How to do alpha-amylase conversion first and beta second?

dmtaylor and Mr_road have good answers, and aiming for a temperature in-between is the easiest way to achieve your goal.

However, you can increase or decrease your temperature if you want to try. Here are a few tips (some that I found here: https://homebrewacademy.com/missed-mash-temp/ )

To increase temperature

• Add boiling water
• Heat the mash directly (if in a kettle setup or using heating element)
• Perform a decoction mash (heat part of the liquid you already have and add it back)

To lower temperature

• Add ice cubes or cold water
• Use a wort chiller
• Use a frozen container (like an ice-pack)
• Open the mash tun and stir a little (less effective, but still)

For most of our brews we shoot for 65°C (148°F)for 90 min, we find this gives us a highly fermentable wort with enough longer chain dextrins for good mouthfeel. This is the usual temp I see most of my colleagues going for apart from those doing the more weird stuff.

For stouts we shoot for 68°C (155°F), for more mouthfeel.

I believe that in older less well modified malts, one of the things that decoction mashing helped with was a way to allow all the enzymes their time and temperature. Also, a nice reliable way to brew when you did not have a thermometer.

The best way is a hard one to answer, but I would suggest as dmtaylor suggests aim for 65°C for 90 min and this is a great balance between ease and efficiency.

Excellent question, which I know every detail-focused brewer wonders about at some point. The reason we don't go up to alpha temperature right away and then drop down is that the beta enzyme denatures relatively very quickly above about 150°F (65.6°C).

You could look into the actual science on this but in my estimation it seems the majority of beta enzymes are gone after the first 30-40 minutes at a temperature of say 155°F (68°C).

On the other hand, alpha amylase enzyme is very stable and actually active in a very broad range from way down in the 130-168°F (55-75.5C). Therefore, if you want the benefits of the beta enzyme, all you really need to do is prolong the mash TIME at a temperature of anywhere from 140-150°F (60-66°C), which will speed up the beta activity without denaturing it too quickly. I find a good compromise at about 148-152°F(64.5-66.5°C), or at the nice round number of 150 F (65.5°C). This is where I mash nearly all my batches, typically aiming for 148-150°F(64.5-65.5°C), and for just 45 minutes typically or about 60 minutes maximum, unless I want very high attenuation then I will mash for 75-90 minutes. I think mash TIME is too often overlooked as a tool for the brewer. Temperature is important, if you want to understand where the beta and alpha amylase are denatured.

As long as you have the temperature in range for both to be very active at say 145-152°F(63-66.5°C) or thereabouts, then the primary variable I think is TIME, not temperature.

Cheers.