I do not usually get to attend many technical sessions at the IFT Annual Meeting because I generally focus on the Expo, but this year I did have the opportunity to hear some interesting papers and observe some useful posters.
A session sponsored by the Citrus Products and the Fruit and Vegetable Divisions on juice and beverage spoilage by thermophilic aciduric bacteria (TAB) was especially relevant as I consult with acidified food and beverage manufacturers. Alicyclobaccilus is not a health issue, but it can be an economic issue because it can produce guaicol, a distinctively unpleasant compound, which has been described as tasting “medicinal, antiseptic, or like a disinfectant.” The U.S. Food and Drug Administration is not particularly interested in TAB because it is not a public health issue.
In general, the species grows in acid, can form spores, is an aerobe, does not produce gas, does not produce spores under anaerobic conditions, and has great genetic variability. It will not grow at 15°C, preferring warm conditions, so refrigeration is one protection. The specific variety that produces guaicol is A. acidoterrestris, which has been found in apple juice. It has variable D and z values, and so no one thermal condition will kill it. It is difficult to control so the best practice is to clean raw fruit before processing. Hypochlorite and hydrogen peroxide can reduce microbial loads. High pressure processing at 50°C for 20 min gave almost five decimal log reduction. E-beams and gamma radiation alone are not adequate, but they enhance thermal treatments.
A poster from a group at the U.S. Dept. of Agriculture Eastern Regional Research Center in Wyndmoor, Pa. (Sampedro, F., McAloon, A., Yee, W., Fan, X., and Geveke, D.), concludes that nonthermal processes such as pulsed electric fields (PEF) and high pressure (HPP) are more expensive and have higher greenhouse gas emissions in treating orange juice than do conventional thermal processes. The study actually treated juice using lab-scale units and measured energy use. The researchers estimated carbon dioxide production from the energy consumption of each process.
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Pasteurization Process Validation and Verification
Another session discussed pasteurization process validation and verification for flour and other dry ingredients. Validation means a process has been proven to work and that each step is controlled. Verification assumes the process is validated. The target pathogen is often salmonella. Biovalidation may involve testing against a surrogate or against the actual pathogen, though most manufacturers are understandably reluctant to introduce a pathogen to the plant. Desirable characteristics of a surrogate organism are that it be at least as resistant as the pathogen, that it be easily prepared, stable, easy to enumerate, and have similar attachment ability to the substrate (product). The Almond Board of California recommends E. faecium as the preferred surrogate. They also advise that test products and untreated controls be stored and shipped to simulate real world conditions.
A significant issue in dry pasteurization is how to handle the product after treatment to prevent recontamination. One solution is to treat in a protective package. Treating in bulk then requires a high level of sanitation, dedicated people and equipment, protective clothing, and frequent hand washing by personnel. FDA requires that pasteurization processes target the most resistant microbe of public health significance and that data on its destruction be provided. As there is little published data, FDA believes that challenge studies are probably necessary.
A group from the University of California-Davis (Taitano, L.Z., Singh, R.P., and Lee, J.H.) presented a poster with experimental data on the kinetics of water uptake in almonds during storage. Water uptake is the major constraint on shelf life of almonds. The data could be expressed either by Fick’s law of diffusion with a diffusion coefficient that varied with temperature or by a simple kinetic equation with a coefficient that also varied with temperature.
Dry Pasteurization Equipment
At least four competing processes were exhibited at the Expo. Safesteril (www.safesteril.com) is a French firm whose technology uses Joule effect heating of a transport screw in a steam-filled trough to heat dry powder product. Product that reaches target temperature is transferred to a cooling chamber, where cold, dry air is contacted. Product is screened, if necessary, and packaged. Rates are 50–10,000 kg/hr.
Florent CHEINET (www.fcdsystem.com) offers a slightly different continuous system in which transport is as a thin layer cascading over several vibratory conveyors that are heated by electrical resistance while steam is injected at the transition between conveyors. Cooling occurs on similar conveyors. Rates are 50–500 kg/hr.
Buhler Barth AG (www.buhlergroup.com/global/en/products/controlled-condensation-pasteurizing-system-ccp.htm) won an Innovation Award from IFT in 2010 for its controlled condensation system (CCP) in which a pressure vessel with a vertical ribbon agitator treats successive batches of product in a steam atmosphere. Cooling occurs in an external unit. Rates are 75–20,000 kg/hr depending on density and equipment size. The key to operation is preheating product before steam is introduced to minimize condensation. In all these systems, steam is required to make the target pathogens vulnerable to heat, but at the same time, moisture can damage the typical dry product such as nuts, spices, and herbs.
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Imtech-Steri AG (www.imtech-steri.ch) treats packaged products in batch stainless steel chambers using saturated steam. After the required short exposure, the product is quickly dried and cooled in the same chamber.
It is interesting that there are such varied options to implement essentially the same fundamental process.
Developments in Heat Transfer
Scraped surface heat exchangers (SSHE) have been around a long time, and they have their admirers and their critics. They do require regular maintenance and have wear parts. On the other hand, they are effective in heating and cooling viscous fluids. The Terlotherm from Terlet (www.terlet.com) is a unique SSHE in which both the outer shell and the inner rotor are heat transfer surfaces, significantly increasing the heating or cooling capacity of a given unit. Scrapers are easily replaced, and the metal parts are solid stainless steel, unlike some other devices in which surfaces are coated or plated.
Littleford Day (www.littleford.com) is best known for plow mixers in which a horizontal shaft holds triangular heads that fluidize the contents of a cylindrical vessel. Often there also are individually driven choppers entering through the side walls, between the paths of the plows. The vessel is usually jacketed so the contents can be heated or cooled. The combination of highly turbulent mixing and effective heat transfer means that the device can be used for many functions including extraction, drying, and biochemical reactions. Two new applications are sterilization and inoculation of fermentation media, avoiding contamination, and solvent extraction and drying.
AmeriQual Foods (www.ameriqual.com) is installing the first commercial microwave assisted thermo sterilization (MATS) process developed at Washington State University by a consortium that included the Natick Soldier RD&E Center, Kraft Foods, and others, who were recognized by IFT in 2010. AmeriQual co-manufactures foods in pouches and bowls for food companies and the Dept. of Defense (for military rations). The military, through Natick, has a continuing interest in technologies that preserve foods while retaining high quality. They have supported development of retort pouch foods, half steam table trays, high pressure processing and, now, MATS.
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A poster from the mechanical engineering department at the University of California-Berkeley (Berkeley, P.M. and Sunter, D.A.) measured temperature profiles during the baking of cake batter, and found that the process had three distinct phases: heating of the batter with no change in mass or volume; expansion (due to leavening) while losing minimal mass; and loss of mass due to evaporation with some shrinking. Temperature rises most steeply in the first phase. There is an abrupt change in the slope of the temperature curve at the start of the second phase. The cake is considered done at the end of the second phase. In practice, cakes are often over-baked, so the insights from this work could improve energy efficiency in baking.
CPM Wolverine Proctor LLC (www.wolverineproctor.com) offers the SCF II dryer with an ultra-sanitary design developed initially in cooperation with breakfast cereal manufacturers who were dissatisfied with conventional designs. It is all welded stainless steel construction with minimal legs, externally mounted hardware on sanitary spacers, no product seals, sanitary bearing mounts, and other features.
Bepex (www.bepex.com) showed the PCX dispersion dryer intended for wet cakes, pastes, and slurries. It has a rotor in a circular case that includes static or dynamic classifiers and application-based liners. It can use any of several external heat sources and collects the dry product in a cyclone and baghouse (like a spray dryer).
Dycem (www.dycem-cc.com) showed a unique Clean Zone floor mat that uses a polymer surface treated with silver to attract particles from shoes and vehicle wheels to prevent contamination of clean rooms. It has application in pharmaceuticals, electronics, and foods. The mat is easily cleaned when necessary, and the company routinely tests to determine when it may need replacement. It replaces peel-off mats that are said to be less effective, create waste, and release particles when they are torn up.
J. Peter Clark, Ph.D., is Contributing Editor to Food Technology magazine, and Consultant to the Process Industries, Oak Park, Ill. ([email protected]).