One batch had more CO2 or gunk in the test samples. Temperature's 156 and 160F are in the Alpha II and Alpha I range. Find out what Alpha does and the picture will get clearer.
This is what I think happened. Primary fermentation occurs due to glucose. The 156F batch had a higher percentage of glucose due to enzymatic action of Alpha. The temperature was very close to optimum for Alpha during the rest. During the rest Alpha released a better balance of sweet tasting, non-fermenting sugar and glucose and yeast went to work on the glucose. In the higher temp mash Alpha released more non-fermenting sugar and less glucose. It is difficult to measure efficiency, ABV and attenuation accurately without using the device that does it. There are a lot of variables. A hydrometer measures pressure and that's about it. The device is quite expensive. Something else to consider, the black malt reduced the mash pH and assuming everything was the same in both batches, brewing water and everything except for temperature, mash pH may have been more optimum in a batch due to the rest temperature being optimum for the enzyme. What I mean is that since there is an optimum temperature for an enzyme the pH must be optimum as well. If they are not in sync enzymatic action changes.
A note about efficiency. When malt is soaked in hot water for an hour or so a certain type of complex starch called amylo-pectin is floating around in the mash. The thing about the starch is that it is heat resistant and it doesn't rupture and begin to enter into solution until mash temperature reaches 169F. During single infusion mashing at no time are temperatures high enough to rupture the starch and if the high temperature were used enzymes would denature. The starch goes with the spent mash to the compost heap, it's the small white pieces. The starch is amylo-pectin and it is responsible for the body in beer. Alpha releases limit dextrin as it liquefies the starch chain. The types of sugar are called A and B limit dextrin. The sugar is tasteless and non-fermenting. So, when considering efficiency, consider that you are throwing money on the compost heap. Also, consider the beer that you are producing, it is missing the starch that beer kind of needs to be beer. There's little more to focus on than efficiency.
Mash is boiled in the decoction method without denaturing enzymes.
Quote...."This totally makes sense. At mash temperatures of 148-151 you get more alpha amylase and 153 -157 you get more beta amylase from your grains. 152 is typically viewed as the mash temp where you get approximately equal amounts of alpha and beta amylase.
Alpha amylase is an enzyme from the grains that produces more fermentable sugars which will increase attenuation and thus your ABV. It will also produce a drier beer as the yeast can process further down the sugar chains."
If what you wrote came from a book on making beer throw the book away and if it came from the internet, the person that put it there knows nothing about enzymes or you are pretty much confused, which does happen and that's OK because you are probably just beginning. I'll help you out.
You might not realize this but, Alpha works very well at 98.6F, saliva is loaded with it. Chew up a big chunk of bread for about 10 minutes and it begins to taste sweet.
Alpha amylase is the most prolific and strongest enzyme in the kernel of grain. It activates at temperatures from cool, earth temperature in the spring up to 168F where it denatures. The enzyme releases two types of sugar from simple starch called amylose. When Alpha liquefies the starch chain it does it at a place called a 1-4 link. When that happens two chains are created. Alpha continues to work on the chain called the reducing end because there are more 1-4 links in the chain. When there are no longer any 1-4 links for Alpha to liquefy, sweet tasting, non-fermenting sugar is left. The other chain is the fuel for yeast, people and the plant from where the kernel came from, it is a simple sugar called glucose. Alpha converts starch into nothing, it liquefies/softens starch. The correct term is mash conversion.
We'll get into conversion next.
Beta is a low temperature activated enzyme. Activation range is around 130ish and optimum temperature range is 140 to 145F. Beta is a weak enzyme and temperature above 145F denatures the enzyme quickly. Beta is the enzyme responsible for first conversion. Beta converts the glucose that Alpha releases from amylose into complex types of sugar called maltose and malto-triose, di and tri-saccharides. Conversion occurs when Beta converts simple sugar into complex sugar and it occurs during the maltose rest.
Yeast cannot use complex sugar for fuel, only simple sugar and due to that a type of second conversion occurs during the second fermentation cycle. During second fermentation yeast absorbs maltose and enzymes within yeast converts the maltose back into glucose. The glucose is expelled through the cell walls and it becomes fuel. Gravity reduces. During the aging phase yeast does the same thing with malto-triose and natural carbonation occurs.
The malt that works with single infusion and which most home brewers purchase is high modified malt. The enzyme richness isn't the best in high modified malt and because of it the low temperature enzymes are pretty much non-existent or they are very weak and will not do anything. There are no reasons to use any temperature except for the optimum temperature for Alpha. It would be a waste of time. Vis versa, purchasing a more expensive, higher grade malt and soaking it in hot water at one temperature for an hour would be a waste of money.
