Raw materials must in principle be regarded with the possibility of infestations as they are received, and they should always be examined for pests before put in a warehouse. If the products are heavily infested you will almost always be able to find some of the pests on the outside of the packaging.
It is obvious that holes in packaging are indications that something is wrong. It may be jagged edges or holes at the corners due to mice or rats. Small round holes indicate infestations of drugstore or tobacco beetles, while more irregular holes often are due to pupating moth larvae that have left the packaging.
Abnormally high dust content should always give cause to scrutiny. It may be beetle excrement. Note whether there is dust on the outside of packaging and on the floor and shelves around the products. Dust that is gray-violet or form ridges may be flour mites.
Gnawings on goods show of course that it is, or has been infested. It is often possible to see which pest did the damage simply by looking at the gnawings.
Smell the product. A sweet, honey-like odour can be rotting flour mites. A phenol-like odour may come from dark flour beetles. Flour which is infested with confused flour beetles will gradually become greyish and it smells mouldy. Moth larvae in groceries reveal themselves by the products being spun into clumps. If flour is spun to a solid block, it can also be caused by fungal hyphae.
A suspiciously horizontal surface in flour that has stood untouched for some time suggests flour mites. Their digging eventually results in more dense products and smoothing of surfaces.
Biological activity in foods with low thermal conductivity can be found when there are temperature differences in the products. Insert your hand between packagings, bags or in the product itself. If the products are warmer than the room, the reason for this should be investigated. It may be fungi, bacteria or pests, and in any event these are organisms which are undesirable.
About cereal storages: Granary weevils are rarely seen until there are many of them and the temperature is so high that they become very active. Therefore, do not inspect each and every one of the millions of grains. Instead, you should look at wall surfaces. If there are shovels or sticks in the grain then there is a tendency for the weevils to gather at the very top of these. In the cleaning of grain you will usually be able to see various pests better than in the grain pile itself. Thermometer measurements with remote equipment or, for lack of a better way, a hand stuck into the stack may disclose heat changes, which may be due to weevils and especially their larvae inside the grains. Green budding grains in the surface can be an indication that there is a place within this takes place a heavy perspiration. It may be a “warm pocket” with lots of granary weevils and other pests.
In aerated flat storage, mites will be driven up to the surface when aerated from below. On the surface, they will form shapes following the most humid areas. These shapes are clearly visible as violet-gray dust piles. The colour is due to the reddish legs and gray-white bodies of flour mites. These shapes disappear when the aeration stops and the mites begin to seek down again. It is possible to smell whether there are mites in food. Put your hand down into the grain so that a watch would be covered and take a handful of seeds at this depth or slightly deeper. Roll the grains together to crush any flour mites and snuff. Flour mites have an acid and unpleasant smell. The smell is believed to come from two large glands of unknown function, which are crushed when you squeeze the mites into pieces.
So there are several options to get an idea of the condition of the goods, but in most cases it will be necessary to take samples for further scrutiny in the laboratory.