Chestnut is mainly grown in Europe and Asia and the latter covers 70% of the world production. China is the leader with 445000 ha, followed by South Corea 430000 ha, Turkey 392000 ha, Japan 283000 ha, and Italy with 275000 ha. Other areas are beginning to become producers such as USA, Chile, Australia, New Zealand, and Hungary.
The main species are: Castanea mollissima or chinese chestnut, Castanea sativa or european chestnut, Castanea crenata or japanese chestnuts, and Castanea dentata or american chestnut. Interspecific hybrid (Castanea crenata x sativa) is also diffuse. In Fig. 1 all species of chestnuts are reported.
Fig. 1 Botanical species of chestnuts
C. crenata (Japanese chestnut);
C. mollissima (Chinese chestnut);
C. seguinii (China);
C. davidii (China);
C. henryl (China);
C. dentata (Eastern states);
C. pumila (Eastern states);
C. ashei (Southern states)
C. floridana (Southern states);
C. alnifolia (Southern states);
C. paupispina (Southern states);
Chestnut fruit is a starchy nut, slightly hard, with intermediate characteristics between fresh and dry fruits. In Table 1 we can observe the characteristics of perishability of horticultural commodity and it is possible to note the difference among chestnuts, fresh, and dry fruits.
Tab.1 Perishability characteristics of fresh and dry fruits, and chestnuts.
|Characteristic||Fresh fruit||Dry fruit||Chestnut|
|water content %||90-95||5-15||50-60|
|physical structure||Soft||hard||slight hard|
High transpiration rate almost similar to that of some fresh fruits, the relatively high water content and the living aspect compare to dry fruit, make chestnut fruit a very special product to deal with after harvest depending on the botanical formation, chemical composition, physical structure, and physiological behaviour.
From a botanical viewpoint, chestnut is a fruit (achen) with a starchy nut with cream-colored cotyledons (the seed, the edible part) covered with an astringent membrane called the pellicle (episperm).
Brown peel wraps the nut which is protected by a spiny burr. When the fruit starts to ripe, the burr changes colour becoming yellow-brown and breaks in 2-4 lengthways lines releasing three nuts (Fig. 2). Sometime the burr opens on the tree, more often the burr drops and opens completely on the ground due to the high humidity, releasing chestnuts.
Fig. 2 Spiny burr with chestnuts
(Source: Ferrini, 1997)
As all the seeds, chestnuts have a dormancy phase which is depending on the temperature. When the temperature is low dormancy is prolonged and sprouting starts in the early spring.
Different from other dry fruits such as walnut, hazelnut, almond, chestnut is very rich in starch (37-45%) and low in lipids (2-3%). Chestnut is rich in soluble sugars, monosaccharides and disaccharides, such as mainly sucrose, glucose, fructose, and, in less amount, stachyose, and raffinose. Chestnuts are rich in fibre and very rich in potassium. All these features together with the low amount of calories make chestnut a good dietetic, natural, product. The presence of disaccharides can provoke fermentation in the human intestine giving flatulence.
In Table 2 the chemical composition is reported.
|soluble sugars:||8.1 g|
|energy: 189 kcal|
The value are indicative and can change depending on species and growth conditions.
Chestnut, differently from the other dry fruit, has not stony, hard, seed involucre. The external, brown-black, or brown-red, involucre which is commercially called "peel" is very porous which promotes water absorption but, in the same time, provokes fast water loss from the seed.
One of the negative commercial quality factor of chestnut is the softening which is not due to the softening of the seed (unless it is decayed) but to the air remaining between peel and flesh because of seed water loss through the peel. This peel, which wraps the cotyledons, ends up with a tuft (in Italy it is called "flame").
The break of the tuft, which is the main entrance for fungi spores
(Fig. 3), and the delicacy of the soft, leathery, peel represent the main problems
for the use of mechanical harvest of chestnuts.
Fig. 3 The break of the tuft favours the pathogen attack
(Source: ARSIAL, 1999)
Under the peel, cotyledons are tightly covered by the pellicle (episperm). This pellicle in some varieties, after steaming, is released easily, while in other varieties of chestnuts is difficult to remove and compromises the use of these varieties for processing (i.e. marrons glacés).
Flesh is firm compare to fresh fruit. If the average firmness (measured by a penetrometer) of a commercially ripe fresh fruit is 3-8 kg/cm2, for chestnut it can be more than 20 kg/cm2. Compare to dry fruits such as walnut, pecan, hazelnut, and almond fresh chestnuts has the same or higher firmness but also higher elasticity (hardness) which make them less crispy and tougher at the first bite. This different behaviour with dry fruits is due to water content, indeed dry chestnuts become firmer but even less elastic like the other dry fruits. When chestnuts are boiled, they absorb water which is strongly bound by starch, giving a glassy appearance to the texture which breaks at the first bite and after chewing becomes floury.
Chestnut shape varies according to its location in the burr. In Italy the top quality, big size chestnuts are called "marrons" and their flavour, easy peeling, episperm not intruded or just a little bit, and absence of double cotyledons are appreciated.