Kitchen Essential

Notes on measurement

Two ordinary mercury-in-glass thermometers are required with a range of about 50-100° F. They must be inserted through holes in the wall of the kiln a few inches apart so that the bulbs project well into the smoke stream. One of the thermometers (the ‘wet bulb‘) has a muslin bag or sleeve tied round the bottom inch or two of the stem, so that it completely surrounds the mercury bulb.

Before a reading is taken, this bag is moistened by dipping in clean water, preferably distilled water, and after putting back in position, the reading of the thermometer is taken when it is steady after about half a minute. The ordinary temperature of the dry bulb should be read on the other thermometer at the same time as that of the wet bulb. The smoke should not pass over the wet bulb immediately before the dry bulb in case the air gets cooled by evaporation before affecting the dry bulb.

The temperature of the wet bulb is normally lower than that of the dry bulb, because, as the water on the muslin sleeve dries, it cools the mercury bulb and lowers the temperature read on the scale. The drier the air, the greater the ‘wet bulb depression’, ie the dry bulb temperature minus the wet bulb temperature.

If the air is completely saturated with moisture vapour, ie the relative humidity is 100%, no further evaporation can take place, the wet sleeve cannot dry and thus the wet bulb reading is the same as that of the dry bulb.

While the flavours are mostly due to the smoke and the initial salting treatments, the flesh texture is largely dependent on the drying process. The amount of drying to which the product has been subjected before smoking will, of course, also affect the final product texture.

Efficient drying processes depend on the humidity of the surrounding atmosphere. The degree of saturation of the air with water vapour is termed the relative humidity (RH). Air which is completely saturated is at 100% RH, and air containing half as much water vapour as it can hold at any temperature is said to be at 50% RH. The best range of relative humidity for cold smoking purposes, where temperatures of 85^°F are used, is between 60 and 70%. Where RH is above 70% the drying process is too slow, and if below 60% the fish surface dries too quickly.

In other words, the air and smoke draught speed and their capacity to dry products depends on how much the air entering the smoke unit is warmed by the fire. Where cool, dry air enters and is warmed, it will be much lighter than the air outside and will thus travel rapidly up the smoke unit and over the product. The quantity of water vapour which this air will absorb depends on its temperatures, so that if the cool dry air is warmed on entering the smoke unit, its drying capacity will be considerably increased. Where humid ambient conditions exist, the already warm, saturated air cannot be further warmed quickly enough to reduce the water vapour content and thus its capacity for drying the product is reduced.

Thus in the first stage of any smoking process moisture is removed at a rate dependent on the drying capacity of the air and smoke and their speed over the product. During the second stage of the process, when the surface has dried somewhat, its temperature gets closer to that of the air and smoke. The rate of drying becomes slower, however, as moisture has now got to move out from the inner layers of the flesh. If the initial drying stage is carried out at too high a temperature, too quickly, the result will be to harden the fish surface, making it difficult to remove inner moisture. In this instancethere may be no drying effect at the centre of the product.

In hot smoking, the product must not at first be subjected to the higher temperatures involved. The process should start at a relatively low temperature to enable the product to dry partially and the skin to harden. Only then should the heat be increased to cook and smoke the product. Where too high a temperature is reached in the early stage of the process, the product cooks quickly, and as it is still soft and moist, it will break and fall into the fire.

When smoking fatty fish, the initial drying period can be reduced, using a slightly higher temperature, as the presence of some oil prevents hardening of the surface. However, the oil also retards the movement of moisture from below the flesh surface and thus the overall time of the drying process becomes longer.

As spoilage bacteria are more active in moisture-laden products, the drying process is very important. The keeping time of the product will depend, to a large extent, on the amount of moisture removed. Products which have been lightly smoke cured will spoil almost as fast as the raw material from which they were made. Products which have been very heavily cured may keep for many months at normal temperature.

Food will, of course, lose weight during the drying process as the water content is evaporated, and cold smoke cures with a weight loss of 12-18% on drying will result in good-quality products. Cooked products, obtained when hot smoked, will generally lose considerably more weight than this. The length of the drying process will depend, to a great extent, on the experience of the smoker regarding a particular smoke cure, and will involve consideration of such factors as flesh texture, prevailing weather conditions and whether cold or hot smoking is involved.

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