity conditions need to be maintained respective of the fruit to keep the fruits fresh for longer. There are techniques and systems available to maintain such conditions such as modified atmospheric storage (MAS) and controlled atmospheric storage (CAS). These systems are difficult to maintain and also increase the cost of the fruits, thus making it difficult for common people to afford such products. Thus, the most common approach to preserve the nutrients in the fruits and pass on to the consumers is by processing them and converting them into finer products such as juices and wine.
The major processes in juice processing include harvesting, crushing, extraction, clarification and pasteurization. Wine processing includes the similar basic steps plus fermentation after crushing and aging after clarification step. In both the processes, clarification/filtration is the most important step to attain desirable texture, clarity, consistency and taste of the final product. The crushed fruit is extracted to get the fruit juice. There are suspended particles in the fruit juice extracted in the form of suspended particles, seeds and other extraneous foreign particles. Filtration is a mechanical process to absorb/ retain the suspended particles for aesthetic quality and microbial stability. For coarser particles straining is done but to remove finer particles to get a clear juice, other filtration techniques are required (Fig. 1). Earth filtration, pad filtration, membrane filtration, cross-flow filtration are some of the filtration techniques used for filtration of juices over the years.
Fig. 1: Wine manufacturing process
Various filtration techniques
Use of diatomaceous earth for filtering out the suspended particles, also known as earth filtration is one of the oldest techniques for filtration. Earth filtration material is also used as a coating on other filtering materials such as on pad filtration, or other filtration screens. The principle of earth filtration includes passage of liquid through finely ground media coated on a mesh screen also known as septum. It is commonly used to filter particles with diameter more than 1 µm. Diatomaceous earth media provides high permeability which is why it is considered a good filter medium. Various advantages of using this technique including simplicity, less use of chemicals, low cost, recovery of particles and reusability (with 90% recovery of the media), make it the most affordable till date. However, it is applicable only for low turbidity fluids and is also not suitable for dissolved solids and taste and odor, etc.
Another important and convenient way for wine filtration is pad filtration, which is based on the principle of absorption, sedimentation and inertia to achieve complete filtration process. Filter pad consists of filtering cloth or fibres made of materials such as cellulose, nylon, cotton, etc. with holes of particular size fitted in a plate and frame setup. Sizing of filtering media is the first and most important step towards deciding filter pads which can filter a known amount of wine/ juice without clogging the media and the need to stop the filtration process. In general, a filter media should be able to filter 10 to 15 gallons of wine/juice per pad in the filter.
Membrane process has become the most important filter medium in the past 30 years for food-processing industry. In wine filtration, membrane processing is used for microbial stabilization of the wines. Membranes filtration in different ways including ultra-filtration, reverse-osmosis, nano-filtration, cross-filtration, etc. is used to clarify wine extract for different sizes. Ultrafiltration is considered ideal for juice and wine processing for separating heavy press from light press fractions. The major advantage of ultrafiltration is that the output is free of components imparting color and bitterness such as tannins, which allows it to be added back to other processing units, if required. It represents similar permeate production and particle removal as that by microfiltration, with easy cleaning as compared to microfiltration.
Apart from ultrafiltration, cross-flow filtration is the most common membrane filtration method for wine and juice processing. This is based on the perpendicular movement of the liquid product across the porous surface of the membrane, as opposed to parallel movement in dead end filtration (Fig. 2), thus resulting in less fouling. There are several benefits that cross-flow filtration technique provides over the conventional techniques that make it more convenient to use in the latest wine/juice industries. The process provides filtration and microbial stabilization in single process thus reducing the cost of different treatments and also the product losses. By avoiding the use of material such as diatomaceous earth, production cost is further reduced by the process. Cross-flow filtration is a highly automated and continuous process thus reducing labor cost and saving time as well.
Fig.2: Difference between dead end and cross-flow filtration
Considerations while selecting filtration system
To select the filtration systems, several factors need to be considered among the different options available. These factors include membrane type and configuration, plant design and function, final quality of product required, power consumption and the overall cost.
- The membranes can be asymmetric or symmetric membrane with different configuration for eg. Spiral wound, tubular, hollow fibre, capillary, etc.
- The size of the plant decides the capacity of the filtering equipment to be used, size of the feed tank, filter tank and pump capacity, etc. quality of wine required plays a major role when selecting the filtering system.
- The filter material should be inert to the product, maintain the composition of the product and avoid losses of the product.
- Power consumption adds to the cost of the overall processing unit, thus, the choice of filtering systems; number of pumps required should be considered while selecting the filtering systems.
- Automated plants avoid the operating cost as they reduce the labor requirements for the whole unit. Thus automated filtering units that can be integrated with the processing unit are of much help to the operator.
- The cost of the processing system is to be taken care of while designing and selecting the unit operations. The cost may include the cost for water and cleaning chemicals, electricity cost and labor cost, etc.
Filtration thus is a very important unit operation in the processing of wine and fruit juices to make it clear, tannin free and microbial stability. There are various systems that are available for filtration of the concentrate for juice and wine processing. Filtering systems have developed from conventional diatomaceous earth filtering to cross-flow filtration in the present scenario.
*Dairy Engineering Division, ICAR-National Dairy Research Institute,
Karnal-132001, Email: firstname.lastname@example.org