Chapter

Recent progress in aseptic processing of food

Thomas E. Szemplenski

Aseptic Resources, Inc., Overland Park, Kansas, U.S.A.

1. INTRODUCTION

Aseptic processing and packaging of food was invented by C. Olin Ball in the 1930's as the Dole Aseptic Canning system [1], For many years the system was improved upon by Dr. William McKinley Martin and others, but the Dole Canning System still remained the only equipment for aseptically containing food and dairy products.

Aseptic processing and packaging continues to be one of the most dynamic areas of food processing. New methods of processing and packaging are continually being researched, invented and commercialized. Even during the stages of publication of this book other improvements will be made, thus making it difficult to introduce a comprehensive collection of the latest technology.

The focus of much of this research centers around the aseptic processing of low acid foods containing particulates. Until recently, only low acid homogeneous foods were aseptically packaged, but continued research with products containing particulates has paid off and the result of this research is affording tremendous opportunities for growth. Acid foods containing particulates of significant size have been aseptically processed since the early seventies, thanks mainly to the development of the Scholle bag-in-box filler that allowed the filling of particulates [2], Products such as fruit for yogurt and ice cream toppings, sliced fruit in syrup, diced peppers and even diced tomatoes have all been aseptically filled for years with the Scholle and other similar bag-in-box fillers. The sterile products going into these packages are all for institutional use or for the reprocessed market. The processing equipment development and design changes that allow significant particulate processing for acid foods are now starting to be applied to low acid foods. It appears that the largest market for aseptically processed low acid foods is not in the larger bag-in-box containers, but in the retail sized and institutional pouch sized containers. In this regard, additional developmental work is needed to aseptically fill particulates into these containers. In pursuit of this goal several new developments are under way. Some of the more important ones will be considered in this chapter.

2. NEW DEVELOPMENTS 2.1. Ohmic heating

The Ohmic heater was developed by EA Technology at Capenhurst, Cheshire in the United Kingdom [3]. It is direct electrical resistance heating of food products by the passage of current through the continuous flow of product. The passage of current generates heat which sterilizes the food. The depth of heat penetration is unlimited.

As mentioned above, it is one of the food industry's goals to continuously sterilize food products containing larger and larger particulates. Present aseptic processing systems most often utilize scraped surface heat exchangers for heating and cooling food products containing discrete particulate matter. In this regard, most of the damage that is done to the particulates in aseptic processing systems is caused by heating with scraped surface heat exchangers. Due to the inherent design of the Ohmic heater, larger particulates are able to be processed with improved identity. The Ohmic heater is already being used to sterilize low-acid particulates up to 25mm [4],

Particulate identity is not the main advantage of the Ohmic heater. The main benefit the Ohmic heater offers is in the method of heating. The Ohmic heater heats and sterilizes the food quickly and uniformly as the electrical energy is transformed into thermal energy volumetrically throughout the bulk of the food product. This is unlike scraped surface heating which is accomplished almost entirely by conduction. The degree of penetration with Ohmic is unlimited and the extent of heating is controlled by special uniformity of electrical conductivity throughout the product and its residence time in the heater.

A typical in-line heater and power supply will consist of three separate modules: the heater assembly, the power supply, and the control panel. In its simplest configuration, there will be four electrode housings and three interconnecting tubes. The electrode housings are machined from a solid block of polytetrafluoroethylene (PTFE) and are encased in stainless steel. Each contains the working electrodes which are connected to each phase of the secondary side of the transformer.

Electr

Electr

Product To Be Heated

Alternating Current Power Supply

Elect!

Elect!

Figure 1. Principle of Ohmic heating

(Courtesy of APV)

The interconnecting tube sections of the heater are of stainless steel with suitable electrically insulated plastic lining. The suitable lining material is capable of withstanding temperatures in excess of 145°C. The tube sections are bolted onto the electrode housing by means of flanges. Product sealing between tubes and electrode housing is by the use of flat rubber seals. With some heat sensitive food products, the interconnecting tubes will be jacketed and cooled to counteract the effect of longer residence times near the inner surface of the tubes. The complete Ohmic assembly is mounted in a vertical or near vertical position to ensure complete filling.

At the onset, the energy costs to use the Ohmic heater might seem to be more expensive than conventional tubular or scraped surface heat exchangers as electricity is thought to be a very costly energy source; however, the conversion efficiency is greater than 90% compared to only 45-50% for alternative energy forms, so the overall cost to heat a comparable amount of food product is similar.

The Ohmic heater cannot be used for all food products. The applicability of Ohmic heating depends on each product's electrical conductivity and on whether the product is an insulator or a conductor. Insulators cannot be heated with the Ohmic heater. Some insulators include non-ionized fluids such as bone, fats, oils and alcohols. The Ohmic heater generally cannot be used to heat tap water unless some salts are added to increase the conductivity. Fortunately, a great many foods contain moderate amounts of free water with dissolved ionic salts and are therefore conductors. Most pumpable food products with water in excess of 30% conduct sufficiently for the Ohmic heater to be applied.

Product Outlet

Figure 2. Diagram of Ohmic heater electrode wiring. Source: APV

Pre-sterilization of an aseptic processing system utilizing an Ohmic heater is done by recirculating a solution of sodium phosphate or sodium citrate at a concentration which approximates the electrical conductivity of the food product to be processed. The temperature necessary to achieve sterilization is reached by heating the recirculated solution with the Ohmic to the superelevated temperature with an aseptic back pressure valve in the system.

Although there are obvious advantages to this new technology, there are also some drawbacks. The Ohmic cannot be used to create a phase change such as gelatinizing starches or other stabilizers. Starches must be pregelatinized prior to processing. The optimum temperature the food product should be at prior to processing is 55°C. Therefore, the heat-stable stabilizer must be heated to between 80°C and 85°C to effect gelatinization and then cooled back to 55°C by more efficient alternate heat exchangers. The particulates are then added and the temperature brought to equilibrium prior to entering the Ohmic for sterilization. This is very time consuming and not energy efficient.

The obvious advantages of Ohmic heating are:

* Particulates and carrier medium are heated virtually simultaneously

* Very rapid heating (1°C per second)

* No moving parts

* No hot heat transfer walls

* No obstruction in the heating area like scraped surface heat exchangers

* No fouling of the heat transfer walls

* No noise

* Low maintenance

There are 12 commercial installations in the world heating and sterilizing such products as strawberries, ready meals, meat, poultry and pet food. Expect many more installations as the learning curve is expanded. Commercial systems are available in 75 kilowatt (kW) and 300 kW sizes rated at 1650 and 6600 lbs. per hour, respectively [5,6].

2.2. High voltage pulsed electric field heating

Another heating/sterilizing system under development is the use of high voltage pulsed electric field process heating. Research centered around this technology is taking place in both the UK and the United States [6]. In the UK, Sale and Hamilton found that pulsed high voltage electric fields (up to 25 kV/cm) produced different stages of kill to vegetative bacteria cells in yeast depending upon pulse length, number of pulses and field strength [7]. The lethal effect was not due to heating and was independent of current density and energy output; damage to the cell membranes appeared to be the cause. The pulsing action of the electric field causes an irreversible loss of membrane function because of selective osmotic barrier between the cell and its environment.

Although electric field sterilization has not been commercialized, the research continues in the United States, the UK and Japan. Electric field sterilization offers the possibility of cold sterilization followed by aseptic packaging.

23. Isostatic high pressure sterilization

One of the most exciting new technologies for commercial sterilization of food products is the use of isostatic high pressure. Isostatic means the pressure is equalized from all directions so that neither the product nor the package is damaged. High pressure sterilization is in it's infancy, but it is being researched in the UK, Sweden, France, Germany, Belgium, Spain, Japan and a number of places in the United States [8], There are a couple of high acid products on the market in Japan and the first low acid product will soon be introduced in France [9].

High pressure processing not only destroys microorganisms, but also deactivates enzymes and can gel diced fruit to create preserves. Depending upon the product to be processed, commercial sterilization with pressure can be affected with as little as 3,000 bar to as much as 200,000 bar of pressure. Vegetative microorganisms can be inactivated at pressures as low as 3,000 bar at room temperature.

Figure 3. Schematics of high-pressure processing of food. (Courtesy of Yamauchi, Food Eng. Magazine)

The lethal effect is strongly influenced by the food product's composition (especially pH and water activity). Certain proteins, polysaccharides and organic acids can have a protective or synergistic effect on inactivating microbes. Bacterial spores, however, are extremely pressure resistant at room temperature. Research indicates that high pressure coupled with a moderate temperature of 70 to 80°C is very effective (Chapter #9 in this book deals exclusively with high pressure processing) in producing a commercially sterile food (10).

2.4. Corrugated tubular heat exchangers

Tubular heat exchangers have been used for aseptic processing for years to sterilize and cool liquid products such as milk and coffee creamers. It was not until recently that design changes allowed tubular heat exchangers to be used for viscous products and products containing particulate matter.

In the late 60's there was a boom in the number of aseptic processors packaging puddings. All the systems utilized scraped surface heat exchangers to sterilize and aseptically cool the pudding prior to packaging. Scraped surface heat exchangers were used to facilitate the high pressures incurred by other types of heat exchangers and to

Figure 3. Schematics of high-pressure processing of food. (Courtesy of Yamauchi, Food Eng. Magazine)

prevent protein burn-on in the systems. Aseptic product with particulates was not an issue as there were no aseptic systems for processing particulates until the mid 70's. The first aseptic systems for processing acid products such as fruit for yogurt and ice cream mix also were designed and installed using scraped surface heat exchangers. Scraped surface heat exchangers are expensive, costly to maintain and are not very energy efficient is not feasible to capitalize on regeneration with them.

Figure 4. Corrugated tubular heat exchangers (Courtesy of Cherry Burell Corp.)

In an effort to lower the capital and operating costs of aseptic processing systems, improvements were made in tubular heat exchange designs. One of the most significant design changes has been the use of corrugation [11]. Corrugation has allowed higher heat transfer rates, less burn-on resulting in longer production runs, and wider tube design allowing the tubular heat exchangers to be used to aseptically process significant particulate. Temperatures up to 185°C and pressures up to 100 bar can now be attained in aseptic systems with the use of corrugated tubular heat exchangers [12]. These newly designed tubular heat exchangers are now replacing most of the systems using scraped surface heat exchangers that were installed in the 60's and 70's. The new tubular heat exchangers are also being used to aseptically process products such as diced tomatoes (2 centimeters), sliced and diced peaches, sliced strawberries, blueberries, etc. [13]. The corrugated design also facilitates the processing of viscous products containing protein, such as pudding, without the loss of efficiency due to protein burn-on that resulted with older designed tubular heat exchangers. Product-to-product and water-to-water regeneration is easily attained with tubular heat exchangers, thus making them far less costly to operate than scraped surface heat exchangers.

2.5. Marlen pump

The Marlen reciprocating piston pump is doing more for the growth and renewed interest in the aseptic processing of particulates in the food industry than any other new technology. The inherent design of the Marlen has allowed the largest particulates to be processed. It also has zero slip regardless of viscosity of the product being pumped and it has made the change from initial sterilization water to product extremely simple by the changing of one switch on the control panel.

The operating principle of the hydraulically driven Marlen is unique. It consists of two reciprocating stainless steel pistons. Each of the pistons has a stainless steel sleeve around it. With either water or product in the hopper of the pump, one sleeve loads while the other piston is pumping. During loading the sleeve and piston retract together, filling the cavity with product. The sleeve then moves forward to fill and trap the product. The product is also automatically precompressed to the operating pressure of the system. When the pumping piston finishes its stroke, a valve automatically switches and the other piston starts forward while the first sleeve reloads with product. During the switching of the pistons there is negligible upset in pressure. The Marlen pump comes with either 15 or 18 cm diameter pistons which pump up to 5,442 and 8,164 kgs/hr respectively, at up to 30 bar pressure.

Figure 5. Marlen pump

(Courtesy of Marlen Research Corp.)

Figure 5. Marlen pump

(Courtesy of Marlen Research Corp.)

Another unique feature of the Marlen which facilitates aseptic processing is a sterilization mode in which the sleeves stay sealed shut, but the pistons are in operation. During loading, instead of the sleeve retracting, a valve in the front manifold opens and allows water to fill the sleeve while water is being pumped out the other side. This feature allows the Marlen to be used for pumping the initial sterilization water with zero slip and allows the operator to set production flow rate and temperature with the sterilization water.

Product to be processed is in the hopper of the Marlen while sterilization is taking place. When sterilization is completed and product is ready to be processed, all the operator must do is change one position of a switch on the control panel. On the very next time to load a sleeve the water valve remains shut and the sleeve retracts to load the product. When the other sleeve pushes the last water out, the product immediately follows the water making the switchover extremely easy.

2.6. Combibloc Comitop

PKL Verpackungssysteme.manufacturer of the Combibloc aseptic filler for pre-formed cartons, announced a new technology in a resealable fitment. The fitment is an injection molded, single-piece fitment that is attached to the carton after filling. This allows the carton to be opened, some product poured out, and the carton resealed. Of course, after opening, the remaining product is not shelf-stable and in most cases must be refrigerated.

Figure 5. CombiTop carton

Courtesy of Combibloc USA

Figure 5. CombiTop carton

Courtesy of Combibloc USA

Trademarked and patented, the ComiTop fitment consists of an easy open clip closure with a sealing tongue, a vent hole, unsealing flap, channels, and a pouring lip. All one has to do is "lift" the lid and "push" on the tab. The internal tab serves as an opener to break the package seal. The resealing is strong enough to sustain stacking, shaking and even being stored on it's side without leaking. It's also strong enough to protect the contents from migration of other flavors.

The ComiTop fitment is applied to the package by downstream equipment after filling and sealing. The carton is specially manufactured to accommodate the ComiTop. The outer polyethylene layer is score-cut with a special tool that leaves the inner layer of aluminum and polyethylene untouched. Sterility and shelf life are not compromised. After scoring, filling, and sealing, the cartons are conveyed to the applicator.

Although not available yet, competitor Tetra Laval advises they will have a reclosable container in the near future. Tetra will start with new technology with the 1 liter size package.

2.7. Aseptic pouches

Within the last several years, two packaging manufacturers have developed and commercialized aseptic pouch fillers. Inpaco, located in Nazareth, Pennsylvania, USA and Robert A. Bosch located in Waiblingen, Germany, introduced aseptic pouch fillers for food products. Bosch's first introduction was for acid food products, whereas Inpaco's first introduction came as a result of building an aseptic pouch filler for pharmaceutical products. Each now has US Food and Drug approval for aseptic packaging of low acid foods.

Figure 7. INPACO aseptic pouch filler

(Courtesy of INPACO Corp.)

Figure 7. INPACO aseptic pouch filler

(Courtesy of INPACO Corp.)

Each of the fillers sterilizes the packaging material with hydrogen peroxide followed by sterile hot air for drying. Filling rates are approximately the same with Inpaco filling at 32 #10 size pouches per minute and Bosch at 24/25 pouches per minute. Each of the fillers can fill from 1 to 5 liter packages.

As the cost of rigid containers continues to rise, it is expected that the demand for aseptic pouch fillers will be the fastest growing segment of the aseptic packaging industry. This market is economically driven, as the cost of #10 (3 liters) sized pouches is 45 to 50% less expensive than a case of #10 cans. With this kind of cost difference economic justification of the cost of the filler is usually in months depending upon production use.

There are other advantages of aseptic pouches that will contribute to the worldwide demand for these filling machines. Some of the other more paramount advantages of flexible pouches compared to #10 cans are:

* Greater safety

* Lower warehousing cost

* Lower shipping costs

* Less space required

* Less residual product loss

* Easier to open

* Disposal savings

2.8. Aseptic ship

One of the newest innovations in aseptics was introduced in 1993 [14], The Ouro do Brasil is a ship built and dedicated to aseptically transport citrus juices for Citrosuco Paulista of Brazil to Europe, Japan and the United States. The ship was built in Norway with technology that was originally developed at Purdue University in West Lafayette, Indiana, USA.

Figure 8. Aseptic ship

(Courtesy of Citrus Coolstores, Inc.)

Figure 8. Aseptic ship

(Courtesy of Citrus Coolstores, Inc.)

The ship can aseptically transport single strength juice or concentrate in 16 vertical stainless steel tanks each holding 200,000 gallons. The ceiling, walls and floor in the hold of the ship are all stainless steel. In that, all 3.2 million gallons can be shipped. The Ouro do Brasil is 564 feet long. It is so large that it had to be built in two sections and welded together in the water. The success of this venture has been so great that another ship is being built.

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