UHT, or Ultra High Temperature, treatment uses high temperature for a short time to kill micro-organisms in a food or beverage product. It is widely used in the dairy industry where products can easily lose their nutritional value, flavor and appearance as micro-organisms multiply. These organisms thrive at certain temperatures but if they are not present in a product, it can be stored for many months without the need for refrigeration. This state is known as “commercial sterility.”
UHT processes are designed to achieve commercial sterility. They use heat to treat the product, killing viable microorganisms. This aseptic processing also requires equipment and packaging to be sterile, achieved with chemical or other sterilization treatments.
There are many technologies available for the UHT treatment of products. Direct heating methods, such as steam injection or steam infusion, mix the heating steam with the product. Indirect heating methods, such as plate or tubular heat exchangers, keep the product and heating medium separate with a stainless steel barrier between them.
The choice of direct or indirect heating and the system utilized depends on many factors including product specification, viscosity, and sensitivity to high temperatures. Along with the quality of the raw materials used, the system chosen has a big impact on the characteristics of the final product.
Steam infusion and steam injection are both direct heating systems. Steam injection injects steam into the product to be treated and steam infusion adds the product to the steam. The steam must be high-quality culinary steam and not contain any off-flavors that could change the taste of the product.
Steam infusion is one of the more expensive options to install so why do those who try it tend to repeatedly use it?
STEAM INFUSION PROCESS
Product is pre-heated and then pumped into an infuser, which is a pressure vessel with cones at either end. A steam infusion chamber typically has multiple nozzles through which the product is distributed and enters a steam atmosphere in a number of jets without hitting the wall of the vessel until it reaches the bottom cone.
Product heating is almost instant (typically 0.2 seconds) due to the latent heat of vaporization of the steam. The steam dilutes the product but is later removed in a vacuum chamber as the product cools.
The steam infusion chamber provides instant, gentle heating—achieved because of the short time at high temperature and a low-pressure differential between the product and the steam. This gives exceptional results with minimal chemical change to the treated product compared with other conventional UHT systems. The short time at high temperature helps a product to maintain its fresh taste while providing exceptionally high bacteria kill rate for long shelf lives.
A cooling jacket on the bottom cone keeps the wall temperature below the product temperature and limits any burn-on and fouling. If the product leaves the infusion chamber through a pump mounted directly below the chamber, this ensures sufficient pressure in the holding tube to keep a well-defined, single-phase flow, which is substantially free from air and steam bubbles. This is important to ensure precise control over holding times and temperature—both of which greatly impact the final quality of the product.
Flash cooling takes place in a vacuum expansion vessel with precise control to ensure the correct amount of water is removed and the product is not diluted or concentrated from its original state.
Steam infusion can be used for many products including milk, creams, puddings, ice cream, baby food, condensed milk, processed cheese, and sauces.
Using gentle heating of steam infusion sterilization causes minimal heat degradation compared with other UHT processes which helps to protect essential components such as vitamins and results in fresh tasting products. It provides the necessary kill rate for commercial sterility and can handle a wide range of products and viscosities ranging from thin soya milk to thick custards and puddings. A steam infusion sterilizer may have a capacity from 150 l/h (with pilot plants) up to 44,000 l/h.