An Unexpected Drop in Gravity
TroubleShooting
Mark Mathews — Lapeer, Michigan asks,
I’ve been brewing for 7 years and have noticed a strange trend that no one has been able to explain to me. I always see a pretty significant drop in gravity between ending my boil and transferring the cooled beer into the primary. For instance in the last beer I made, I checked the gravity using a refractometer (sample cooled to approximately 70 °F/21 °C) about 5 minutes before flame out and it was 1.104 (making a big Belgian Strong). I cooled with a copper coil and when I sampled the beer going into the fermenter my gravity had dropped to 1.088! This happens to me every time. I typically use fining agents.Do I have sugars dropping out during cold break?
I want to begin with a true confession about how I write this column. Using no special system, I select questions for discussion from those that are sent into BYO. The best questions are those with enough wiggle room to find some fun rabbit holes and angles. And most of the questions I select are answerable! I selected your question because it sparks my interest and have been sitting on this for a while hoping to come up with something really clever. But I haven’t, and, in the process, I let your question get a bit long in the tooth before answering. Sorry for the delay! I will give you what I have.For starters, I have never experienced gravity drops between kettle samples and fermenter samples. But have observed exactly the opposite, where the gravity goes up from kettle to fermenter, both in 5-gallon (19-L) and 500-gallon (19 hL) brew kettles. As crazy as it may sound, wort can stratify in brew kettles for a number of reasons, including how kettles are filled during wort collection, how solid and liquid sugars are sometimes added to the kettle towards the end of boiling, and how top-up water is sometimes added towards the end of the boil. All of these practices may result in heavier wort on the bottom of the kettle compared to the top of the kettle. Hmm, by simply writing this paragraph, I think I may have a plausible suggestion to explain why your observations are the opposite of mine. And without further ado, let’s get into what may be happening.
Until about five minutes ago, the only explanation that made sense to me was a leak in your wort cooler that you never noticed. Most homebrewers don’t run cooling water through an immersion chiller when not in use, making pinhole leaks difficult to spot. Another possibility is that you have a leaky connection on the water supply and are leaking water into the kettle from the connection. The latter is much less likely than the pinhole idea.
Although this general idea may sound a bit far-fetched, leaks in heat exchangers are a real concern for food and beverage producers because these tools can and do develop leaks. Flash pasteurizers used in the dairy industry are designed to ensure that pasteurized milk flows into the raw milk side of the heat regeneration section of the flash pasteurizer in the event of a leak. And differential pressure is monitored to make sure the raw pressure is never higher than the pasteurized pressure. My gut feeling, however, is that this is not your problem.
Your problem is probably related to your sampling method. Many brewers grab wort samples from the brew kettle using some sort of sampling device. Indeed, there are plenty of action shots of brewers grabbing wort samples from copper kettles with fancy looking tools because this part of the brewing process is important to all brewers. If wort gravity is not homogenous, this method usually pulls a lower gravity sample because the heavy stuff sinks. In your case, I am betting that you pull your sample from your kettle’s outlet valve. And by doing so, your sample may be different than wort at the top of the kettle, thus explaining your issue. Next time you brew, drain some wort into a clean pot, pour the wort into the top of the kettle, and repeat a few times. If this solves your issue, let us know!
