How to make sorbet from fruit like watermelon?

Some suggestions in the event that the questions doesn't get closed as a duplicate...firstly, get a copy of McGee's book, as suggested in the first possible duplicate. It is an excellent resource with too much information to reproduce here. Referring to McGee, you will want to decide what kind of texture and what level of sweetness you desire. The proportion of sugar in the mix and how fast it is frozen impacts the final consistency. It's not really about pulp content. Again, this is all outlined in McGee's excellent book. What McGee calls "moderately sweet ice" (20% sugar) becomes rock hard when frozen, and needs to be used before that state...or allowed to temper after. Sweet ices contain about 35% sugar and need little, if any thawing. Creating the desired texture is a matter of balancing acid and sweetness given a specific ingredient. Watermelon and lemon will behave very differently, and McGee has a table that helps to see why. The ratio of sugar to acid in lemon is 0.4. For Watermelon it is 45. For example, he suggests the following for a sweet lemon ice: 1 C juice, 14 TBS sugar, 1 cup water.

Serious Eats has covered this :

By contrast, watermelon and pomegranate juices are thin with no body, so they need some special handling to make their textures as thick and creamy as berry or stone fruit sorbets. It's even trickier with citrus like lemon, lime, and grapefruit; not only does their juice lack pectin or fiber,** they're so tart they need extra sugar to balance their flavor, and even when you add enough, the resulting sorbet isn't as rich.

** Whole citrus fruit has plenty of pectin but it's all in the rind, not the juice or flesh.

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The master ratio above works great with any fruit purée that has some body and viscosity. But what about thin juices like watermelon, pomegranate, and citrus? Without any fiber or pectin they tend to produce a thin and icy sorbet, even when made with the correct amount of sugar. What's more, they're less forgiving than berry or stone fruit sorbets, because there's nothing in them besides sugar to inhibit the growth of big ice crystals.

If you're dealing with citrus juice you have another problem: the juice is so tart it needs to be diluted and sweetened with care. Go ahead: try making lemon sorbet with four cups of lemon juice and one cup of sugar: you'll get something so lip-puckeringly sour you'll barely be able to choke it down.

The solution to both of these problems is an alternative kind of sugar, one with different sweetening and freezing properties than sucrose, a.k.a. table sugar.

Sucrose is fairly sweet and doesn't add much body to a syrup. That's why pastry chefs look to liquid sugar like invert sugar, glucose, or dextrose, which all make sorbet creamier when used properly. The easiest alternative sugar—the one you can find in any American supermarket—is plain 'ol non-high-fructose corn syrup. Trust me: it's lemon sorbet's best friend.

I've written a whole article on the benefits of corn syrup in sorbet, but here are the Cliff's Notes: 1) corn syrup is highly viscous, so it makes for richer, creamier sorbet; and 2) it's only one third as sweet as sugar, so you can use three times as much of it as sucrose—making your sorbet three times as creamy—without over-sweetening the end result. In a blind taste test, tasters almost universally preferred lemon sorbet made with corn syrup compared to sugar. You can see the difference in texture here.

Lemon sorbet made with different proportions of corn syrup to table sugar. The more corn syrup you add, the smoother and creamier the sorbet becomes.

Even small amounts of corn syrup (or other liquid sugars) can add body and creaminess to a sorbet made with sucrose. How much you use, and in what proportion to sucrose, will vary from fruit to fruit, but this lemon sorbet recipe is a good starting point for super-sour citrus.

Oh, and because I know you'll ask: no, honey, agave nectar, and maple syrup aren't good alternatives. For one, they bring strong flavors of their own that may or may not jive with your other ingredients. They're also not very effective; honey has more body than sucrose, but it's so sweet you can't use much of it; maple and agave don't have much body at all.

The article also talks about other ways to soften it (e.g., alcohol), but oddly, only links to a lemon recipe, nothing for watermelon.

Instead of using a normal ice cream maker, it's possible that you might be able to get away with food-processor sorbet, as the blades break down the ice crystals:

• 1 lb of frozen fruit
• 1/4 tsp xanthan gum
• syrup (optional; can be a flavored syrup, heavy syrup or corn syrup)
• juice (about a cup)

Place the frozen fruit in the food processor, and pulse it a couple of times. Sprinkle the xanthan gum on top and pulse a couple more times. With the food processor running, add the syrup and liquid until you get a good flavor & texture.

If it's too soft for your liking, you can put it in the freezer to firm up.

1. Blend Watermelon till smooth.
2. Add sugar, stir until you're pretty sure the crystals dissolved.
3. Stir in the lemon.
4. Let it sit in the fridge for about 10-15 minutes.
5. Pour it into the Ice Cream Maker

Marmiton suggests 500g watermelon to 75g sugar with a tablespoon of lemon juice.

Since it turned out the store had neither glucose or fructose in stock, I ended up doing some more research that lead me to what I ended up trying, which I'm offering as a supplemental answer to the other good ones:

Apparently it's possible to make invert sugar syrup at home by cooking sugar with a bit of acid. The basic process is to take 2 parts sugar, 1 part water, 1/500th part citric acid or cream of tartar, simmer this syrup without stirring until the temperature reaches 114C. (E.g. 1000g sugar, 500g water, 1g acid.)

For 1kg of sherbet base (sour cherries and evaporated milk) I ended up adding 375g of the syrup, which, accounting for some of the water boiling off during the cooking, should have landed me somewhere north of 20% added sugar by weight of the mix. (I started with 250g syrup and added an additional 125g to taste, hoping that the additional water binding power of the invert sugar will offset whatever is lost by not knowing the exact sugar percentage - especially since recipes mention that substituting even part of table sugar with corn syrup that's less concentrated than my invert syrup.)

Subjectively the texture was an improvement over my first sorbet attempt, which used strawberries that yielded a distinctly thicker puree than sour cherries, so I'll tentatively call this a success.