Some examples of food products processed under UHT conditions are:
• Liquids such as milk, juices, yogurts, cream, and salad dressings.
• Foods with small particulates like baby food, soups, sauces, and stews.
• Soy based products in order to inactivate bacteria and reduce off flavors.
The traditional problems or difficulties in using a UHT process have been:
• Sterility: The complexity of the equipment requires more highly skilled operators to maintain sterility through out the aseptic process.
• Particulate Size: With larger particulates comes the danger of overcooking of the product surface.
• Product Quality: Heat stable lipases or proteases can lead to flavor deterioration. In a number of cases flavor deterioration has caused a more pronounced cooked flavor for UHT milk as an example.
A major consumer complaint about UHT products has always been the so-called unpleasant “cooked” taste and sometimes-brown color of the finished product. This is understandable when we remember that dairy products in general, and specifically milk, are a colloidal mixture of water, lipids, carbohydrates, and proteins. When the mixture is heated under pressure to ultra-high temperatures, the protein structure is altered in such a way that some of the proteins are denatured and off flavors or browning can occur.
The conventional method for heating products to ultra-high temperatures has been to use indirect heating such as 1) plate and frame heat exchangers, 2) tubular-type heat exchangers such as shell and tube, or 3) scraped surface heat exchangers. The other current method for UHT production is steam infusion.
The goal of the equipment manufacturer today is not only to design equipment that can process product at increased product flow rates (over 35,000 liters per hour), operate continuously for more than 20-hours a day, and be easily inspected and cleaned, but also to design equipment that can minimize off flavors and browning.
Current UHT Cooking Methods
Indirect Heating
With this method the heating medium and product are not in direct contact with each other. As mentioned earlier, the types of heat exchangers are plate and frame, tubular, and scraped surface. The advantage of using these types of exchangers is that you do not need culinary steam since the two media are kept separate. However, each type of exchanger has its own disadvantages.
• Plate and Frame Exchangers: While they are typically easy to inspect and take up less floor space than other types of indirect exchangers, they are limited by gasket temperatures and pressures. In most cases the EPDM gasket is limited to a maximum 160˚C. The plates, while easy to take apart, can over time become fatigued as they flex from the constant high temperature steam and lower temperature product passing over the contact surface. Liquid velocities are usually lower for a plate and frame exchanger arrangement and as such can lead to uneven heating and potential burn-on and browning.
• Tubular Exchangers: While they have fewer seals and therefore do not suffer as much from gasket limitations or plate fatigue, they typically take up more floor space and are not easy to inspect. Heating may be more uniform, however browning or burn-on is possible because of the large surface area required to achieve the desired set point.
• Scraped Surface Exchangers: This type of exchanger forces product through a jacketed tube in which a set of rotating blades is constantly moving product from the outer walls toward the center. The product is more evenly heated and there is less opportunity for product browning or burn-on. Scraped surface exchangers are also more suitable for highly viscous products and products containing particulates. A general negative has always been the time and cost required for inspection and maintenance of this equipment.
A final point concerning indirect heat exchangers is that while they do a good job of heating product at a fixed liquid flow rate, they suffer when the liquid side flow rate varies. The potential for burning increases as the liquid flow rate decreases.
Steam Infusion
The general concept is to take the liquid product stream and have it pumped at a higher pressure through a distribution nozzle into a chamber filled with slightly lower pressure, culinary quality steam. This system is characterized by cascading a small volume of product through a large steam chamber. The product then collects at the bottom of the chamber and is fed forward via a timing pump.
Product temperature is generally controlled by pressure. Additional holding time is accomplished through the use of hold tubes, plate and frame exchangers or tubular exchangers. This is followed by flash cooling in a vacuum chamber where all added moisture is removed as needed. Variations of this method involve 1) pre-heating the product to a desired set point before the addition of direct contact steam, or 2) using the steam infusion method first followed by flash steam removal and perhaps reheating to a uniform set point.
All of these steam infusion methods accomplish the same thing:
• Instantaneous heating and rapid cooling.
• Lack of overheating or burn-on.
• Heating of low and high viscosity products.
• Use of variable product flow rates.
The negatives of steam infusion are:
• Size: The infusion chambers take up a sizeable amount of useable production floor space.
• Sanitation: These systems are not easily cleaned.
• Capital Outlay: Units require high, initial capital investment.
• Operations: A fairly skilled work force is required to monitor pressures, feed pump flow rates, etc.
• Need for a Timing Pump: Added equipment and operating cost.