Cutting-edge separation and purification techniques can determine whether a product or process rises or falls. Dr S Sefa Koseoglu gives an update on the extraction field
One of the key problems in the processing of biomaterials and foods, and indeed in all chemical processes, is the purification of specific components by separating them from a complex, multi-component matrix. This may be as simple as clarifying a functional beverage by removing small suspended solid impurities and precipitated impurities, or as complex as the fractionation of protein fractions from liquid extracts of oilseed flours. The cost of these separation and purification steps often determines the economic viability of a product or process.
Separation may be based on physical properties such as particle/molecular size, molecular structure, physico-chemical characteristics such as boiling point differences or surface charges, or on chemical properties including solubilities, polarities and specific chemical affinities. Nutraceutical products must be purified to meet regulatory requirements and to minimise side effects. Energy required for separation may be supplied in the form of gravity, potential energy, pressure drop or thermal energy.
The overall objective of the process engineer or separations scientist is to create a fully integrated process system to assure raw ingredients, supplements and foods contain consistent nutraceutical bioactivity during processing, packaging, storage and retail shelf life. Many issues related to separation of bioactives from biomaterials include a good understanding of the following three items:
- the mechanisms of bioactivity loss due to processing, storage, decomposition or reactions with other components;
- deficiencies that compromise retention of bioactive components through optimisation of process conditions, storage and chemistries;
- target levels of nutraceutical bioactives that benefit health.
Many factors influence the active content of biomaterials. Plant-to-plant variations occur in all food ingredients, botanicals, grains, fruits and vegetables. The factors that influence variability include climate, seasonal differences, age, harvest and drying conditions. Therefore, the amount of minor components - anthocyanins, phenolic acids, catechins, flavonones, flavonols, nonflavonoid polyphenols and other phenolic compounds - must be analysed accurately before selecting the appropriate separation/purification techniques. Such methods include precipitations, membrane separation processes, ion exchange chromatography processes, desalting and bipolar membrane, electro dialysis processes, liquid/liquid separations, solid/liquid separations, supercritical fluid extraction, crystallisation, or distillation.
Normal food processing activities compromise quality and composition of most components, such as macronutrients, vitamins and minerals. Just adding health-promoting bioactives to food systems does not ensure their delivery to consumers. Multiple chemical reactions, enzymatic activities, biological degradation processes and physical changes could cause deterioration of the functionality or bioactivity of nutraceuticals in foods and supplements.
Extracting bioactives from biomaterials can be done by equipment that can be classified by the method used to connect the solid with the solvent. The dispersion method employs sufficient quantity of solvent to suspend the solids within the solvent system. In the percolation extractor, solvent is circulated through a fixed bed of properly sized biomass.
A successful large-scale extraction process design requires a variety of bench tests such as sink, column, swelling, attrition and subjectivity tests plus extensive continuous tabletop tests. Effect of variables such as time, particle size, swell of the product, material preparation, maceration and destruction of cell walls, flaking or palletising, solvent composition, solvent temperature, solvent feed ratio, and solvent retention of the solids must be determined to minimise solvent use with maximum recovery of the active components.
Also, extraction equipment must be designed according to GMP regulations, depending on the final product specifications and end use. In addition, selection of extraction equipment must include appropriate and economical solvent recovery systems to remove the solvent from solid and liquid residues. The three main pillars of any extraction process are biomass preparation, the extraction system and solvent recovery unit.
Liquid Separation And Fractionation
Over the years, many industries have come to accept cross-flow filtration such as ultrafiltration, microfiltration, nanofiltration and reverse osmosis as standard technologies for clarification or concentration (See Table 1). In some instances, more than one type of membrane process may be used in a series to achieve the desired performance. The term 'membrane filtration' is the common denominator for the following pressure-driven filtration processes.
These membrane filtration processes are carried out using the concept of cross flow, in the sense that the solution to be filtered is flowing across the membrane surface at a certain velocity at the same time that the filtrate is going through the membrane.
The cross-flow technique is used to increase the rate of mass transfer away from the membrane surface and thus ensure reasonable filtration conditions at the membrane surface, contrary to conventional dead-end filtration.
In nutraceutical beverages and bioactive extracts, most of the membrane products are used for clarification to replace diatomaceous earth (DE) depth filtration. DE is a traditional method for clarifying beverages. However, its use presents a number of issues in terms of product quality, disposal and operator exposure to a potential health hazard.
There are three processing modes: batch, topped-off batch and stages in series. All are used in the beverage industry. Thermo-dynamically, the batch or top-off batch process is more efficient than a stage-in-series process. Although the stage-in-series design is more expensive, it is used when continuous, constant composition of the product stream is required or when trying to minimise product hold-up time before moving on to the next operation. For many applications, most of the systems are topped-off batch design. For single-strength, high-quality specialty and tropical juices, stages-in-series processes allow for rapid passage through the separation process to minimise possible product change.
The ability to separate efficiently, along with low energy and low capital and labour costs, have all been deciding factors in the success of newly developed crops. As an example, we might not have as much interest in development of biotechnology techniques to increase concentrations of valuable components in crops by 10 to 100 or more times than normal if we could afford to separate these components from the currently available varieties.
Today's separations processes have two typical limitations: energy costs and problems and costs of handling the residual by-products, which typically are larger in volume than the components extracted. Within the last 10 years in the US food and nutraceuticals industries, energy costs have moved ahead of labour costs, into second place behind raw materials. The processors are fortunate if their by-products have ready markets as animal feedstuffs or as raw materials for chemicals and other manufacturing industries. Charges often have to be paid to dispose of wastes, or expensive facilities must be built to convert them into forms that can be used or disposed of safely.
Industrial membrane technology is becoming increasingly attractive as a low-cost generic separation technique for concentration, purification and removal of solvents and recovery of solutes. The typical yield of 93 to 98 per cent may be achieved with various types of presses. In some cases, to achieve even higher yield, water is added to wash out the remaining juice sugar. This process is called diafiltration, and with it, yields as high as 99.6 per cent can be achieved.
Dr Sefa Koseoglu is co-organiser of Worldnutra, the International Conference and Exhibition on Nutraceuticals and Functional Foods series. He is also a founder and owner of Filtration and Membrane World, which uses membrane-based technologies in food, nutraceuticals and pharmaceutical industries.