Selecting the best heat exchange solution for food pasteurisation

The texture and viscosity of food is not only an important quality parameter, it also influences how food behaves during production and processing. Therefore, it should be considered when choosing the handling and treatment equipment used in the manufacturing process. This is particularly true when this process has the potential to change the viscosity of the product itself, as can be the case with pasteurisation or cooking.

Pasteurisation

Pasteurisation differs from sterilisation in that it only aims to reduce the number of bacterial pathogens to a level where they are very unlikely to cause disease before the specified use-by date and potentially extend the shelf life of the product in question. For alkaline foods it will typically extend shelf life by several days, while acidic foods (with a pH above 4.5) can have their shelf life increased by months. In contrast, sterilisation aims to kill all pathogens present in the food or drink to prevent spoilage for long periods.

There are various different types of pasteurisation based on different combinations of heat and time. Two of the most common are high-temperature, short-time (HTST) and extended shelf life (ESL). When determining the factors required for effective pasteurisation, there are two key variables to consider:

1. The heat required to raise the temperature of the foodstuff by the right amount to destroy sufficient pathogens without changing other characteristics. This is known as the sensible heat.
2. The timing (or extent) of heat treatment required to stabilise the foodstuff based on the most resistant enzyme or microorganism present — the so-called ‘D-value’.

Other elements which characterise the quality of food, including flavours, colours and vitamins, also have their own D-values. It is often the case that using the HTST method will best preserve the quality of the foodstuff, but this is not true in all cases.

Viscosity

Viscosity is a measure of texture, which is itself one of the key quality characteristics of food and drink. It is a measure of a substance’s resistance to motion under an applied force based on the amount of force required to remove one layer in relation to another (shear stress) and the change in speed of the layers relative to each other (the shear rate). It is measured in units called centipose (cP), with one cP being equal to 1 mPa·s^-1 (millipascal per second).

Most fluids are classified as being either Newtonian or non-Newtonian according to the way that their viscosity changes due to shear stress and temperature. Newtonian fluids have the same viscosity irrespective of changes in temperature or shear stress; the most common example of this is water. Non-Newtonian fluids (which are further divided into five categories) have viscosities which fluctuate depending on the shear rate applied.

As viscosity is a key part of the texture of food and drink, processing operations should not have an adverse effect on it. Factors such as pumping, heating, cooling and passing through pipework all have the potential to affect viscosity, with the exact effects depending both on the liquid itself and the way it is handled.

Whatever foodstuff you are looking to heat or cool, understanding its viscosity properties is the first step to specifying a system which will not result in adverse effects on the quality of your product.

Pizza sauce problems

HRS Heat Exchangers was contacted by an international customer that was experiencing just such a problem. One of its products is a pizza sauce (a non-Newtonian fluid) for a popular restaurant chain. After cooking, the sauce was pumped through a multitube heat exchanger to cool it. However, the system pressure was so great that the generated shear stress on the sauce permanently changed its viscosity, making it thinner than it should be. In the worst case, the sauce was so thin that that it would not stay on the pizza base when used in the restaurants.

The original unit passed the sauce though a series of ½″ and 1″ tubes which generated high pressure. Replacing it with an HRS Unicus model heat exchanger using seven 3″ tubes resolved the pressure issues, while the Unicus unit’s oscillating cleaning system is much gentler than a rotary scraped surface heat exchanger, further helping to maintain the quality of the pizza sauce.