Saccharification in very basic terms is the conversion of starches to sugars. When it comes to all grain brewing, saccharification is a critical conversion process that occurs during mashing. As the mash tun’s temperature is increased to a range of 120° F to 158° F, the diastatic enzymes of the malted grains begin to activate and break the starches of the grains and adjuncts into sugars. The alpha amylase enzymes break apart complex starches into sugars that the beta amylase enzymes break apart even further into easy-to-ferment maltose sugar.
Precision is critical when it comes to the temperature of a mash and 10 degrees makes a massive difference. Beta amylase is more temperature dependent than alpha amylase, and when the temperature in the mash begins to rise above 158° F, the beta amylase is no longer capable of breaking apart the more complex sugar chains into maltose. So if your target mash temp is 152° F and you instead conduct your mash at 162° F, you will be left with a massive amount of unfermentable sugars in your finished beer, and it will have a fuller body and overly sweet finish.
Beta amylase thrives in a temperature range of 140° F to 150° F, so if your target mash temp was 152° F and you conducted your mash at 142° F, you would end up with a beer with a very thin body and dry finish due to a deficiency of unfermentable sugars. This is the reason why the typical mash saccharification rest temperature is in a range of 152° F to 154° F; it provides a good temperature compromise for both alpha amylase and beta amylase to carry out their required starch and sugar conversion processes.