Featured Story – Thermal processing of egg products

The majority of egg products we come across in the supermarket have been pasteurised. Perhaps you grew up with the advice to never eat raw egg as they would make you sick, only to discover a decidedly low risk of illness having licked clean a bowl of leftover cake batter after a Sunday spent baking with family. The likely reason for that is the standard pasteurisation of eggs sold in shops in most countries to combat the risk of salmonella in such raw products.

But the method of pasteurising processed egg products is complicated. Fresh eggs have a thick white and an upstanding yolk. Over time, the white thins and the yolk spreads and enlarges as water passes through the membrane from the white into the yolk, weakening it. Because of this, eggs must be refrigerated and processed quickly, usually within a week.

Liquid egg is a very delicate product as the proteins in egg are more sensitive to heat than other products, such as milk or juices.

This is because the white and the yolk are distinct components with different compositions and behaviours.

When mixed, they interact mutually –for example, egg white is denatured at 58°C while yolk is denatured at 65°C.

These low temperatures also make it hard to aseptically process natural liquid egg products; the eggs are frequently cooked before the required time and temperature minimums are achieved.

There may be a number of reasons to process egg products, including convenience, to extend shelf life or as part of other food processing and manufacturing operations. The main reason to pasteurise egg products is for food safety, but other reasons include ease of use, improved hygiene and product uniformity. Depending on the exact combination of treatment time and temperature used, it is possible to produce a shelf life of up to 16 weeks for refrigerated liquid egg products. Irrespective of the treatment method used, it is important to use fresh, clean and sanitised eggs, and to chill and filter them immediately after breaking. The contents of an egg are essentially sterile until broken, so one of the aims of processing is to reduce or eliminate any bacteria or contamination which may be introduced once the egg is cracked. Liquid whole egg and yolk should be held at or below 4°C, and egg whites below 7°C.

Yolk and whole egg products are generally pasteurised in their liquid form, while liquid egg white may be pasteurised when sold as a liquid or frozen product. In contrast, dehydrated egg yolk (with the glucose removed) is normally pasteurised by holding containers in a large chamber over several days.

For most liquid egg products, pasteurisation using heat exchangers remains the main form of heat treatment. Various time and temperature regimes are used to pasteurise eggs depending on the product, which could be whole egg; separated egg (whites or yolks); or a treated product, like salted yolk.

Each type of product presents a different challenge in terms of viscosity, and products with added salt also introduce a higher likelihood of equipment degradation or corrosion.

Pasteurisation can have a number of unwanted effects, including gel formation and softening of the yolk, or irreversible denaturation of the proteins and changes to the appearance. If not handled correctly, thermal pasteurisation can decrease protein content, change physical characteristics such as texture and colour, and increase product viscosity. Choosing the right pasteurisation regime and equipment is therefore vital to minimise and prevent such unwanted effects.

In the past, many processors have used plate heat exchangers to pasteurise egg products, but these allow product to coagulate on the plate surface, fouling the heat exchanger so that frequent cleaning-in-place (CIP) is required to maintain operational efficiency. This adds time, energy and cost to the processing, and also reduces overall capacity.

Tubular heat exchangers overcome some of these problems (for example, the larger diameter helps the product to run through the heat exchanger more easily) but there can be issues around heat transfer efficiency and the necessary size of the exchanger to achieve effective pasteurisation.

Fortunately, these issues can be overcome with the use of corrugated tube technology as employed by HRS, which uses turbulent flow to reduce fouling. Because a corrugated tube has an increased heat transfer rate compared toa smooth tube of the same length, the heat exchanger can be made smaller. It is also important that the equipment chosen allows regular inspection and suitable CIP. Not only do HRS corrugated tube heat exchangers facilitate this, but because their design helps to prevent fouling in the first place, they also reduce downtime.

Therefore, the operational run times between cleaning cycles are generally much longer with corrugated tubes than smooth ones, further increasing the overall efficiency of the process.