I do not have any information on the change itself, but neither does that claim in Wikipedia.
However, I would refer you to the discussion page of lentil's and read over the community's discussion regarding the article's nutrition claims generally. They are not pleased with many of the claims, and have been active in moderating it. Also of note, the claim has no citation.
Upon my reading, I would not take for granted that the minor act of sprouting in water changes nutritional value substantially, especially as compared to deep-frying them or other processes.
That said, you might refer to the general sprouting article:
These nutrients are essential for human health. To clarify, the nutritional changes upon germination & sprouting have been summarized below. Chavan and Kadam (1989) concluded that - “The desirable nutritional changes that occur during sprouting are mainly due to the breakdown of complex compounds into a more simple form, transformation into essential constituents and breakdown of nutritionally undesirable constituents.”
It goes on to explain that, according to studies done by Chavan & Kadam (1989), Cuddeford (1989) and Shippard (2005), changes during the sprouting process result in:
- Increases in protein quality
- Increases in crude fiber content
- Increases in essential fatty acids
- Increases in vitamin content
The article cites a claim from the Shipard (2005) study: "“When seeds are sprouted, minerals chelate or merge with protein, in a way that increases their function.”
It sounds like you pretty clearly had too much water, and maybe cooked too long and too hot. As for what to actually do: using a microwave doesn't mean you can't be flexible. You can figure out how to cook them the same way you could figure out how to cook many things in the microwave.
Start just enough water to cover the lentils. Cook it on high until it's nearly boiling; then reduce the power until it can keep going without boiling much. (That depends completely on your microwave; on mine it'd probably be in the 20-50% range.) Every couple minutes, have a peek. Make sure there's still enough water; stir them and see if they're done.
If you keep track of the total time, and the amount of water you had to add as you went along, you should be able to do it without much trouble next time. (And if you like, leave out the initial heating on high; that just saves some time.)
Mushiness can be caused by cooking too long, obviously, but also by cooking too hot. A rapid boil can help disintegrate things. It's especially tricky to avoid boiling in a microwave, since reducing power just makes it go off and on. Pay attention to how much it's boiling while it's on; while it's off it'll still be plenty hot.
Best Answer
Dried lentils don't go bad as such if stored sensibly (i.e. dry), but they do go hard and take longer to cook. It's always possible they were stored for some time before you cought them. An overnight soak before cooking can help. Other tips include:
It's not definitive, so you need to apply common sense, but when pulses do spoil they smell terrible as soon as you open the container. They'd also be slimy, because they have to be wet to spoil.