FAQ: Clarification of Fruit (Apple) Juice

Over the years we have seen an increased use of filtration equipment in juice processing, particularly regarding ultrafiltration (UF) or microfiltration (MF) for the clarification of apple juice.  Since it has been demonstrated that membrane filtration can produce yields of 95%-99% - compared to only 80-94% through conventional processes – it is no wonder that filtration methods are growing in prevalence.  The greater yield combined with the reduced time and labor costs have translated to hundreds of thousands of dollars saved for juice processing plants!

If you are considering juice filtration, here a couple of tips to keep in mind:

• The juice must be clear.  Of the four common types of apple juice produced – natural, crushed, clarified, and clear – only clear juice is suitable for membrane processing.    
• Consider ceramic membranes.  More and more fruit juice installations are installing ceramic membranes.  While these do have  a higher cost than other materials, they do offer a higher flux, much longer life, and better resistance to aggressive processing and cleaning conditions. 
• Know your operation.  Since fruit juices have a very low level of retained solids, the optimum mode of operation is the modified batch operation with a partial recycle of retentate.
• Not just for apples.  Other fruit and vegetables that have benefited from membrane filtration include: apricot, carrot, cherry, cranberry, grape, lemon, lime, orange, peach, passion fruit, and tomato.

References:
Ultrafiltration and Microfiltration Handbook.  Cheryan, Munir.  Technomic Publishing Company, 1998.
Microfiltration and Ultrafiltration: Principles and Applications.  Zemon, Leos & Zydney, Andrew.  Marcel Dekker, 1996.

Water Sterilization & Silver

From this recent article in NanoLetters, the American Chemical Society Journal, comes information about a new form of water sterilization out of Stanford University that takes advantages of the unique bacteria-killing properties of silver (the vampire and werewolf killing properties of silver have yet to be proven).  Basically, the proposed multiscale device would perform high speed electrical sterilization of water using a combination of silver nanowires, carbon nanotubes, and cotton.  The end result is that when operating at 100,000 L/(h m²) this device can inactivate greater than 98% of bacteria with only several seconds of total incubation time.

The author’s of this paper mention two interesting reasons for why silver is used in the device.  The first:        

Taking advantage of silver nanowires’ (AgNWs) and CNTs’ [Carbon Nanotube] unique ability to form complex multiscale coatings on cotton to produce an electrically conducting and high surface area device for the active, high-throughput inactivation of bacteria in water.

The other reason described for using silver in water sterilization:

Silver is chosen since it is a very well-known bactericidal agent, and recently a large amount of interest has been spurred by the discovery that silver nanoparticles work extremely well at killing bacteria and can be attached to various surfaces with chemical techniques.    

The outcome of the silver treatment in the author’s experiment provides further evidence of these properties:

The results clearly show that filters not treated with silver, including CNT-only cotton, showed a robust growth of bacteria, while the bacteria concentration in the solutions incubated with AgNW-treated material was reduced to the detection limit of the absorbance system used, at least a 2 to 3 order of magnitude reduction.

All in all, the findings in this paper are encouraging that implementation of this approach can kill microorganisms which cause biofouling in downstream filters.  The authors of the paper state, “Such technology could dramatically lower the cost of a wide array of filtration technologies for water as well as food, air, and pharmaceuticals, where the need to frequently replace filters is a large cost and difficult challenge.”

Their next step is to expand their experimentation to other microorganisms beyond the E. coli that was used for this study.  In their conclusion the authors note that, “Silver is known to be an extremely general agent so it can be expected that this device will also work over a wide array of organisms.” 

We’ll continue to monitor their progress and hope for the best!

New Technique to Improve Crossflow Filtration

One of the biggest issues for crossflow filtration is figuring out how to control the loss of permeate flux in the process. Whether using reverse osmosis (RO), ultrafiltration (UF), or microfiltration (MF), the loss due to polarization and membrane fouling prevents many potential users in the biological or chemical processing fields from adopting this method.

If you are using crossflow filtration, or considering using it, and fear the effects of permeate loss, then you may want to consider this technique courtesy of North Carolina A&T University and the U.S. National Energy Technology Laboratory. Their study (see here) produced drastically improved results by implementing flow reversal to enhance the membrane flux.

They found that by periodically reversing the flow direction of the feed stream at the membrane surface results in prevention and mitigation of membrane fouling. This particular study conducted experiments with bovine serum albumin, Detran T-70, and apple juice. We’d love to hear from any of you in the field that may have tried this technique to see how it worked out!