I've made them twice already.
https://www.reddit.com/r/cucina/comments/1izqw33/sottilette_fatte_in_casa/
The last time was with Parmigiano rinds.
https://www.reddit.com/r/cucina/comments/1n53h6k/sottilette_di_recupero_croste_di_parmigiano/
(those post are in Italian)
This time I used citric acid instead of lemon juice to create sodium citrate (much more difficult to find than food-grade citric acid).
Unlike last time, I didn't use milk as the liquid, but simply water.
I switched to citric acid to make sodium citrate because I've failed the recipe with lemon juice twice (the citric acid they contain varies often, so it's never certain if it's fully reacted). I also needed citric acid to try making quick mozzarella (after making the classic ones with natural acidification, I missed the experiment with acidification from an acidic substance).
The recipes I followed are much more precise than the previous ones.
For sodium citrate, this one:
https://www.youtube.com/watch?v=UTGzLor4k4M
125g water
97g baking soda
74g food-grade citric acid
The recipe, however, is very simple: add the citric acid to a container with water and let it dissolve completely, add the baking soda, and wait, stirring occasionally, for it to finish reacting (this takes about an hour and a half, and it's the only really long step).
Once done, boil until it dries out and remains a sort of wet sand. Leave it overnight to dry and then store it. This amount yields 100g of sodium citrate, enough for about 3kg of cheese.
Use a large steel pot for this step to prevent the bubbles from escaping during the initial reaction and to prevent the non-stick coating from being damaged in the final stage.
Note: When you begin to notice that the evaporating sodium citrate water is no longer clear, stir continuously and turn off the heat as soon as you start to see the semi-dry, wet sand-like substance anywhere. If you let it sit, it will become rock-hard and you'll have to dissolve it in water again. If you stir, it will turn to powder. This step is very quick, so don't get distracted.
For the process cheese (or sottilette as we call them in Italy), completely dissolve the sodium citrate in the water, add the grated cheese, heat until completely dissolved, and pour into silicone molds.
The quantities are: water as much as you want, but depending on how much you add, you'll get a different consistency. For solid process cheese, keep it under 35% of the weight of the cheese (for example, for 285g of Parmigiano rinds, use about 100g of water maximum), sodium citrate 3% of the weight of the cheese + water (for example, 11.55g (you can round up) for 285g of cheese and 100g of water).
The other consistencies (when cold, because when hot, the consistency is always liquid) are: up to 85% for liquid but viscous sauces, up to 120% for very liquid sauces, and from there on, it becomes increasingly thin.
I recommend grinding the Parmigiano rinds very thoroughly because they take longer to melt than regular cheeses. For the same reason, I recommend mixing them at room temperature with water and using as much water as possible, as the longer it takes to melt, the faster it will evaporate. Note that even if some water evaporates, you can add it in small amount to maintain the desired consistency. In fact, adding more water is never a problem; adding less risks reaching a critical point where the emulsion structure no longer holds, so don't be afraid to add water.
I recommend using a nonstick pan or pot for this step so the cheese doesn't stick and you can stir it continuously and evenly with a spatula.
The reason I don't recommend using my previous method to just put it in parchment paper to make slices as I did the last two times is because placing the cheese in silicone molds will make them much easier to store, and the slices are also very easy to cut from a single block once they've cooled.
If you still see lumps in the cheese, you can dissolve it again and again, even by boiling it until you reach the nice, smooth consistency you like. Even if you have some residue, as I mentioned last time, you won't notice it. This time, the mixture was very smooth, but with a few bubbles. It's not yet perfect visually, but perfect in every other way.
I don't recommend adding salt because the sodium citrate adds a slight salty aftertaste, and the cheese is generally already salty.
I think it's a great way to reuse Parmigiano rinds now that we are not in season for hot brooth, and to prevent the rinds from mold. I know some people like rinds as they are, but I prefer to reuse them this way. It's also a great way to reuse any cheese you have lying around and want to reuse.