Although the chestnut is classified as a nut, it differs from it both for its low lipid content (approximately 3 percent), and for its difficult storage. Furthermore, the nut pulp (more than 50 percent of water) and the porous epicarp, non-lignified, favour a gaseous external exchange.
The nuts are prone to decay due to insects and fungi; to extend the storage, chestnuts damaged by parasites have to be eliminated.
In the past, chestnuts were preserved by means of a traditional husk mound method, which consisted in the laying out of the chestnuts preferably still inside their husks in mounds of 0.5-1 m in height, and the covering of them with leaves, ferns, other vegetation and earth and regularly dampening them. Inside the mounds the nuts post-mature as the result of a fermentation process and can therefore be stored for some months without any deterioration. When ready for use the nuts are separated from the prickly husk using a wooden hammer.
Nowadays the classic method to preserve the nuts is curing (or hydrotherapy). The method consists of the immersion of the nuts into water at room temperature for a four to ten day period and drying them after curing.
The curing process consists of putting the chestnuts into stainless steel or vitrified concrete tanks, so as to bring about the floating of the spoiled nuts (empty, worm-eaten, aborted) and their consequent elimination; which is the initial sorting stage.
After treatment the chestnuts are then laid out in layers of 40-50 cm in well-aired premises and then subjected to continuous manual turning until they are completely dried. Drying can be done using forced ventilation.
The sterilization or thermo-hydrotherapy consists of placing the nuts in steel cages and then pouring them in baths containing hot water at a temperature of 50°C for 45 minutes and then pouring them in cold water to lower the temperature. The temperature chosen for sterilization is the maximum that the nut protein can withstand without the risk of denaturing, while the duration of the treatment is determined according to the survival capacity of the insects; 45 minutes are needed to devitalize the larvae and eggs of any insects which might be present inside the product.
The chestnuts are then laid out to dry.
This process either precedes or substitutes the curing process and is undertaken especially on chestnuts destined for export. Unlike the curing and hydrotherapy it does not significantly affect the preservation qualities of the product itself.
Before sale, chestnuts are also subjected to other treatments, namely:
the use of methyl bromide fumigation, although it is an officially recognized sterilization method and the treatment required by international export norms in order to ensure that the chestnuts do not contain any live insect larvae, it is aimed to meet increasing restrictions of a legislative-sanitary nature. Storage at a temperature of between 0-2°C with RH of 90-95 percent ensures product storing for three to four weeks, providing that the ventilation of the storage premises are adequate. Fridge conservation can also be combined with other methods such as curing and the use of controlled atmosphere conditions;
controlled atmosphere (CA) storage consists in the storage of the chestnuts in cold rooms at a temperature of between 0-1°C, with RH of 90-95 percent, C02 level of 5-10 percent and O2 at 5 percent. In such conditions the storage can last about three to four months;
whole or peeled chestnuts can also be frozen and stored at a temperature of between -18°C and -20°C for long periods (six to twelve months) without any deterioration. This method is commonly used to preserve nuts for candying and for marrons glacés as it simplifies the organization of the industrial processing schedule during the course of the year and thus ensuring a more rational marketing of the processed product.
The sizing is undertaken using rotating drums both before and after curing treatments; at the beginning of the "line of sizing" the small sized chestnuts are separated for their intended destinations and for eventual drying, so as to divide the product according to size.
The sorting process eliminates the worm-attacked, broken or imperfect nuts. The chestnuts are placed on conveyor belts between two rows of workers, who carefully sort them.
Brushing is undertaken by means of rollers equipped with thick short bristles to eliminate any surface moulds which may have developed on the epicarp and to remove any dust and impurities.
Chestnuts destined for fresh consumption are firstly divided according to size, type and treatment and then packed in plastic mesh sacks with a capacity of between 5-10 kg or jute bags of 5-10-25-30 kg. The front of the sack must be marked with the marketing name, address where the nuts have been processed and packed, and any trademarks.
Roasted or boiled chestnuts are mainly eaten during the autumn, while during the rest of the year the demand of chestnut is usually for processed products available on the market all year round.
As well as being destined for fresh consumption, chestnuts and marrons are also industrially processed according to two processing chains, namely:
The semi-processed category mainly consists of peeled chestnuts, chestnut and marron purées, raw ingredients for the confectionery and processing industries.
The finished products include candied marrons, marrons glacés, marron cream and natural conserved marrons. Other traditional products are dried chestnuts and flour.
Dried chestnuts are obtained by means of the drying process by which the nuts acquire and become easier to digest as the water content is reduced from 50 to 10 percent with an increase in the concentration of active ingredients and mineral elements. Dried chestnuts may be stored for long periods (over 12 months) without any risk of spoiling.
Smaller chestnuts are usually destined for flour: small, sweet, modestly divided and easily peelable nuts are preferred.
The drying operation is often carried out in a traditional manner in special drying sheds consisting of two-storey constructions being either square or rectangular in shape, made from the local stone or bricks. The lower level being used as the heating area where a fire is kept burning and fed with wood, chestnut peel, or with forest waste products. While on the upper storey there is a trestle grating, traditionally made of wood, but which can also be made of metal, on which layers of chestnuts are regularly placed (every four to five days when the previous 15 cm layer has dried) until a maximum height of 30-50 cm is reached.
The chestnuts are turned over several times during the drying process and the internal temperature is checked daily, to ensure that it remains constant. The whole process usually takes an average of 30 days. The dried chestnuts are then peeled using various techniques. Formerly the epicarp removal (peeling) was done manually, by placing them inside a jute sack, beating it against a tree stump or on the ground or by stamping them using special shoes with wooden spikes or by using wooden cylinders with spikes and a handle. Nowadays, these operations are mechanized: drum husk-removers are used for the rapid removal of the dried chestnuts from the epicarp and the episperm. The operation is completed with manual sorting to eliminate any defective or damaged nuts.
For the drying operation, modern ovens are used which operate with electrical resistances and heat dispensers, which are able to reduce the drying time without altering the product quality.
Chestnut flour is obtained by grinding dried and peeled chestnuts; the product has a high nutritious value, with a high saccharose and starch content and is rich in mineral elements.
Mills are usually used and the flour is passed through several times in order to obtain a homogeneous and very fine flour. Industrial milling is carried out using electrical hammer mills of high capacity.
The scrap (worm-eaten nuts) is used as livestock fodder. If dried fast, in order to avoid any mould formation, it provides a wholesome and highly nutritious food which is easy to conserve.