The main advantage of direct heating is that the product is held at the elevated temperature for a shorter period of time.For a heat-sensitive product such as milk,this means less damage.Direct heating can be accomplished with injection or infusion heaters.With injection,high-pressure/high-temperature steam is injected into preheated liquid by a steam injection,leading to a rapid rise in temperature.After holding,the product is flash-cooled in a vacuum to remove water equivalent to the amount of steam that condensed into the product.This method allows fast heating and cooling but is only suitable for some products.It is energy intensive and because the product comes in contact with hot equipment,there is potential for flavor damage.Infusion heaters operate by pumping the liquid product stream through a distributing nozzle into a chamber of high-pressure steam.Thissystem is characterized by a large sream volume and a small product volume,distributed into a large surface area of product.Product temperature is accurately controlled via pressure.Flash cooling in a vacuum chamber follows,leading to instantaneous heating and rapid cooling,with no localized overheating.
In indirect heating systems,the heating medium and product are not in direct contact,but separated by equipment contact surfaces.Several types of heat exchangers can be used,but plate and tubular systems are most common in the dairy industry.Plate heat exchangers are similar to those used in HTST but operating pressures are limited by gaskets.Liquid velocities are low,which could lead to uneven heating and burn-on.This method is economical in floor space,easily inspected,and allows for potential regeneration.Tubular heat exchangers,for example,shell and tube,shell and coil,or double tube,have fewer seals involved than with plates.This allows for higher pressures,thus higher flow rates and higher temperatures.The heating is more uniform but surfaces are difficult to inspect.