Cross-flow filtration of fruit juice

Summary

Current filtration of a wide variety of process fluids from the Food, Pharmaceutical and Chemical industries is accomplished using filter aids. These filter aids have excellent filtration qualities, however there are some disadvantages involved in their use:

	Some filter aids are classified for provoking lung diseases due to the dust,
	There are environmental effects of the deposition of sludge or filter cake obtained,
	The use of kieselguhr results in high costs since it mostly is imported, a lot of water is needed and the deposition of the filter cake is quite expensive.

The general aim of this project is to develop and implement a system to filter various types of process fluids. The applied aim of this project is to develop a filtration system to clarify juice keeping and / or improving the quality of the juice.

The two main quality demands in juice processing are to preserve the organoleptic quality and clarify the juice for storage. Current filtration of a wide variety of juices is accomplished using filters aids. However, a final polishing is achieved by means of ultrafiltration.

Membrane technology is currently a "proven technology" within a few main areas, i.e. food and dairy industry, water purification and treatment of liquid fluent streams, and it is presently being introduced into a wide variety of other applications. The recent development of membrane technology will strongly influence the way industry evaluates separation processes in the immediate future.

The advantages of using membrane technology in the beverage industry are related to economy, working conditions, environment and quality (Hägg, 1998):

	Low energy requirements and costs,
	Avoids dust and sludge (formation/deposition),
	Possibility of lower temperature processing (hence reduction of thermal damage to food during processing),
	Simpler process design.

Cross flow microfiltration is one of the most recent developments in membrane technology, and it is replacing a number of traditional clarification and sterilisation operations in a wide variety of industries (Forbes, 1987). This technique is today used with success in some applications in the pharmaceutical and the biotechnological industry in Europe when purifying products of high value and low volume. The increase of disposal and purchase costs will make it economically feasible to invest in alternative filtration methods for several industry sectors with a large use of filter aids, such as the brewing and the beverage industries, and cross-flow microfiltration can be one possible alternative. Furthermore, some studies have indicated some losses in flavour when clarifying apple juice by means of ultrafiltration compared to microfiltration, and other advantages of microfiltration are better efficiency and shorter processing periods (Wu et al., 1990).

This new technology could offer a competitive and attractive alternative to the filtration technique that currently dominates the market because of its efficiency and because it is harmful to public health and environment.

The project was divided in five parts:

	Pre-treatment dealing with removal of big particles to improve the efficiency of Cross-flow microfiltration
	Cross-flow microfiltration dealing with the optimisation of the microfiltration efficiency keeping/improving the quality of the final product
	Up-scaling dealing with the evaluation of the physical conditions to facilitate the application of this technology in the food industry
	Fouling & Cleaning studying different methods and techniques in order to avoid fouling and improve cleaning
	Feasibility studies dealing with the analysis of the investment extend and profitability of Cross-flow microfiltration at a social and economical level

The research institutions and companies involved in the project were:

	Department of Biotechnology at The Technical University of Denmark in charge of co-ordination, research and development in the five areas
	Vallø Saft A/S in charge of delivering juice and adviser in juice and juice processing
	Gustav Fagerberg A/S in charge of delivering Filtomat thread filters and filtration systems and technical adviser in pre-treatment
	PLS Consult A/S in charge of collecting data to the Feasibility studies
	Novo Nordisk Enginneering A/S participating as a technical adviser

The investigations carried out during this project indicated that Filtomat thread filters could be successfully used to produce black currant juice of excellent quality, since these can replace the vacuum filter. Further microfiltration was required when filtering sour cherry juice. Good performance and high fluxes were achieved with the Filtomat thread filters (pre-treatment) and the polymeric membranes from X-Flow (cross flow microfiltration). Furthermore, the quality of the juice obtained was improved since running at low temperatures avoids the precipitation of polyphenols, which results in undesired turbidity. The feasibility studies confirmed the potential of this new technology, since it results in lower production costs and it is also better for the environment when compared to the technology used nowadays. When applying this technology at larger scale, the shadow effect did not result in complications. However, the pressure distribution and the control of the backshock should be further investigated.

It has also been demonstrated during this project that the backshock technique and high power ultrasounds had a positive effect on the microfiltration performance and efficiency, and that these were feasible methods to reduce fouling when filtering sour cherry juice.

The composition of the juices investigated during this project is quite complex and these contain several components that could disturb the filtration and the cleaning processes. Some studies have verified that the nature of the foulants is complex and that the role of the sugars in membrane fouling should be further investigated. Furthermore, it has been proven that the cause for pore blocking was the amount of juice filtered, and not the concentration.