Crack the egg processing issue

Eggs are a key foodstuff and ingredient. According to industry data, Canadians eat an average of 242 eggs a year. While shell-on eggs accounted for the bulk of this consumption, processed egg products represented 30 per cent of the market in 2020. Processed egg products can take several forms, but the most common one is liquid.

Egg characteristics

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. Therefore, eggs are 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 due to the fact that the white and yolk are distinct components with different compositions and behaviours. When mixed, they interact mutually. For example, egg white is denatured at 136 F while yolk is denatured at 149 F. 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.

The importance of pasteurization

There may be a number of reasons to process eggs, including convenience, to extend shelf life or as part of other food processing and manufacturing operations. The main reason to pasteurize egg products is for food safety, but ease of use, improved hygiene and product uniformity are other motivating factors. Depending on the exact combination of treatment time and temperature, 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 sanitized 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 bacteria or contamination that may be introduced once the egg is cracked. Liquid whole egg and yolk should be held at or below 39 F, and egg whites below 44 F.

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

There can be many reasons, such as convenience and improved shelf life, to process liquid egg. Photo courtesy HRS Heat Exchangers

Technical challenges

For most liquid egg products, pasteurization using heat exchangers remains the main form of heat treatment. Various time and temperature regimes are used to pasteurize 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.

Pasteurization 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 pasteurization can decrease protein content, change physical characteristics such as texture and colour and increase product viscosity. Choosing the right pasteurization regime and equipment is therefore vital to minimize and prevent such unwanted effects.

Limitations of plate and smooth-tube heat exchangers

In the past, many processors have used plate heat exchangers to pasteurize egg products, but these allow products to coagulate on the plate surface, fouling the heat exchanger so that frequent cleaning-in-place 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). However, there can be issues around heat transfer efficiency and the necessary size of the exchanger to achieve effective pasteurization.

Corrugated heat exchangers

All of these issues can be overcome with the use of corrugated tube technology, which uses turbulent flow to reduce fouling. A corrugated tube has an increased heat transfer rate compared to a smooth tube of the same length, so the heat exchanger can be made smaller.

Matt Hale is international sales and marketing director, HRS Heat Exchangers.