Yeah, I would agree with the people in the comments, and make the assumption that it had to do with concentrating the ethylene gas from the bananas.
After a bit of googling, it looks like the rice bag trick might be popular because it apparently results in a nearly ideal, stable environment for ripening.
I imagine that the rice would keep the moisture level at a lower point, the bag would contain the gas, and the mass of the rice would decrease the temperature variants (assuming that is conducive to ripening)
I found an informative article which mentioned rice bags here:
http://monsterguide.net/how-to-ripen-bananas-quickly
If you simmer milk with acid, the milk will curdle. I wouldn't go this way.
If you insist on a sweet application, you have to add sugar (or another sweetener) to the mango. A sweet taste will cover acidity perfectly. You can either cook it with sugar syrup, or macerate it. After that, pureeing is probably best, because you want to avoid hard sour pieces in sweet sauce. From then on, your fantasy is the limit. Jam (maybe in combination with another fruit - how about that nectarine and some Grand Marnier?), sorbet, candy, yogurt-based smoothie, jello in molds or as a cake layer, mixing with cheese to create a spread or a mango cheesecake - everything is possible. The taste, however, will be less than optimal, because unripe fruit is not only sour, it doesn't have yet its full aroma. Still, it will work - not as well as a ripe mango, but it will be OK.
Sweet isn't the only option. The classic use for an unripe mango is to put its sourness to good use and prepare a mango chutney. Again, you can stay traditional and make it pure, or experiment with additional fruit and spice combinations. Then use the chutney as a dip or sauce in savory dishes.
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I think you just need to dehydrate them some more to get that effect. I assume granola and muesli tend to use drier raisins because they will keep longer, and not affect the rest of the cereal with extra moisture. The amount of moisture left in raisins can be variable, it depends on how they're made and for what purpose - the moister ones are more accessible, the drier ones keep longer - that would be why some are softer than others.
To dry out your raisins, you might sun-dry them, or use a dehydrator, or even lay them out in a warm oven to chase of the extra moisture. You would have to keep checking to figure out when they reach the consistency you want. You might even try toasting them, for a higher temperature and a quicker time - though I'm not sure if the higher heat will change the flavors, or if the changes would be any good... I have not heard of toasted raisins as an ingredient.