PROCESS EXPO | Expert in Residence
Direct Heating Methods for Beverages
Steve Smith | Purdue University
Indirect heat exchangers exchange energy from a heating media (steam or hot water) into the product through a barrier such as stainless steel. This prevents the heating media from contacting the product directly.
Indirect methods of heating beverages, such as plate, tubular and scraped surface heat exchangers, have been traditionally used for the pasteurization and sterilization of beverages for many years. While plates and triple tubular heat exchangers are used for thin fluid beverages such as milk and juices, double tube and scraped surface heat exchangers are used for more viscous and even particulate foods such as cheese sauces and purees.
Lately, direct heating methods have shown to be a viable option for heating beverages such as dairy products. This method involves injecting steam into the product (steam injection) or injecting the product into a bath of steam (steam infusion). For both categories of direct heating, the steam will condense in the product. Therefore, precautions must be made.
First the steam must be of culinary (edible) quality. To create culinary steam, first we need to make sure that only GRAS listed chemicals are used in the boiler making the steam. One method is simply filtering the steam through a 2 micron mesh screen. However, some flavors from the GRAS listed chemicals may affect finished product flavor. So, often we can make steam from potable water, which should not affect finished product quality.
Second, we need to account for the added water. Generally, for every 18°F (10°C) rise in temperature, a 1% pick up in water occurs. We need to account for this water either by the formulation or by vacuum flash cooling. If, after heating and then holding the product, we run the diluted product into a vacuum chamber, water is flashed off and the product is rapidly cooled. Again, for every 18°F (10°C) reduction in temperature, a 1% loss in water occurs. So, if the temperature going into the direct heater equals the temperature exiting the vacuum flash cooler, there is no effective moisture change.
Compared to indirect heating methods, direct methods have the following advantages:
- Efficiency– Since almost 100% of the steam’s energy goes into the product, the efficiency of direct methods is much higher than indirect methods.
- Instantaneous heating/Rapid cooling– Because of much higher surface area and turbulence heating and cooling is almost instant.
- Less flavor change– Due to rapid heating and cooling.
- “Heating Methods for Ultra-High Temperature Pasteurization” by Michael Campbell: http://www.pickheaters.com/assets/uht_pasteurization_final.pdf
- “Handbook of Aseptic Processing and Packaging” 2nd edition David, Graves, Szemplenski.