I am planning on doing some veggie brews and would be interested to know what the gelatinisation temps are for potato, sweet potato, carrot, parsnip, beetroot, etc...
This page from Oregon State shows some of the science behind gelatinization and mentions that:
Starch begins to gelatinize between 60 and 70C, the exact temperature dependent is the specific starch.
However, it also mentions:
At some point between 60-95C we would likely have gelatinization occur.
In some instances, when heated to 90C the starch granule could reach optimum gelatinization and be a nice swollen granule sack. In other cases, this may allow the sack to "implode" and loose their contents as there is not enough structure and hydrogen bonding to hold the polymers together. It is interesting that overcooking, as with overstirring, will decrease the starch paste colloidal sol viscosity.
I take this to mean that somewhere between 60°C and 90°C you should see most starches gelatinize, though 60°C to 70°C should work in most cases/with sufficient time.
Similarly, this BYO article on brewing with potatoes mentions the following:
The starch in potatoes is arranged in larger granules than those found in most plants, including barley. Potato starch is about 20% amylose and 80% amylopectin. In comparison, barley starch is 20–25% amylose and 75–80% amylopectin. Potato starch gelatinizes at relatively low temperatures, around 130 °F (54 °C) for most mealy varieties. As such, the starch is accessible to the saccharification enzymes at typical mash temperatures — 148–158 °F (64–70 °C).
From what I recall reading in the past, the same process works for other (root) vegetables, as implied later in the BYO with sweet potatoes.
Interestingly, this page on post-apocalyptic brewing, for what it's worth, suggests a three-step process for producing a sweet-potato beer: gelatinizing at 140°F (60°C) for 60-90 minutes, boiling for 30 minutes, then performing a standard saccharification rest for 30 minutes.
I couldn't find any detailed resources on using carrots, parsnips, etc.
In Homebrew Beyond the Basics, while it has a nice table of gelatinization points for various grains, in the section with the heading "Potato, Carrot, and Other Root vegetables" it says:
The use of vegetables in beer is still largely uncharted territory. Other than the ubiquitous pumpkin beer or the rare sweet-potato beer, few veggies have been utilized.
That said, earlier in the section on cereal mashing it mentions:
There's no harm in boiling a starch to ensure complete gelatinization, so when in doubt go for a higher temperature.
So boiling for 30 minutes may be a good default until better information comes along for each vegetable.
As a final side-note, the book also recommends roasting most vegetables so they develop a caramelized flavor. For pumpkins it recommends 90+ minutes at 350°F.
I've found that when looking for low gelatinization temperatures, a good guide are sous vide recipes. For instance, in this recipe for carrots it recommends 183ºF (84ºC)
sous vide is basically cooking at the "doneness" temperature of meats or veggies.