Thermal treatment for product

Aseptic filling is a complex process in which both the filling and the product thermal treatment play a central role.

For effective aseptic filling it is essential to start with well-treated products. If a product is stable it will remain so in the bottle. If it is not stable, no aseptic filling machine can change its microbiological load. Aseptic filling is not possible for products that have high microbial loads; on the contrary they will contaminate the filling area.

Products generally need to be thermally treated before filling to kill or inactivate the microorganisms that are inside the products and to ensure stability throughout their shelf life. Heat kills microorganisms very effectively and has been widely used by food processors throughout history to ensure shelf life stability. Thermal treatment is much easier with liquids that are free of particles as it can sometimes be difficult for heat to penetrate the fibres. Beverages that contain particles or fibers require turbulent flow to prevent the flow channeling in preferential passages and to ensure an even heat dispersal.

**Afb.3.1. Thermal treatment for product**

3.1.

Heat Exchangers for Liquid Products

There are a number of different heat exchangers for liquid products including: plate heat exchangers; tubular heat exchangers which include single tube, multi tube, triple tube, spiral tube; and scraped surface heat exchangers.

When evaluating heat exchangers it is essential to match the capability of the technology with the needs of the process, for example: type of products, particles, fibers, viscosity, total solid content, thermal conductivity and salt. The temperature difference between the media and product sides is important as are the surface area of the heat exchangers, the surface material (typically stainless steel) and devices that increase the flow properties of the product.

3.1.1.

Plate Heat Exchanger

Plate Heat Exchangers (PHE) have multiple, thin, slightly separated plates that provide a very large surface area and multiple fluid flow passages for heat transfer. They provide the most efficient method of heat transfer because plates can be stacked together to provide a large surface area. Although the velocity of product through a PHE is lower compared with a tube heat exchanger, the narrow flow passages and thin transfer material can have higher heat transfer properties.

The thin parallel passages between the plates means that PHEs are not ideal for products containing a high level of fibers or for high temperature applications.

3.1.2.

Single Tube Heat Exchanger

The Single Tube Heat Exchanger (S-THE) is a tube within a tube. The broad tube diameter makes it ideal for processing viscous products such as puree, sauces, and products that contain large particles. They are ideal for all liquid products including those with high viscosity. Single tube heat exchanges can operate at high pressures but their low surface area means that their heat transfer capabilities are limited.

3.1.3.

Multi Tube Heat Exchanger

The Multi Tube Heat Exchanger (M-THE) consists of a bundle of inner tubes with the media flowing in the shell tube. Tubes can have various hole patterns, can be longitudinally plane or corrugated, and have different diameters to meet the needs of the application. A special flow device can be used to change the hole pattern thereby increasing the Reynold values. They are ideal for use with all liquid products including those with a high viscosity or that contain particles. They can operate at high pressures and are almost as efficient at PHEs.

**Afb.3.4. Inside a multi tube heat exchanger**

3.1.4.

Triple Tube Heat Exchanger

Triple Tube Heat Exchangers (T-THE) consist of three concentric tubes that provide a surface area of around twice that of the S-THE. Product flows in the annular space between the inner and the shell tube. They are ideal for viscous products, with or without particles, and can withstand high pressures.

3.1.5.

Spiral Tube Heat Exchangers

Spiral Tube Heat Exchangers (Sp-THE) have a coiled inner tube that carries the product, inside a shell tube. The heat exchange surface temperatures are lower than other designs, they are suitable for viscous or non-viscous products with or without particles and can withstand high pressures. Sp-THEs are not easy to inspect.

3.1.6.

Scraped Surface Heat Exchangers

Scraped Surface Heat Exchangers (SSHE) are mainly used for heating or cooling high viscosity products. This typically includes products such as stews and is used in crystallization processes, evaporation and high fouling applications. The continuous scraping of the surface allows longer running hours, avoids fouling and achieves sustainable heat transfer during the process. SSHEs are not easy to inspect, products have a long residence time and servicing costs are high.

3.2.

Indirect and Direct Heating

Technologies for UHT (Ultra High Temperature) and ESL or HHST (Extended Shelf Life, High Heat Short Time):

Ultra High Temperature (UHT) treatment for aseptic or ESL production can be achieved using either direct or indirect systems. All UHT and ESL plants have some part of the process that includes indirect heating using one of the above types of heat exchangers. Generally there is a barrier between the product and the media preventing any direct contact with the steam or heating media and the product. CIP cycles are determined by the extent of product fouling. Cleaning usually takes place under aseptic conditions (AIC) every 8 -20 hours; infusion can provide up to 30 hours of operation.

**Afb.3.7. Indirect Ultra High Temperature (UHT) system**

3.3.

Direct Heating UHT and ESL Designs

Technologies for UHT (Ultra High Temperature) and ESL or HHST (Extended Shelf Life, High Heat Short Time):

With direct heating the steam does come in direct contact with the product. The two methods of direct heating of product used for UHT and ESL are Injection and Infusion. Typically the product is heated directly from 75°C-85°C to 145°-150°C, held for 4 to 8 seconds and then flash cooled back down to between 75°C and 85°C.

3.3.1.

Direct Injection

Direct injection for the rapid heating of a product requires an indirect preheating process prior to the injection of saturated steam. After the temperature has been maintained for the required time the product passes through an aseptic de-aerator with vapor cooler to remove excess water and maintain the product solids at a constant level. The product then returns to the indirect regeneration zone before going to the aseptic filler or being stored in an aseptic tank. The injector pushes steam directly into the product resulting in an instant rise to the target temperature. The main advantage of this method is that the product is less denaturized compared with the indirect process. Furthermore the injector can be supplied with an automatic gap control to prevent fouling.

The heat holding period determines the killing rate of germs and spores. A sophisticated pressure control valve or orifice then flash cools the product in an aseptic de-aerator. The product immediately gives up its enthalpy as product vapor. It is necessary between the inlet temperature of the injector and the outlet temperature from the de-aerator must be accurately controlled to ensure that the product is not adulterated with steam condensate.

Heat recovery is more difficult and control, more complex than the indirect process, so costs are generally higher.

**Afb.3.9. Direct injection ultra high temperature (UHT) system**

3.3.2.

Direct infusion

Direct infusion is an alternative method for the rapid heading of a product.

**Afb.3.11. Injection nozzle with inside and outside steam**

The process uses a pressurized tank filled as required with almost saturated steam. The product is heated as it flows down through the tank without touching the inner surface. There are various infuser models including the optimized model that has the product stream and the steam supply in the identical direction from the top to the bottom: product flows in an open ring with an adjustable gap between the internal main steam and a surrounding support steam. This guarantees that no product touches the hot inner surface of the infusion tank. A centrifugal pump empties the infusion tank and pushes the product by over pressure into the heat holder.

The infusion mixes steam directly with the product resulting in an instant rise of the target temperature, by identical pressure, with nearly no delta T. After heat holding the process is identical to injection technology (above). Heat recovery is more difficult and control is more complex than the indirect process so costs are generally higher.

The biggest advantage of this method is that there is minimal damage to the product compared with the indirect process and injection. This method uses very low delta T compared with injection and indirect heat exchange between the steam and the product outlet which ensures optimal product quality.

3.4.

The best heat exchanger for your application

It is very important to know as much as possible about the product prior to selecting the correct process technology.

3.4.1.

Heat Damage to food

In general products need to be thermally treated to kill germs and spores thereby maintaining their shelf life, however the treatment can often cause product damage.

The following graphs show the approximate relationship between temperature and time: the two key factors that cause product damage. It is clear that there is a significant difference in the time that products are exposed to heat between the direct and indirect process. This is because the efficiency of indirect heating is much lower than direct heating.

**Afb.3.13. Aseptic comparison between direct and indirect heat treatment**

3.4.2.

System Selection Criteria

The indirect process is much less expensive and more flexible making it suitable for use with various products. Although energy consumption is much higher some products are better suited to direct processing therefore the choice of technology should be driven by the application.

Direct or indirect technology is chosen for each application depending on the products, markets, and desired level of quality, freshness of taste, etc.

Pros for Processing with Indirect Heating Systems:

They deal well with fibers, and particulates
Do not require culinary steam
In fluid milk up to 90% of energy can be regenerated
Does not require flash cooling of fluid milk to remove the condensate from the milk
Overall system cost is much lower than direct, even though an injector costs less than a PHE

Cons for Processing with Indirect Heating Systems:

Causes greater damage to protein and other solids
Cannot usually achieve high production hours >16 h
Product is much more likely to be burned, the residence time is typically minutes compared with seconds in direct processing
Greater protein denaturing in fluid milk
Accelerates the Mallard Browning Effect in fluid milk at high heat penetration

Pros for Processing with Direct Heating Systems:

The product is only kept at elevated temperatures for a few seconds so there is less product damage. This reduces protein denaturing, retains flavor, and helps preserve the original color
The flash cooling also removes other undesirable volatiles such as gasses, O², onion grass flavor, etc
Virtually eliminates fouling

Cons for Processing with Direct Heating Systems:

Requires min. 2,5 times more steam which must be of culinary quality
Requires min. 3 times more cooling media
Requires more capital investment
Has higher variable costs
High noise level for injection

Advantages of Injection over Infusion:

Lower capital cost
Less floor space
Possibly easier to clean in place

Advantages of Infusion over Injection:

Longer running production
Easier control of infusion
Noise level below 75 dB
Minimum denaturation
Best quality
Minimum damage
Less fouling because of lower delta T

Selection of Technology by Application:

There is no one system that is ideal for all aseptic food processing. The system selection must be driven by the requirements of the application. The following chart shows some typical products and a suggestion for the best suited technology for each application.

Aseptic Processor Type

**Tabel. 3.1.**
Product Family	Indirect tubolar	Indirect plate	Direct stem heating with flash cooling	Scrape-surface heat exchangers	Raw-side deaeration
Clear juices	x	x	(x)		(x)
Fruit juices with limited pulp	x	(x)			x
Fruit juices with long fibers < 10 mm	x			(x)	x
Thick products	(x)			x
Fruit puree with large particles > 10 mm	(x)			(x)
Milk and milk-based beverages	x		x		x
Tea	x	(x)
Shake mix	x		x		(x)
Custard and puddings	x		(x)		(x)
Soups and stews with particles	x			x

3.5.

Conclusions

All applications are different and technology should be selected to meet specific needs depending on the product, markets, and budgets. The wrong choice can adversely affect product quality and characteristics. The selection of technology should focus on optimizing product quality, productivity, and both capital and operating costs.

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