Agriculture Reference
In-Depth Information
However, when small and large tubers were har-
vested from the same plant under the same en-
vironmental conditions, folate concentrations
were not significantly different, suggesting that
tuber maturity, but not size, was the main factor
responsible for changes in folate content.
the efficiency of deconjugation of polyglutam-
ylated folate to monoglutamates for normal ab-
sorption in the small intestine. Folate covalently
bound to the food matrix must be released before
absorption by the intestine. It is not clear how
much matrix-bound folate is in potato. The add-
ition of protease and amylase did not increase
folate content in potato significantly (Konings
et al
., 2001), but we found that protease treat-
ment followed by amylase and conjugase re-
sulted in folate values ~20% higher, indicating
that a significant amount of protein-bound fol-
ate was present in tubers (Goyer and Navarre,
unpublished data).
Bioavailability is influenced by the pres-
ence of certain plant molecules, some of which
protect folate from degradation during process-
ing, cooking, and digestion. Folate bound to
folate-dependent proteins greatly improves fol-
ate stability (Rebeille
et al
., 1994), as do antioxi-
dants such as ascorbic acid (McNulty and
Pentieva, 2004). Potatoes can contain high
amounts of vitamin C and other antioxidants,
as detailed later in this chapter.
Postharvest and cooking
Losses of folate from
17
to 40% on a dry weight
basis during an
8-
month storage period were re-
ported by Augustin
et al
. (Augustin
et al
., 1978),
while up to 87% increase in folate concentra-
tions over a
7-
month period were reported in
some genotypes (Goyer and Navarre, 2007).
This discrepancy may be due to differences in
storage conditions (e.g. humidity), genotypes
used, and the physiological stage of the tubers
(e.g. dormant versus sprouting).
Loss of folate after boiling has been reported
in several studies (Augustin
et al
., 1978;
Ko nings
et al
., 2001; McKillop
et al
., 2002), with
overall loss percentage between
19
and
25
per-
cent. An overall 29% loss of folate in oven-baked
potatoes, with loss as high as 52%, was reported,
but the overall retention of folate after cooking
exceeded 70% (Augustin
et al
., 1978). Thermal
processing can increase the digestibility of pro-
teins and carbohydrates, and therefore the re-
lease of folates from the food matrix.
Vitamin B summary
As the information listed above shows, potatoes
are a good source for the B vitamins. Data on
processed potatoes indicate tubers contain ei-
ther modest (B
1
, B
3
, B
5
) or even high (B
6
, B
7
)
amounts of the vitamins, with the exception of
vitamins B
2
and B
9
, which appear to be only
poorly available in potato. Vitamin B
7
(biotin)
clearly had the highest values, but surprisingly
little data are currently available for this vitamin
in processed potato.
Potato chips consistently have the highest
content of the vitamins shown in
Table 18.2
,
probably due to the low moisture content of
chips, which effectively concentrates phyto-
nutrient content relative to fresh potatoes. Con-
sequently, baked chips with reduced fat and salt
could be promoted as a comparably healthy
snack food.
Given the RDA values for the different vita-
mins in combination with their content in pro-
cessed potatoes, a major effort could be in focus-
ing on increasing B
2
and B
9
contents in potato
tubers, because deficiencies of these two vitamins
Forms and bioavailability
Raw potato tubers contained
21
µg
100
g
-
1
FW
of
5-
methyl-tetrahydrofolate (THF),
3
µg
100
g
-
1
FW of THF, and traces of the oxidation product,
10-
formyl-folic acid (Vahteristo
et al
., 1997).
More than 90% of potato folate was polyglutam-
ylated and 95% of folates were present as the
5-
methyl-THF derivative, with the rest com-
posed of
10-
formyl-folic acid and folic acid,
(Konings
et al
., 2001). Polyglutamates must be
hydrolyzed to monoglutamylated forms by folate
deconjugase before intestinal absorption, but it
is controversial whether polyglutamates are less
bioavailable than monoglutamates (McNulty
and Pentieva, 2004).
In addition to cooking, other factors af-
fecting the bioavailability of folate include the
instability of labile folate derivatives during di-
gestion, the food matrix, the presence of food
constituents that enhance folate stability, and
