Why the register cannot answer this one
This site’s fruit register works on three readings: how much sugar a fruit brings, how acid it is, and how much pectin it has. For chestnut, all three are missing from the reference sources, and not for want of looking.
USDA FoodData Central gives chestnut 44.17 g of carbohydrate per 100 g peeled, or 45.54 g unpeeled, and does not break out the sugars in either entry. So how much of that carbohydrate is sugar is not established from that source. The FDA pH tables do not list sweet chestnut at all. And there is no pectin figure.
Note also that USDA files chestnut under nuts, not fruit. That is not a filing quirk: a chestnut is starchy in a way no fruit in the register is, and the framework that describes a fruit preserve was not built for it.
The chestnut row in the pH table is a different plant
There is a trap here worth naming explicitly, because it would be easy to fall into and it produces a confident wrong answer. The FDA table used throughout this site does contain a row reading Water Chestnut, 6.00–6.20.
That is not a chestnut. A water chestnut is Eleocharis dulcis, an aquatic sedge tuber with no relation to Castanea sativa beyond the English word they share. Reading that figure as a chestnut pH would put a number on this page that belongs to a completely different plant, which is the same class of error as the original 2015 table’s claim about guava that this register exists to correct.
So the pH stays empty. A page that says not established is doing more for a reader than one that borrows a number from a sedge.
What a chestnut preserve therefore is
Whatever body a confiture de châtaignes has does not come from a pectin set. It is a sweetened purée: the texture is the chestnut’s own starch and flesh, thickened by reduction and held by sugar, rather than a gel formed by pectin and acid meeting at the right concentration.
That is why French keeps two names for closely related things. Crème de marrons is the smooth sweetened purée; confiture de châtaignes is the chunkier preserve. Neither is trying to be a jelly, and neither will respond to the corrections that fix a fruit jam, adding lemon does nothing useful here, because there is no pectin network for acid to help form. Contrast fig, where the acid correction is the whole answer.
Marron, châtaigne and marron d’Inde
The naming here is not a nicety. ANSES, the French food safety agency, reports that confusion between horse chestnuts and sweet chestnuts accounts for more than one in ten of all plant mistaken-identity cases in French poison centre records for 2012 to 2018, and it gives the burr as the way to tell them apart:
A sweet chestnut burr is brown with numerous long bristly spines, and holds two to three fairly small, flattened, triangular nuts. A horse chestnut burr is thick and green with small, short, more widely spaced spikes, and generally holds one larger rounded nut. Horse chestnut causes digestive problems such as abdominal pain, nausea, vomiting and throat irritation.
Separately from that, and much less dangerous: culinary marron is not a different species. It is the grade given to a chestnut that grew as a single round fruit with no internal partition, selected for size. So marrons glacés and crème de marrons are made from edible chestnut varieties, and never from the tree that drops the toxic ones. The word marron on a jar is a quality claim, not a species.
The rest of the setting rules, and the fruits they do apply to, are at preserves. The French terms are in the glossary.
Questions about chestnut preserves
Does chestnut jam need pectin?
The question does not quite apply. A chestnut preserve is a sweetened purée thickened by reduction rather than a gel set by pectin and acid, and chestnut has no published pectin figure at all, so there is no network for added pectin or acid to help form.
What is the pH of chestnut?
Not established. The FDA pH tables do not list sweet chestnut. They do list water chestnut at 6.00–6.20, but that is Eleocharis dulcis, an unrelated aquatic tuber, and reading it as a chestnut pH would be simply wrong.
How much sugar is in chestnut?
Not established. USDA gives 44.17 g of carbohydrate per 100 g peeled and does not break out the sugars, so what fraction of it is sugar cannot be taken from that source.
What is the difference between a marron and a châtaigne?
In cooking, a grade rather than a species. A marron is a chestnut that grew as a single round fruit with no internal partition, selected for size. Crème de marrons and marrons glacés are made from edible chestnut varieties.
How do you tell a sweet chestnut from a horse chestnut?
By the burr, per ANSES. Sweet chestnut: brown, numerous long bristly spines, two to three small flattened triangular nuts. Horse chestnut: thick and green, short widely spaced spikes, generally one larger rounded nut, and it causes abdominal pain, nausea, vomiting and throat irritation.