Boil gently for one and a half hour, not more. NO PRESSURE COOKER. Longer cooking, pressure cooker, microwaving at any stage will turn this into a death brew of glutamate.
A home pressure cooker should not have the ability to turn folate into glutamate. In the US, a home pressure cooker is only rated to a
maximum 15 psi relative to your current barometric pressure. So, at sea level, we are talking about
an absolute pressure of 29 psi inside the pressure cooker, which is about 0.2 MPa (Megapascals).
However, the pressure from high pressure canned/vacuum packed food (known as "HPP" in the food industry), is put under
much higher pressure.
FDA.gov said:
FDA.gov: High Pressure Processing
High pressure processing (HPP), also described as high hydrostatic pressure (HHP), or ultra high pressure (UHP) processing, subjects liquid and solid foods, with or without packaging,
to pressures between 100 and 800 MPa. Process temperature during pressure treatment can be specified from below 0 °C (to minimize any effects of adiabatic heat) to above 100 °C. Vessels are uniquely designed to safely withstand these pressures over many cycles. Commercial exposure times at pressure can range from a millisecond pulse (obtained by oscillating pumps) to a treatment time of over 1200 s (20 min). In contrast to thermal processing, economic requirements for throughput may limit practical exposure times to less than 20 min.
To give you an idea of what kind of high pressure we are talking about...
100 MPa = 14,503 psi
800 MPa = 116,030 psi
That's a ridiculous amount of pressure. Home pressure cookers (with only 29 psi of absolute pressure, at sea level) are nowhere near that kind of pressure.
So, here's what the research says about folate when it's exposed to the extreme "HPP" high pressure in the canning industry:
Leuven Food Science and Nutrition Research Centre said:
From:
High-pressure treatments induce folate polyglutamate profile changes in intact broccoli (Brassica oleraceae L. cv. Italica) tissue
In plant matrices, folates exist largely as folylpoly-c-glutamates requiring deglutamylation to monoglutamates prior to absorption, which might impair dietary folate bioavailability. This study investigated folylpoly-c-glutamate stability and conversions in broccoli tissue during thermal (25–90 °C, 30 min) and high-pressure treatments (0.1–
600 MPa, 25–45 °C, 30 min) after vacuum packaging...this study demonstrated that high-pressure treatments
above 200 MPa resulted in depoly-c-glutamylation of folylpoly-c-glutamates at 25–45 °C with an accumulation of mono- and di-c-glutamate folates at most conditions, whereas this was not observed for the thermal treatments at the reference temperatures (25–45 °C,
0.1 MPa, 30 min).
But let's not confuse High Pressure Processing (HPP) with a home pressure cooker. As we see above, they are completely different. A home pressure cooker can only obtain
15 psi above your current atmospheric pressure (= absolute measurement of ~29 psi at sea level). That's the equivalent of 0.2 MPa. According to that study, the degradation of folate would not happen until 200 MPa (or 29,007 psi) or above.
In fact, it appears that home pressure cookers are very good at preserving folate, as this study points out.
J. Dang J. Arcot A. Shrestha said:
Folate retention in selected processed legumes
The effect of soaking, boiling and pressure cooking on the retention of folates in whole chickpeas (
Cicer arietinum) and field peas (
Pisum sativum) was investigated.
Pressure-cooking allowed significantly higher (p<0.05) retention of folates compared to boiling in both the legumes. Retention of folates in chickpeas was greater than field peas irrespective of the processing procedure used.
Leaching into the cooking medium was identified to be the major cause for losses of folates during soaking and cooking. Leaching losses were greater in field peas compared to chickpeas.
According to that study, you'll retain
more folate by using a pressure cooker than by boiling (it's because you use less water with a pressure cooker). A home pressure cooker is nothing like those industrial canners—not even in the same universe!