Aseptic Processing: New and Old

The next new thing in aseptic processing is low-acid beverages in plastic bottles, according to Charles Sizer, Partner in Universal Foods and Beverages, Naperville, Ill. (phone 708-308-8903). Sizer, who recently left the National Center for Food Safety and Technology in Argo, Ill., and previously directed a laboratory for Tetra Pak, started his new firm to manufacture waters, beverages, and foods in a plant in Virginia.

While keeping his specific plans confidential for now, Sizer provided some hints by observing the opportunity for flavor improvement in packaging teas, now acidified, at closer to their normal pH of 4.8–5.6. These higher pH values make the beverages low-acid foods, requiring more rigorous sterilization processes and attracting more stringent scrutiny of the filling equipment by the Food and Drug administration.

Two manufacturers now have linear filling machines for plastic containers accepted by FDA. These are Tetra Pak, Vernon Hills, Ill. (phone 847-955-6000), and Stork, Gainesville, Ga. (phone 770-535-1875). Linear machines operate at relatively low filling rates of 200–300 bottles/min. A higher-speed technology is rotary filling, often used in Europe for fillers considered to be aseptic there but not yet accepted by FDA. The issues are the use in Europe of a sanitizer composed of peracetic acid and hydrogen peroxide and the concern that in a rotary filler, air flow is turbulent, not laminar. So far as is known, no rotary filling machines have yet been accepted as aseptic by United States regulatory agencies.

The objection to the chemical blend stems from some published research that asserts that the mixture is a sanitizer, not a sterilant. Straight (30%) hydrogen peroxide and heat is used in the U.S. for sterilizing aseptic containers and closures. However, hydrogen peroxide seems to have some interaction with polyethylene terephthalate (PET), a desirable material of construction for bottles. Current bottles used for aseptic filling are usually high-density polyethylene (HDPE), but it does not have the same clarity as PET.

Aseptic Processing
Some of the issues alluded to earlier may need to be clarified. Aseptic processing is the separate sterilization of a pumpable food, containers, and closures, followed by the cooling of the food and the filling and sealing of the containers in a sterile atmosphere. Once the container is hermetically sealed, it can leave the aseptic filling zone for further operations, such as labeling, cartoning, and case packing.

• Food. Sterilization of the food is normally achieved by heating, often in scraped-surface heat exchangers or tubular heat exchangers, or by direct steam injection. After heating, the food is held for a specified time, usually by flowing through a holding tube, and then cooled. Viscous foods may be in laminar flow in a holding tube. Since many foods are non-Newtonian fluids, the velocity profile, which determines the residence time distribution in the tube, may be flatter than for a Newtonian fluid in laminar flow. When particles are present, there is concern about the temperature profile that may exist between the fluid and the center of the particle.

Typically, it takes about twice as much heat-exchange surface for cooling as it does for heating. Scraped-surface heat exchangers are efficient, but can incur high maintenance costs, as they have bearings and wearing surfaces. Tubular heat exchangers have fewer moving parts. A common design is a triple tube in which food flows through an annular space between a center tube and an outer tube, both of which contain heating or cooling media. Hot water or steam is used for heating, and chilled water or glycol solutions for cooling.

• Containers. Sterilization of containers may be by heat or chemicals. Metal cans are sterilized with steam. Thermoformed containers are sterilized by the heat used to soften and deform a sheet of plastic. Paperboard or preformed containers are sterilized by chemicals—hydrogen peroxide in the U.S. Heat is used to dry and decompose the residual hydrogen peroxide.

• Closures. Closures may be metal lids for cans, sterilized by steam, or heat-sealed films for thermoformed packages or bottles, sterilized with chemicals as roll stock that may be perforated and from which the closure is cut or pushed.

• Bulk Containers. A special case of container is bulk containers, which may be composite bags or rigid tanks. The bulk bags are sterilized by irradiation, while the bulk tanks are sterilized by chemicals.

• Filling System. The filling environment is sterilized by heat or chemicals and then maintained by sterile filtration of air or inert gas. The filters themselves must be validated and sterilized, lest they become a source of contamination.

All fittings, valves, and penetrations of the filling area must be designed for ease of cleaning and for the ability to be sterilized in place. One of the reasons there are relatively few accepted aseptic fillers is the challenge of meeting the stringent requirements for FDA acceptance.

FDA does not “approve” aseptic processes or equipment. Rather, it accepts a filing detailing the design, process conditions,tests, and control procedures. Often, FDA raises questions or suggests additional tests until it is satisfied. At that point, FDA sends a letter saying the filing is accepted.

Sizer, while at Tetra Pak, led an effort to demonstrate how to verify a thermal process for a model food containing larger particles, but so far it does not appear that any commercial effort in the U.S. has been made to do so. Food particles smaller than 0.25 inch in diameter or on edge are considered homogeneous. Existing aseptically processed foods with particles include tapioca pudding, oatmeal, and a cheese sauce with jalapeño pieces.

Aseptically Processed Foods
Some foods that are aseptically processed include:
• Baby Foods. Gerber Products Co., Fremont, Mich., is not officially disclosing all of its plans to use aseptic processing, but it may in fact be one of the largest users of the technology. The company is committed to replacing most, if not all, of its traditional glass jars with plastic containers. Most fruits and some vegetables are already on the shelves in the new, thermoformed packages. Fruit juices have been in hot-filled plastic bottles for several years.

Gerber’s purees come in three main serving sizes (2, 4, and 6 oz), and some differ in particle size as well, for the different eating skill levels of different-aged babies. Fruits tend to be high-acid foods, while vegetables and meats are low-acid. Viscosity is also a factor in thermal processing and in filling.

Baby foods, compared to other foods that might be aseptically processed, have the advantage that they are inherently homogeneous and so do not pose the complexities of determining thermal processes for foods with larger particles.

• Cheese Sauces and Puddings. These are some of the older products to be successfully aseptically processed. Before plastic packaging became widely used, cheese sauces were processed for food service in No. 10 cans that had been sterilized with superheated steam and filled in a steam-sterilized environment. Lids were likewise sterilized with steam and seamed in the same environment. Puddings have been filled into small cans, but now are most often found in thermoformed polymer containers. These processes are still used.

• Tomato Paste. Tomato paste has been aseptically processed and filled into 55-gal drums for some time. The advantage is the improvement in quality from cooling externally rather than trying to cool a large container after hot filling.

Typical Process Equipment
Pat Foley, an owner of Advanced Process Systems (APS), Jeffersonville, Ind. (phone 847-821-9700) described his firm as a single-stop source for aseptic processing equipment. He and some of his partners were with Waukesha Cherry-Burrell in Louisville, Ky., but left amicably to form their own firm, focusing on aseptic processing of viscous foods using corrugated tubular and spiral-coil heat exchangers. The corrugation refers to embossments of the surface of the tubes to enhance the heat-transfer efficiency. The company partners with Custom Fabrication and Repair of Marshfield, Wis. (phone 715-387-6598), to fabricate complete process systems, including all types of tanks, pumps, valves, and necessary controls for the process requirements. They typically use Waukesha pumps and valves. Their equipment is used for purees, tomato products, coffee creamers, whipping cream, and other viscous foods.

Foodservice Opportunities
Jairus David of Rich Foods, Buffalo, N.Y. (phone 716-878-8768) believes that a very promising area for aseptic processing is foodservice, for semi-solid foods such as soups, sauces, cheese sauces, puddings, ice cream mix, and purees, especially in quick-serve and family-style restaurants, where refrigerated storage is short. Typical packages are 5- to 10-lb bags in boxes. Critical issues are filling and dispensing fitments, increasing processing rates, and maintaining product integrity and safety of low-acid foods at the point of sale. He points to the success of bag-in-box wine as a good example of the potential.

Bulk Aseptic Storage
Philip E. Nelson, Professor and retired chair of the Dept. of Food Science at Purdue University, West Lafayette, Ind. (phone 765-494-8256), has been responsible for major developments in bulk aseptic storage of foods. He described how, early in his career, he was interested in prolonging the tomato processing season in the Midwest, because of his own background with a family tomato business. A company then known as Bishopric, now Enerfab, Cincinnati, Ohio, agreed to help. Bishopric then was a fabricator of tanks for breweries.

Nelson and his group over a number of years developed valves, filters, and procedures to enable the storage of high-acid foods, such as tomatoes, in bulk tanks. A key discovery was the need to flood the tanks with a sterilant solution of iodophor. Spraying, as in cleaning, did not work because it did not sterilize the atmosphere. After the solution was drained, it was replaced with sterile nitrogen.

The iodophor was used at no more than 25 ppm, so that it did not need to be rinsed. Purdue researchers found that an epoxy lining was superior to even stainless steel, which also meant that tanks could be made of epoxy-coated carbon steel and therefore be less expensive than stainless steel.

Bishopric and Purdue were joint recipients of the Institute of Food Technologists’ 1976 Food Technology Industrial Achievement Award for the development and commercialization of the aseptic bulk storage system.

In parallel to the research on bulk tanks, work at Purdue helped to develop the bulk bag system commercialized by Scholle Corp., Northlake, Ill. (phone 708-562-7290). The original bags were used to store battery acid, but with the application of irradiation sterilization and design of sterile fitments to fill and empty, the bags could be used to store fruits and vegetables for later processing. Now it is common to store purees and concentrates of apples, peaches, pears, and mangoes and later repackage them as such or in other products.

Some low-acid foods are stored in bags, but not in bulk. Citrus juices are stored in very large tanks, up to 1.7 million gal, but must be refrigerated because they are subject to browning. Sea-going tankers with 16 half-million-gal tanks routinely take not-from-concentrate orange juice from Brazil to Europe and the U.S. Recognizing Nelson’s contributions, a scale model of such a ship was presented to Purdue. Separately, Mrs. Scholle has established a $1.5-million endowment in her late husband’s name.

Aseptic processing, from 2 oz to millions of gallons, is a vital and growing contributor to the food industry.

by J. PETER CLARK
Contributing Editor
Consultant to the Process Industries Oak Park, Ill.
[email protected]