Fate of folates during vegetable juice processing - Deglutamylation and interconversion

- We monitored polyglutamyl folate chain length after juicing of several vegetables.
- Turnip and carrot folates were deglutamylated to a large degree with juicing.
- Broccoli and turnip greens were partially deglutamylated with juicing.
- Beets, beet greens, romaine lettuce, and carrot greens juices largely unchanged
- Several vegetable juices were good-excellent sources of bioavailable folate.

A high performance liquid chromatography-tandem mass spectrometry method with electrospray ionization (HPLC-ESI-MS/MS) was applied for quantitatively determining folate species and polyglutamyl 5-methyltetrahydrofolates (5MTHF) of various chain lengths in vegetable juices. 5MTHF polyglutamyls were the focus of this study since 5MTHF was the most abundant form in all vegetables investigated. Mono-glutamyl 5MTHF has been found to be highly bioavailable and native vegetable gamma glutamyl hydrolase (GGH) might effect this conversion. During juicing 5MTHF polyglutamyls in some vegetables were highly deglutamylated with turnip (monoglutamyl 5MTHF increased from 5.9% to 70.9%) and carrot (monoglutamyl 5MTHF increased from 2.9% to 53.4%), partially converted with broccoli (triglutamyl 5MTHF increased from 2.3% to 81.6%) and turnip greens (triglutamyl 5MTHF increased from 3.4% to 77.7%), or mostly unchanged (beet, beet greens, romaine lettuce, carrot greens). Deglutamylation of folates in turnip greens and broccoli juices terminated with 75-80% of 5MTHF species as tri-glutamyl (both Brassica species) while all other vegetables displaying GGH activity progressed to monoglutamyl 5MTHF. Total folate was mostly recovered (there are no statistical difference of total folate in juiced vegetables and unprocessed vegetables) in all vegetable juices except carrot green juice (ca. 1/3 loss). 5MTHF was the dominant folate species for all vegetables except romaine lettuce, carrot greens and turnip greens which also contained significant levels of 10-formyl species and THF. Interconversion of folates was observed during juicing usually enhancing THF levels with coincident decreases in 5MTHF and 10CHO-related species. The total folate in the whole vegetables ranged from 41 to 219 μg/100 FW while ranged from 28 to 180 μg/100 FW in the corresponding juice. In this study of juices from many different vegetables, beet greens and romaine lettuce yielded juices qualifying as excellent sources of folate, mostly in their monoglutamyl form, which may be more bioavailable than their polyglutamyl forms often predominating in raw and cooked vegetables.