I had no time to dive into this in June, but made a note to.
Hi! I butchered a veal calf last fall and saved out presumably the right part of the stomach, washed out the stomach contents, then salted it and placed it on a rack in the sun to dry out with a piece of glass above (to keep the flies off) and a screen below (to hopefully keep it from getting overly hot). First, is there a better way to harvest/preserve rennet on a home-scale?
Yes, the way to do it is to first butcher at the right age. The age depends on the ratio of chymosin to pepsin that you want. The normal range is usually 15-20% pepsin, which is achieved by slaughtering at around 4 weeks of age.
Second, you need to either prepare the rennet as a liquid, paste, or dessicate and preserve the abomasum. To make a paste, you first grind with a commercial grinder to pulverize the heck out of the tissue with 20-30% salt, and then either freeze and use directly over time, or refrigerate and use. If you want, you can also activate the enzymes by decreasing the pH with HCl to around 2.5. This will increase your extraction rate and also help with lowering bacterial counts. Rennin exists mostly as prorennin in the abomasal mucosa and the zymogen is activated by a low pH and moderate temp (22-25C, pH 2-3). If you had any pregastric lipase from sublingual gland secretions, the low pH would inactivate it. Then raise the pH again with NaOH (titrate to know how much), and target a pH of mid 6s. Then you can use the paste.
To make a liquid, you can puree the paste and strain out the solids. There are commercial methods for how to get pure liquid, such as bacterial filtration, centrifugation, using acetone or other solvents, etc. But it's not necessary, you can strain through muslin and that would be OK.
If you dry, do it just like you did. Cut off all the fat, scrub, salt, and leave in a well-ventilated area so it dries out. Then to use, soak beforehand in whey (old traditional method) or an acidic environment to help extract the enzymes, and then raise pH again and use the liquid.
My main question, though, is what to do next? I've had very limited success with my homemade rennet but I've been very pleased with those few seemingly random successes. How do I know how much rennet to use?
You figure out the activity by using an exact, small amount, and a volume of milk and calculating it against a known standard. So take some 200 IMCU rennet, and some of your rennet, and do a side by side and calculate the activity based on flocculation.
I've stored it heavily salted in the fridge. How long can I keep using it?
A long time, actually. The decrease in activity is fairly low, maybe 5%/year.
Do I need to worry about the salt affecting the coagulation or culturing processes?
Not really. It actually helps preserve things, and the concentration is not high enough to make a difference in the milk
What do I need to do to ensure that the rennet gets into solution?
Pulverize, and/or extract in acid and then adjust pH to ensure extraction. Much of the zymogen (actually both prorennin and propepsin) of the enzymes are bound in the tissue, so using strips from the stomach is not really optimal. I suggest using a paste or making liquid rennet right away.
Sometimes it seems like the rennet does nothing. Even though it's calf's rennet my only successes have been with goat's milk; all my attempts with cow's milk have acted as if I hadn't added any rennet at all. Maybe that's because I generally make larger batches of cow's milk cheese and I'm not adding a big enough piece. I've tried soaking the piece in whey (left over from making ricotta) in the fridge for 12-24 hours and using the whey instead of the solid piece, but I'm afraid I have too many variables there to control: the acidity of the whey, temperature, time, etc. In any case, I haven't had consistent success with that. How did rural people make cheese 200 years ago?
Most artisans used a paste, or would make a batch of the liquid and use it for the season. The inconsistency of rennet was actually one huge factor in cheese hygienic quality.
I really want to learn how to make homemade cheese without store-bought ingredients, but I haven't been able to figure out how to use this rennet. Does anyone else on this forum use homemade rennet?
Let me know if you need more help. It's not so hard practically, but you do need some equipment to do it consistently. You can do it without fancy lab gear, but it's more challenging. Not impossible, though.