It's down to what the chicken ate while it was alive. Saturated fat sets, olive oil sets if you chill it, but not otherwise, and a number of seed oils do not set (rapeseed for instance). When you make a stock which has solidified fat on top, that's saturated fat, so I'd hazard a guess that the stock where the fat doesn't set means a healthier eating chicken, because it contained less saturated fat to start with.
UPDATE: Thank you to the person who bothered to do the research and said my answer 'might have some merit'. Chickens are no different from human beings - the fats you put in are the fats floating round your bloodstream and depositing in various places; think about corn fed chickens, where the fat composition is slightly different, not to mention the colour of the flesh itself. That will be a partial explanation; when taken together with the fact that not all chickens, even in the same flock or brood, get to eat the same diet, because the pecking order dictates that some free range birds don't always get the pick of the food, explains differences in chickens from the same supplier. Of course, if you can come up with another explanation, I'd be delighted to hear it...
UPDATE 2: Perhaps I should have been clearer. I am not for a moment suggesting that the fats eaten are deposited in their original form, but if you know anything about biology (chickens or otherwise) then you'll know that certain synergies occur, depending what's put in, which change the composition of any fats deposited within the body system. Hence the connection between eating lots of saturated fat and having high cholesterol in humans, for example.
UPDATE 3: Rumtscho: Can't find any scientific evidence so far to prove this theory regarding chickens, but, for interest's sake, and to prove how much of a difference it can make, farmed salmon in Britain no longer has a balanced omega 3/6/9 ratio, as it should do, and still does in the wild. It's because the feed had to be changed, and the consequence of that has been a much higher level of omega 6 in particular. I'm still looking for something on chicken.
UPDATE 4: Now I've had time to look properly, it's not at all difficult to find scientific evidence, there's plenty of it. There's a study carried out by The American Society for Nutritional Science in 2000 comparing the fat deposition (and other metabolic processes) between chickens fed the same diet, but one lot with saturated fat included in the form of tallow, and the other lot with polyunsaturated fats. The fat deposition in the birds fed tallow was greater, and the composition of the fat contained more saturates compared to the polyunsaturated group. These results reflect previous studies (Sanz et al 1999 and 2000).
Effectively, it's like everything else - you get back what you put in.
Some recipes might be pickier, but Mexican rice is almost certainly the kind of thing where the broth is just there to add a bit of background flavor, and it doesn't have to be specifically chicken. So you can be pretty flexible. Your primary options are:
- Use a different kind of pre-made broth. They'll all work for something like that, just provide slightly different flavors.
- Use bouillon cubes/paste. These are likely to be less expensive since they don't have all that water weight/volume that costs money to ship. You can even buy them online, depending where you live. They do tend to be saltier, so be careful - if the recipe calls for any other salt, you can probably just leave it out.
- Make your own. If broth is expensive but you can get a whole chicken you'll be in fine shape. And again, not-chicken is fine too; you can use any other kind of meat with plenty of bone in it. If you go to a little trouble, you can get something way, way better than you'd ever buy. (Just search online for recipes; there's no shortage.) But even just boiling a whole chicken in water will get you something usable.
And if somehow none of that works... water, a little seasoning (something with some umami in it) and salt if necessary? Water plus some kind of instant seasoning packet, even ramen seasoning? It's hard to say, since I don't live where you do and know what's easy to find.
Best Answer
Both the broccoli stem and the lemon are likely culprits - they probably accentuate each other in a bad way. The lemon (pith) will definitely add a bitter taste on its own - as Spagirl mentioned, the zest and the juice are the non-bitter parts that you would add if you wanted lemon flavor without the bitterness. The broccoli stems are surely not helping either, though - I have added broccoli stems to broth and it was inedibly bitter. Everybody has different bitterness acceptability thresholds, and these thresholds can be shifted for various foods. It's possible you're more accepting of broccoli bitterness on its own than I am, but when combined with the lemon bitterness, it may bring it past your tolerable level. Or perhaps you added too many this time, or the broccoli was more bitter than your previous batch.
I would definitely recommend eliminating the lemon pith from future recipes to be safe, and recognizing that different heads of broccoli can have differing bitterness strengths depending on growing conditions, and act accordingly (See links below). Everything else on your ingredient list are non-bittering soup ingredients (barring food that has gone bad). Fennel, carrots, and sugar snaps sweeten broths, and onions and scallions flavor them (without making them bitter).
Link that explains that broccoli grown in hot and dry conditions appears to taste more bitter: https://qr.ae/TDDHbt
And you could blanch the broccoli stems (in case they're a bitter batch) before adding them to a broth: https://www.livestrong.com/article/476765-how-to-remove-bitterness-from-cooked-broccoli/
Edit to add: if you want to use up lemon peels, you can toss them with sugar to make a non-heated, non-bitter lemon syrup. It's incredibly useful and one of the best kitchen discoveries I've made:
https://www.seriouseats.com/recipes/2016/03/fresh-lemon-syrup-recipe.html