I agree with him, using too much culture and more so, too much rennet are the two most common pitfalls. Using too much culture, especially without a pH meter leads to too rapid of an acidification. And when you follow the time-based recipes that only say to stir for a certain number of minutes, you end up with a crumbly, crappy cheese. DVI culture has something like 100-150 x 10^9 CFUs per gram. That's concentrated stuff.
When you use too much rennet, you throw off the entire proteolysis cascade. Rennet isn't just for coagulating milk. It's a protein-specific enzyme. It acts on casein and breaks it down. Initially, by cleaving k-casein on the outer edge of micelles, but later, by continuing to open up those micelles and making them available to additional proteases in the milk and in the bacterial cell walls and bodies. It's a huge deal to use too much rennet. This is also why flocculation is so important. If you time floc, you can target a rennet schedule to coincide with expected conditions. Meaning for most cheeses, at least 10 mins to floc, or you are simply using too much rennet. It should be closer to 12-15, and in a good number of cheese styles, closer to 18-20. Use too much, you get accelerated proteolysis, in a bad way, meaning bitter amino acids and protein chains. Similar thing happens when your temp is too high -- you throw off the natural sequence that should happen gradually. It's like boiling wine to try and speed up the rate of tannin and SO2 interaction (for you winemakers). Sure, you accomplish that aspect of it, but your wine is then undrinkable.
Which feta recipe did you use, Tom? In general, the rennet schedule, using American double strength standardized rennet (30,000, as opposed to European, which is 20,000), should be .03-.-045 ml per lb of milk for most cheeses. Meaning about 3-4.5 ml rennet per hundredweight. Using a "teaspoon" measure for rennet for homemakers is notoriously inconsistent... just hard to get the ultra small quantities needed.
