Keeping track of process humidity

For a company whose claim to fame is that it 'bakes a better biscuit', McVitie's might be expected to be in the forefront of technological development. This is certainly the case with a new humidity measurement system, Hygrox, jointly developed by parent company United Biscuits and McQueen Cairns International.

According to Dr Gar Evans, recently retired as UB's director of food sciences, McVitie's started looking at ways of continually measuring humidity in its biscuit baking ovens in 1995. 'Early methods of measuring this parameter were largely manual,' he says, 'with the operators, or "ovensmen", inserting "fishslice" probes through doors in the ovens at various points along their length. But what was really wanted was a complete humidity profile along the whole length of the oven.'

Given that a typical production oven can be upwards of 150ft long, with up to six different heating zones along its length, this was clearly no easy monitoring task. But in collaboration with Brunel University and UB's group research and development division in High Wycombe, Bucks, McQueen Cairns has now launched what director Robin Cairns says is 'the first method of its kind available to the food industry to continually measure humidity.'

Humidity is important in baking for many reasons, from governing the spread of products laid down on the steel baking band, to ensuring the right degree of surface cracking on biscuits like ginger nuts. High humidity improves heat transfer by causing condensation on the cold dough as it enters the oven, while front end humidity will decrease the stack height of biscuits, an important factor when it comes to packaging. And an overall knowledge of humidity can give ultimate control of the oven damper settings.

In the past many of these operational settings have been decided by the ovensman in what Evans says has been a traditional craft business. But the industry, now operating on a global scale, 'wants to get away from that and move from subjective to objective measurements of operating parameters.'

In that sense, Hygrox could not be more objective, since the measuring device actually travels through the oven with the product, recording temperature and humidity levels as it passes through the different zones at temperatures up to 400°C. When removed from the band as it emerges from the oven, the Hygrox unit can be connected to a computer to download an accurate humidity profile.

Housed in a robust, thermally insulated, low profile metal box, the Hygrox system is based on two high-temperature zirconia sensors — one measuring oxygen and the other water. The humidity is calculated by measuring the difference between the oxygen in the oven and the oxygen available through water dissociation.

United Biscuits says the humidity profile provided by Hygrox allows the forced convection, gas-fired ovens to be set up correctly, ending the reliance on 'trial and error' and the know-how of experienced operators to get the best out of an oven.PE As well as the portable Hygrox-P unit, McQueen Cairns is introducing a fixed probe unit that can be located permanently for continuous monitoring when particularly tight humidity control is called for.

The Hygrox technology has also been trialed in a test rig at Brunel University on bread and cooked meat products — where the steam-cooking process is especially suited to the technology.

The base price of the portable unit is £12 500, complete with the carrier box. The units can continuously record for up to five hours, which is considerably longer than most industry applications require.