The famous French gourmet bible Larousse Gastronomique advises that a durian is ready to be eaten when its husks begin to crack.
I cannot judge the Larousse Gastronomique on other culinary topics, but on durians, their advice is misleading.
You can wait with a Thai Mon Tong durian, or maybe a Cha Ni, until it cracks, but with all other varieties, it's a foul strategy.
With practically all Indonesian durian varieties (and Indonesia has the greatest local durian consumption in the world), it is best to eat a durian within about six hours after it has fallen from a tree even if it may so difficult to open it that a man with proper tools is needed for the job.
Buying and eating Mon Thong durians in Thailand is a low risk venture. You can buy durians in the market, and for as long as the fruit is ripe, it will be ok. Seldom will it happen that a fruit, which looks all right outside, will be rotten inside.
< In Indonesia, to buy the fruit and take it home is the wrong approach. You will often be disappointed. At least a third of the durians sold in Indonesia are not optimal. The following can happen:
1. Overripe. The flesh is pulpy (beceh), and the sulphuric taste component is too strong. The fruit will cause a lot of burping.
2. Rotten (busuk). When a segment of the fruit is rotten at a side, the whole segment usually has to be discarded.
3 "Cold" (dingin). The fruit hasn't been ripening on the tree. Durian fruits will still ripen after taken of the tree. This means: the flesh of the fruit will become soft and develop the right texture. But the fruit once of the tree, will no longer develop the typical strong durian taste.
Even if you are a durian expert yourself, you should, in Indonesia, always let the "tukang durian" do the choosing for you. The "tukang durian" is the salesman (yes, it's always a man; all other fruit are sold by women) who builds up a roadside stall, usually with a few chairs. Durians are different in different regions. Sometimes one can go by the smell of the closed fruit, but often one can't. And the tukang durian always has an edge over the buyer because he knows from which plantation, or even from which tree, his durians come from.
I have considerable experience with durians. If I have been at a durian place for some time, I develop a feeling on how to select a good fruit. But still, instead of taking over myself the responsibility that a particular fruit I chose appeals to my taste, I rather let the seller do the choosing. If I'm a regular customer, he won't want to lose me by letting me buy fruits of minor quality.
In Indonesia it is common practice to let the customer taste the fruit he is offered to buy. For that purpose it is cut open at one segment (and if not bought, "nailed" closed again with matches). If the customer doesn't like a fruit that is offered, a new one will be chosen and opened in the same manner.
Opening a fruit for tasting is less common in Thailand. There is also less risk with Thai varieties that the taste of a particular fruit just isn't right.
In Thailand, one will often see a durian salesperson hitting a fruit to determine whether it is ripe or not. If the sound is hollow, or resembles knocking on wood, the fruit is not yet ripe. The sound should be low and full for a ripe fruit.
Smelling a fruit is unreliable. Some varieties will smell unopened when they are ready to be eaten, and for some other varieties, when they smell unopened, they are hopelessly overripe.
In Indonesia, the best bet is still to follow the advice of the "tukang durian", and if his choice should be lousy, buy somewhere else in the future.
In Indonesia, northern Sumatra is the best address for durians. For this assessment, I not only take into account the fact that durians are available year-round, but also the generally low price levels. Java has some durians, especially the area around Jepara, but prices in major cities of Java Island are usually around triple the prices of Medan, the main city on Sumatra. I have once traveled to Makassar after having been told that durians there cost on average less than half a dollar a fruit.
What disappointment, after I discovered that South Sulawesi durians are exceptionally small and also not the tastiest variety. I stayed in Makassar only for three days.
Cambodia doesn't have good durians. What is available has mostly been imported from Thailand.
But what you will find on the markets in Phnom Penh is the sorts of durian that cannot be sold in Thailand.
You'll find quality Thai durians in Singapore and Malaysia, though Malaysia also has its own durian produce.