R Au-Young, of Milson, asks :-

How do you dry spring onions and vegetables keeping the colour and flavour?

Michael Parker, a food technologist at Massey University's Institute of Food, Nutrition and Human Health, responded.

Drying is a common method for preserving vegetables as reducing the moisture content slows and can minimise reactions that cause the food to deteriorate. Unfortunately, the drying process itself can cause unwanted changes including loss of colour, loss of flavour and change in texture. The ideal process would result in the necessary drying but without the deteriorative reactions. However, real drying process generally always result in unwanted changes.

The most common method of drying is to place food on a mesh tray and pass hot dry air over the product that picks up moisture from the product. The extent of drying depends on the temperature of the air and the drying time and also the moisture content or humidity of the air. High air temperatures generally result in shorter drying times, but also tend to cause greater unwanted changes to the food. Increasing the air velocity tends to reduce the drying time also.

A type of air drier to maximise the drying rate is fluidised bed drying that involves placing the product on a mesh and passing air at such a high velocity that it causes the product to float which greatly reduces the drying time.

The best method for drying in order to retain the properties of food products is freeze drying, in which the product is placed in a chamber and frozen and then the chamber is reduced to a very low pressure such that heating means the ice sublimes into a vapour and removed, rather than melting into a liquid. However this process is very expensive.

For drying spring onions using hot air there are three possible methods. Place the spring onions (which could be cut into soup size pieces) in a thin layer on a tray and then pass air over it at 80°C for 2 hours and then 67°C for 1 hour, or place them in an oven at 60°C for up to 6 hours, or use a fluidised bed with an inlet temperature of 90°C this should take about 20 minutes. Going higher than these temperatures causes loss of colour.