Concentration without evaporation, pasteurization without heating, demineralization without distillation, fractionation without chemistry-the food applications for membrane separation technology continue to grow since reverse osmosis membranes were first developed 25 years ago.
The process is currently undergoing commercialization in Japan.Clermont cites these advantages of the ultrafiltration system:Juice processing* Ultrafiltration (UF) retains proteins and protein-sized molecules but allows most other molecules to pass through.The greatest potential for membrane separation technologies may lie in combining their capabilities into novel approaches to traditional processes.Membrane separation is analogous to filtration, but using an exceedingly fine filter. Membranes filter molecules rather than particles and the permeability of membranes to molecules is as much a function of the membrane's physical and chemical properties as the size of its pores.Reverse osmosisCurrently, however, applications of these membranes remains limited to microfiltration and ultrafiltration processes, and their acceptance by food processors remains hampered by the membrane's high cost.UltraosmosisA similar system is being used to process fruit juice concentrates. Orange juice is fractionated by ultrafiltration to yield pulp and serum fractions. The pulp is pasteurized to deactivate residual enzymes that could cause off-flavor development while the serum is concentrated with RO. The two product streams are recombined to yield a juice concentrate of superior quality.Reverse osmosis is used mainly to concentrate food product feedstreams or to clean waste streams. Cellulose acetate RO membranes are still widely used. They are very consistent from batch to batch and have predictable and reproducible salt rejection capabilities ranging from 90% to 95%. Alternatively, thin-film composite membranes are much tighter," rejecting more-than 98% of salt molecules, and better withstand harsh processing conditions. They are more expensive, however.* Yields are higher because "we don't have to stop and start the system as frequently as we did before," reports Eischen. This is particularly important considering that some of Clermont's juices sell for over $100 per gallon.* Ultraosmosis (UO) retains sugars, other organic molecules, and some divalent ions, but passes salt and other monovalent ions.One of the topics of discussion at the recent Dairy & Food Industries Supply Association (DFISA) Expo was the "zero-discharge" plant. While no dairy has yet achieved this goal, reverse osmosis (RO) can remove dissolved solved solids and BOD from wastewater to produce water that can be reused in the dairy.* It runs steadily for long periods of time between cleaning. The cleaning itself--which is a chemical clean-in-place flush--takes less than four hours, and then the system is back up and running.Cleanup advantagesTypical applications for reverse osmosis include concentration of skim milk and juices prior to further evaporative concentration, preconcentration of dried egg whites, protein concentration for gelatin production, and alcohol removal from beer or wine.Like filtration . . .* Microfiltration (MF) is the coarsest of the membrane separation options. Microorganisms, fat globules, fine particulates and colloids are retained by the membrane while most molecules permeate through.In addition to eliminating traditional plate and frame diatomaceous earth filters, membrane clarification typically provides 96 to 98 percent juice recovery compared with 85 to 95 percent for other filtering methods. Membrane concentration, done by RO, can increase juice concentration from its original level of 11 to 12 brix to as high as 60 or 70 brix, although this produces a thick and viscous product. To overcome this problem, systems that remove half the water with membranes and then finish the concentration with an evaporator can be installed. This partial concentration by membranes enables processors that already have an evaporator to double its output for a relatively small expenditure.Says Eischen: "The ultrafiltration system has served us better than expected. It takes care of all of our major filtration needs from early June when fresh strawberries start trickling in through the end of pear season at the end of August, when we're cranking six days a week, right through our frozen season in winter."The relatively recent development of sintered metallic and ceramic membranes offers major improvements in membrane durability, and ease of cleaning and sanitizing. Their applicability to high-temperature processes raises the possibility of marrying membrane separation to pasteurization processes.A second development is the nanofiltration (NF) membrane, which removes monovalent ions like sodium and potassium from the liquid stream. In the industry, these systems are used to remove as much as 95 percent of the salt from salty whey. The resulting product can be be blended back into the sweat whey stream and dried or evaporated into whey powder.Industry acceptance accelerated with the development of polysulfone UF and UO membranes. These membranes have a thin "active" membrane surface supported by a more robust supporting substrate. They are more durable, more permeable, more selective, more temperature resistant, and more readily cleaned in-place than their predecessors.Caustic recovery/CIP solution cleaning has been difficult for dairy processors in the past because membranes could not withstand the harsh operating requirement. However, one supplier announced a new stainless membrane at the DFISA Expo that can handle caustic. In addition, new organic MF membranes are also entering the market. The first caustic recovery systems are being installed on evaporator cleaning systems because of the relatively large volumes at one plant location.* System generates minimal heat. Potentially damaging heat is reduced and is easily handled by small, specially designed tube-in-tube heat exchangers integrated into the membrane system.* Costs are lower, especially compared to using diatomaceous earth. During high production times, such as the pear season, Clermont could go through a load of "earth" each week at a cost of almost $5,000 per load.In addition to treating wastewater, membrane systems can be installed on evaporator cow water and the permeate streams from whey processing RO systems, and can reclaim more than 90 percent of the infeed for use in the plant. Not only does this minimize the amount of fresh water needed in a plant - a major boon in dry areas like California and Arizona - it drastically reduces waste treatment charges.
The process is currently undergoing commercialization in Japan.
Author: William Dreier
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