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2 Chemical Structure, Biopotency, and Physicochemical Properties

2 Chemical Structure, Biopotency, and Physicochemical Properties

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nature. It exhibits the full antiscorbutic activity of ascorbic acid on a molar
basis: that is, 1 g of ascorbyl palmitate is equivalent to the potency of
0.425 g of ascorbic acid. Erythorbic acid is cheaper to manufacture on a
commercial scale than L -ascorbic acid. In some countries, it is legal to substitute erythorbate for ascorbate when, for technological reasons, the antioxidant or reducing properties and not the vitamin C activity of the
additive is required. In the United Kingdom and certain other countries,
erythorbate is not permitted as an antioxidant, and it is also prohibited for
use with raw and unprocessed meats.
L -Ascorbic acid is easily and reversibly oxidized to dehydroascorbic
acid, forming the ascorbyl radical anion (also known as semidehydroascorbate) as an intermediate (Figure 15.2). Dehydroascorbic acid possesses
full vitamin C activity because it is readily reduced to ascorbic acid in the
animal body. Dehydroascorbic acid is not an acid in the chemical sense, as
it lacks the dissociable protons that ascorbic acid has at the carbon 2 and 3
positions.

15.2.2

Physicochemical Properties

15.2.2.1 Solubility and Other Properties
Ascorbic acid is an almost odorless white or very pale yellow crystalline
powder with a pleasant sharp taste and an mp of about 1908C (with
decomposition). Pure dry crystalline ascorbic acid and sodium ascorbate
are stable on exposure to air and daylight at normal room temperature for
long periods of time. Commercial vitamin C tablets possess virtually their
original potency even after 8-yr storage at 258C [4]. Ascorbic acid is
readily soluble in water (33 g/100 ml at 258C), less soluble in 95%
ethanol (3.3 g/100 ml), absolute ethanol (2 g/100 ml), acetic acid
(0.2 g/100 ml), and acetonitrile (0.05 g/100 ml) and insoluble in fat solvents [5]. A 5% aqueous solution of ascorbic acid has a pH of 2.2 – 2.5,
the acidic nature being due to the facile ionization of the hydroxyl
group on C-3 (pK1 ¼ 4.17); the hydroxyl group on C-2 is much more
(a)

(b)

CH2OH
HCOH
O



H
O

HCOH
O

H+, e–
O
H

OH

H+,

e–

(c)

CH2OH

O

O

e–


O

e–

CH2OH
HCOH
O
H
O

O

O

FIGURE 15.2
Oxidation of ascorbate. (a) L -Ascorbate (AH2), (b) ascorbyl radical anion (A2†), and (c)
dehydroascorbic acid. Note the delocalized unpaired electron in the ascorbyl radical anion.
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resistant to ionization (pK2 ¼ 11.79) [6]. Sodium ascorbate is freely soluble
in water (62 g/100 ml at 258C; 78 g/100 ml at 758C) and practically insoluble in ethanol, diethyl ether, and chloroform; the pH of an aqueous solution is 5.6– 7.0.
Ascorbyl palmitate is practically insoluble at 258C in water (0.2 g/100 ml),
soluble in ethanol (20 g/100 ml), and slightly soluble in diethyl ether. The
solubility in vegetable oils at room temperature is very low (30 mg/100 ml)
but increases sharply with increasing temperature.
The carbonyl enediol group of ascorbic acid confers strong reducing
properties to the molecule, as indicated by its ability to reduce Fehling’s
or Tollen’s solution at room temperature. The redox potential of the first
stage at pH 5.0 is 110 ¼ þ0.127 V.
15.2.2.2

Stability in Aqueous Solution

Oxidation of ascorbic acid follows first-order kinetics in the pH range 3– 7
in aqueous model systems containing traces of copper. Stability is higher
in the pH range 3.0– 4.5 than in the range 5.0 – 7.0 [7]. At alkaline pH,
ascorbic acid is unstable. At pH 1, the ionization of ascorbic acid is suppressed, and the fully protonated molecule is relatively slowly attacked
by oxygen. Consequently, the rate of oxidation of ascorbic acid accelerates
as the pH is increased from 1.5 to 3.5.
Dehydroascorbic acid in solution at neutral or alkaline pH undergoes a
nonreversible oxidation to form the biologically inactive, straight-chained
compound, 2,3-diketogulonic acid. The half-life for this breakdown is
6 min at 378C [8], 2 min at 708C, and less than 1 min at 1008C [9].
Dehydroascorbic acid is, however, stable for several days at 48C at pH
2.5 –5.5 [5].

15.3 Vitamin C in Foods
15.3.1

Occurrence

The ascorbic acid and dehydroascorbic acid contents of some vegetables
and fruits are listed in Table 15.1 [10]. The values shown are typical of
the observed concentrations found in these samples, but they can vary
greatly (Table 15.2) [11] and should not be taken as absolute. Genetic variation, maturity, climate, sunlight, method of harvesting, and storage all
can affect the levels of vitamin C.
Fresh fruits (especially blackcurrants and citrus fruits) and green vegetables constitute rich sources of vitamin C. Potatoes contain moderate
amounts but, because of their high consumption, represent the most
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TABLE 15.1
Vitamin C Content of Some Vegetables and Fruits
Concentration (mg/100 g)a,b
Food
Vegetables
Broccoli
Fresh, raw
Boiled
Microwaved
Cabbage
Fresh, raw
Boiled
Cauliflower, fresh, raw
Spinach
Fresh, raw
Boiled
Microwaved (4 min)
Peppers
Red, fresh
Green, fresh
Potatoes (without skin)
Raw
Boiled
Tomatoes, fresh
Fruits
Bananas
Grapefruit, fresh
Oranges
Florida
California navel

AA

DHAA

Total

89.0 + 2.0
37.0 + 1.0
111.0 + 2.0

7.7 + 0.6
2.6 + 0.6
4.7 + 0.6

97 + 2
40 + 1
116 + 2

42.3 + 3.4
24.4 + 1.6
54.0 + 1.0



8.7 + 0.6

42 + 3
24 + 2
63 + 1

52.4 + 2.5
19.6 + 1.0
48.3 + 3.7



5.8 + 0.6

52 + 3
20 + 1
54 + 4

151.0 + 3.0
129.0 + 1.0

4.0 + 1.0
5.0 + 0.0

155 + 4
134 + 1

8.0 + 0.0
7.0 + 1.0
10.6 + 0.6

3.0 + 0.0
1.3 + 0.6
3.0 + 0.0

11 + 0
9+1
14 + 1

15.3 + 2.5
21.3 + 0.6

3.3 + 0.6
2.3 + 0.6

19 + 3
24 + 1

54.7 + 2.5
75 + 4.5

8.3 + 1.2
8.2 + 1.6

63 + 3
83 + 5

Values reported are mean + standard deviation based on three measurements. When no
values are listed, the concentration was ,1 mg/100 g sample.
b
Foods analyzed by HPLC together with robotic extraction procedures.
Source: From Vanderslice, J.T., Higgs, D.J., Hayes, J.M., and Block, G., J. Food Comp. Anal., 3,
105, 1990. With permission.
a

important source of the vitamin in the British diet. Liver (containing
10 –40 mg/100 g), kidney, and heart are good sources, but muscle meats
and cereal grains do not contain the vitamin. Human milk provides
enough ascorbic acid to prevent scurvy in breast-fed infants, but
preparations of cow’s milk are a poor source owing to oxidative losses
incurred during processing. Cabbage and other brassica contain a
bound form of ascorbic acid known as ascorbigen, which exhibits
15 –20% bioavailability in guinea pigs [12].
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TABLE 15.2
Range of Vitamin C Values in Some Vegetables and Fruits

Sample
Broccoli, raw
Cabbage, raw
Spinach, fresh
Potatoes, Idaho
Tomatoes
Bananas
Grapefruit, red
Oranges
Florida
California navel

Total Vitamin C
Content (AA þ DHAA)
(mg/100 g)
97–163
42–83
25–70
11–13
14–19
12–19
21–31
53–63
52–78

Source: From Vanderslice, J.T. and Higgs, D.J., Am. J. Clin. Nutr.,
54, 1323S, 1991. With permission.

15.3.2

Stability

Ascorbic acid is very susceptible to chemical and enzymatic oxidation
during the processing, storage, and cooking of food. The catalyzed oxidative pathway of ascorbic acid degradation is the most important reaction pathway for loss of vitamin C in foods. In the presence of
molecular oxygen and trace amounts of transition metals [particularly
copper(II) and iron(III)], a metal –oxygen– ascorbate complex is formed.
This complex has a resonance form of a diradical that rapidly decomposes
to give the ascorbate radical anion, the original metal ion, and hydrogen
peroxide. The radical anion then rapidly reacts with oxygen to give
dehydroascorbic acid [13].
In the absence of free oxygen, an anaerobic pathway of ascorbic acid
degradation leads to the formation of diketogulonic acid [13]. The rate
of degradation is maximum at pH 3 –4 and therefore this pathway
could be responsible for the anaerobic loss of vitamin C in canned grapefruit and orange juices, which have a pH of ca. 3.5. Degradation of
ascorbic acid beyond diketogulonic acid is closely tied to nonenzymatic
browning in some food products [13].
The enzyme mainly responsible for enzymatic degradation of ascorbic
acid in plant tissues after harvesting is ascorbate oxidase (EC 1.10.3.3),
which catalyzes the oxidation of ascorbic acid to dehydroascorbic acid.
This enzyme exhibits maximum activity at 408C and is almost completely
inactivated at 658C [14]. Hence rapid heating, such as the blanching
of fruit and vegetables or the pasteurization of fruit juices, prevents
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the action of this enzyme during post-process storage. The ascorbic acid in
intact plant tissue is protected from ascorbate oxidase and other enzymes
by cellular compartmentation. However, when tissues are disrupted after
bruising, wilting, rotting, or during advanced stages of senescence, the
enzymes gain access to the vitamin and begin to destroy it.
Enzymatic destruction of ascorbic acid in plant tissues begins as soon as
a crop is harvested. Maintaining cool conditions during transport and
storage can markedly reduce vitamin C loss in some vegetables. For
example, newly harvested peas retained 70% of their ascorbic acid
content after 14 days’ storage at 48C compared with 20% retention at
ambient temperature (208C). Corresponding retentions for broccoli were
90 and 30% [15].
Vegetables grown for commercial freezing are processed as soon as
possible after harvest. Some loss of vitamin C takes place during blanching, but little further loss occurs during deep frozen storage. Blanching
losses are greatest in green leafy vegetables with large surface areas.
Thus, steam blanching of broccoli decreased ascorbic acid by about
30%, whereas losses in green beans were only slight [16]. In a study of
frozen stored green beans [17], no significant oxidation of ascorbic acid
occurred during the blanching and freezing steps. During frozen
storage, there was a progressive conversion of ascorbic acid to dehydroascorbic acid, which was almost complete after 20 days of storage at 278C.
The dehydroascorbic acid was very stable at freezer temperatures, the
average loss after 250 days of frozen storage being only 8%. Thus
vitamin C, in the form of dehydroascorbic acid, is well retained during
the frozen storage of green beans.
Chemical oxidation of ascorbic acid is lowered during processing by
carrying out vacuum deaeration and inert gas treatment where feasible.
The headspace in containers should be minimized and hermetically
sealed systems used. Ascorbic acid is very stable in canned or bottled
foods after the oxygen in the headspace has been used up, provided the
food is not subjected to high-temperature storage or exposed to light. In
contrast to glass containers, plastic bottles and cardboard cartons are
permeable to oxygen, so a lowered vitamin C retention is to be expected.
Bronze, brass, copper, and iron equipment should be avoided, while
sequestering agents such as ethylenediaminetetraacetic acid (EDTA),
polyphosphates, and citrates prevent the catalytic action of traces of
copper and iron. The sulfites and metabisulfites, which are added to
juices or beverages as a source of SO2, exert a stabilizing effect on ascorbic
acid in addition to their role as antimicrobial agents. The addition of foodgrade antioxidants such as butylated hydroxyanisole (BHA), butylated
hydroxytoluene (BHT), and propyl gallate also protect the vitamin.
The loss of ascorbic acid from orange juice correlates with the amount
of oxygen initially present in the headspace and that dissolved in the juice,
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Vitamin C

and with temperature and storage time [18 – 20]. In the presence of
dissolved oxygen, ascorbic acid decomposition is predominantly
aerobic, but when the dissolved oxygen is depleted decomposition continues by an anaerobic pathway that is mainly influenced by temperature
[18]. The loss of ascorbic acid also correlates with an increase in the
browning of the juice [20]. Light has little or no effect upon ascorbic
acid levels in orange juice [19]. The vitamin loss is rapid in the early
stage of storage, coincident with the consumption of dissolved oxygen,
and then becomes gradual. The packaging of orange juice in an experimental oxygen-scavenging film reduced ascorbic acid loss in the first 3
days at 258C compared with the loss from juice packaged in a film with
no oxygen scavenger [20]. This demonstrated that the oxygen scavenger
can remove oxygen from the juice before it has the opportunity to react
with the ascorbic acid. Furthermore, the residual oxygen-scavenging
capacity in the experimental film provided an ongoing barrier to
oxygen permeation.
The storage of broccoli spears in elevated (20%) carbon dioxide
atmosphere suppresses tissue respiration and ethylene production rates,
whilst also delaying loss of chlorophyll and ascorbic acid [21]. The
packaging of broccoli spears in polymeric film modified the atmosphere
by elevating carbon dioxide to 8% and lowering oxygen to 10%. While
ascorbic acid retention in packaged broccoli dropped about 15% from
initial values in the first 48 h, losses were minimal during the following
48 h. In contrast, the degradation of ascorbic acid in nonpackaged
samples showed a steady decline over time and decreased 31% by
96 h [22].
Ascorbic acid can leach away from fruits and vegetables during processing or cooking. This is of little importance with canned, bottled, or
stewed fruits where the juice is eaten with the tissue, but may represent
a serious loss with vegetables, where the liquor is drained away before
serving. If vegetables are steamed or pressure-cooked instead of boiled,
the leaching effect is greatly reduced, but a greater loss of ascorbic acid
is to be expected from oxidation. Cold water washing or steeping does
not normally leach out a significant amount of the vitamin in whole undamaged fruits and vegetables. In jam making, when the fruit is boiled with
sugar, ascorbic acid is remarkably stable [23].
Vitamin C in freshly secreted cow’s milk is predominantly in the form
of ascorbic acid, but this is rapidly oxidized by the dissolved oxygen
content [24]. The photochemical destruction of riboflavin accelerates the
oxidation of ascorbic acid in milk through a sensitizing effect [25,26].
Losses of vitamin C content during high-temperature-short time (HTST)
and ultrahigh temperature (UHT) treatment of milk average 20% [27].
Graham and Stevenson [28] studied the effect of ionizing radiation on
vitamin C content of strawberries and potatoes using 60cobalt as the
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source of gamma rays. Their HPLC method of analysis permitted the
separate measurement of total vitamin C, ascorbic acid, and dehydroascorbic acid. Fresh strawberries have a relatively short shelf-life,
mainly because of fungal spoilage. Irradiation at doses of 2 –3 kGy
combined with refrigeration has been shown to extend the shelf-life of
strawberries. Low doses (0.075 – 0.15 kGy) of g-radiation are very effective
in inhibiting the sprouting of potatoes during post-harvest storage.
Strawberries were assayed immediately after irradiation at doses of 1, 2,
or 3 kGy and then after storage for 5 and 10 days at 68C. Irradiation had
the effect of increasing the dehydroascorbic acid content in accordance
with dose, whilst decreasing the concentrations of total vitamin C and
ascorbic acid. During the storage of irradiated strawberries, the total
vitamin C and ascorbic acid levels increased, whereas dehydroascorbic
acid levels decreased. Overall, irradiation of the strawberries caused
some loss of total vitamin C, which increased during storage (Table 15.3).
Raw, boiled, and microwaved potatoes were assayed immediately after
irradiation at 0.15 kGy and then after storage for 1, 2, and 5 months at
128C. The potatoes were irradiated 1 month after harvesting to allow
them to recover from the effects of post-harvest stress. Losses of total
vitamin C in the raw potatoes immediately after irradiation were about
8%; corresponding losses in the cooked potatoes were about 20%. There
was no significant difference in the dehydroascorbic acid content
between irradiated and nonirradiated samples. After 2 and 3 months’
storage, irradiated potatoes contained less vitamin C than nonirradiated
potatoes. However, after 5 months’ storage, both irradiated and nonirradiated potatoes had comparable vitamin C levels. The vitamin C content
in both irradiated and nonirradiated cooked potatoes showed similar
changes on storage to those of raw potatoes. Similar findings for potatoes
were reported by Shirsat and Thomas [29]. The loss of total vitamin C in
nonirradiated potatoes after 3 months in storage (158C) was 26– 45%,
depending on the cultivar. Additional losses of 6.5 –13% were

TABLE 15.3
Percent Loss of Vitamin C in Four Varieties of Strawberry Following a 3-kGy
Dose of Gamma Radiation
Storage (days)
0
5
10

Variety 1

Variety 2

Variety 3

Variety 4

Mean

12.8
14.1
22.7

12.2
3.8
14.0

6.4
7.5
13.6

6.3
20.2
19.5

9.4
11.4
17.5

Source: From Graham, W.D. and Stevenson, M.H., J. Sci. Food Agric., 75, 371, 1997. With
permission.
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recorded in the irradiated samples stored for 3 months. From the fourth
month onwards, the vitamin C levels in the irradiated samples began to
increase. In conclusion, irradiation keeps the potatoes in good marketable
condition during proper storage conditions for at least 5 months, with no
additional loss of vitamin C after this period.
Two studies were carried out to evaluate the possibility of using high
hydrostatic pressure (HHP) treatment as an alternative to the blanching
of vegetables. Potato cubes retained 90% of their ascorbic acid content
after treatment at 400 MPa and 58C for 15 min, but only 35% when the
temperature was increased to 508C [30]. Green peas retained 82% ascorbic
acid after treatment at 900 MPa and 438C for 5 min [31]. Other studies
have evaluated HHP treatment as a possible alternative to the thermal
pasteurization of freshly squeezed fruit juices. Ascorbic acid in orange
juice and tomato juice was shown to be unstable at a combination of relatively high pressure (850 MPa) and temperature (60 –858C). At the same
pressure level and lower temperature (508C), no degradation of ascorbic
acid occurred within 1 h [32]. High-pressure treatment of chilled orange
juice (500 and 800 MPa for 5 min) and storage up to 21 days at 48C
caused no significant difference in vitamin C [33]. Similar treatment of
orange juice (800 MPa at 258C for 1 min) greatly extended the shelf-life,
with less than 20% loss of ascorbic acid after 3 months’ storage at 48C
or after 2 months at 158C [34]. Pressure-processing of guava puree
(600 MPa and 258C for 15 min) and storage at 48C for 40 days did not
alter initial content of ascorbic acid [35].
Sa´nchez-Morino et al. [36] tested three processes that combined
high-pressure treatment with heat treatment for their effect on vitamin C
retention in orange juice: T0, fresh juice (without treatment); T1,
100 MPa/608C/5 min; T2, 350 MPa/308C/2.5 min; T3, 400 MPa/408C/
1 min. Fresh and treated samples were kept refrigerated (48C) and
assayed at intervals over 10 days. T1 and T3 juices showed a small
(,10%) loss of vitamin C just after processing, whereas T2 juices showed
no loss of the vitamin. There was no further degradation of vitamin C
during the 10-day storage period. Therefore, the intermediate pressure/
low-temperature treatment best preserved the vitamin C content.
A major factor contributing to the variability in vitamin C content of
potatoes is the storage time. A sharp decrease in vitamin content was
observed during the first 4 months of storage at 78C and 95% relative
humidity, followed by either a complete leveling out or a less pronounced
decrease [37].
Wang et al. [38] reported on the losses of added ascorbic acid during the
pilot-scale processing and storage of potato flakes, and during the reconstitution and holding of the mashed potatoes prior to serving. Cumulative
losses were: 56% after addition of ascorbic acid to the cooked mashed
potatoes followed by drum-drying; 82% storing the flakes 4.3 months
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at 258C; and 96% reconstituting mashed potatoes and holding them
30 min on a steam table. The total cumulative loss of ascorbic acid was
still severe (82%) when the dried potato flakes were stored for 1 month
at 258C and the reconstituted mashed potatoes were held on the steam
table for 15 min. The initial 56% loss occurred during the cooking and
mashing steps, with little loss during drum-drying. The entrapped air,
high moisture content and high temperature of the mashed potatoes
explain the high losses of ascorbic acid. There was no loss of ascorbic
acid during reconstitution of the mashed potatoes. Adding ascorbic
acid at a level of 251 ppm to the freshly mashed potatoes gave a final
level of 10 ppm ascorbic acid in the reconstituted mashed potatoes at
the point of consumption. In contrast, addition of magnesium L -ascorbate
2-monophosphate or sodium L -ascorbate 2-polyphosphate at about
250 ppm ascorbic acid equivalents produced mashed potato with
ascorbic acid equivalents of 201 or 171 ppm, respectively (20 or 30%
overall losses). These two compounds are more stable toward oxygen
than ascorbic acid.
Williams et al. [39] compared two foodservice systems for their effect on
retention of ascorbic acid in vegetables: (1) cook/hot-hold and (2) cook/
chill, where food is cooked, chilled and held up to 5 days before reheating.
The cook/chill system retained less vitamin C than food held hot for
30 min (50 versus 65%), but more than food held hot for 2 h.
The amount of water used in domestic cooking and, to a lesser extent,
the cooking time affect vitamin C losses more than the source of energy or
the type of cooking [14]. If short cooking times and small amounts
of water are used, more vitamin C will be retained in any cooking
method. Theoretically, stir-frying should provide maximum vitamin C
retention. When the same ratio of water to vegetable (1 : 4) was used in
the microwaving and boiling of frozen peas, ascorbic acid retentions
were similar (70%), but lower than when no water was used in the microwave oven (.96%). In most frozen vegetables, sufficient ice clings to the
product to provide adequate moisture for cooking in the microwave oven.
When microwaving fresh vegetables, it is advisable to add a minimum
amount of water to prevent scorching. When boiling vegetables, boiling
water should be added to the vegetables and boiling maintained in
order to rapidly inactivate enzymes that would otherwise destroy the
vitamin C.

15.3.3

Applicability of Analytical Techniques

In food analysis, a method for determining vitamin C should ideally
account for both ascorbic acid and its reversible oxidation product, dehydroascorbic acid, to give a total value for vitamin C. In addition, the ability
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to distinguish ascorbic acid from its epimer D -isoascorbic acid (erythorbic
acid) is useful in the analysis of processed foods.
The classic titrimetric method using 2,6-dichlorophenolindophenol
accounts for ascorbic acid, but not dehydroascorbic acid. Nonchromatographic methods for determining total vitamin C include colorimetry
and fluorometry, in which ascorbic acid is oxidized to dehydroascorbic
acid and then reacted with a chemical reagent to form a colored or fluorescent compound. Total vitamin C can be determined by HPLC using
absorbance or electrochemical detection after reduction of dehydroascorbic acid to ascorbic acid, or using fluorescence detection after oxidation of
ascorbic acid and derivatization of the dehydroascorbic acid formed.
Capillary electrophoresis offers an alternative to HPLC and eliminates
the need for organic mobile phases and expensive chromatography
columns. Flow-injection analysis coupled with immobilized enzyme
and using electrochemical detection confers high specificity and provides
a rapid automated procedure using relatively simple apparatus. Results
obtained by chemical analysis are usually expressed in milligrams of
pure L -ascorbic acid.

15.4 Intestinal Absorption
Much of the following discussion of absorption is taken from a book by
Ball [40] published in 2004.
15.4.1

General Principles

Approximately 80– 90% of the vitamin C content of a given foodstuff exits
in the reduced form, ascorbic acid; the remainder is in the oxidized form,
dehydroascorbic acid. Ascorbic acid and dehydroascorbic acid are
absorbed by separate transport mechanisms in animal species that
depend upon dietary vitamin C (Figure 15.3). Inside the absorptive cell
(enterocyte) of the intestinal epithelium, dehydroascorbic acid is enzymatically reduced and the accumulated ascorbic acid is transported
across the basolateral membrane to the bloodstream. In addition to
uptake at the brush-border membrane, dehydroascorbic acid from the
bloodstream can be taken up at the basolateral membrane, reduced
within the cell, and returned to the circulation in the form of the useful
and nontoxic ascorbic acid. The serosal uptake of dehydroascorbic acid
from the bloodstream and intracellular reduction to ascorbic acid take
place in animal species which do not have a dietary vitamin C requirement as well as those species that do. The ability of the enterocyte to
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Vitamins in Foods: Analysis, Bioavailability, and Stability
brush-border membrane
of microvillus

basolateral
membrane

enterocyte

DHAA

301

DHAA

DHAA

(H)

intestinal
lumen

AA–

AA–

connective
tissue
AA–
Na+
Na+
K+

FIGURE 15.3
Model of intestinal transport of the L -ascorbate anion (AH2) and uncharged
dehydroascorbic acid (A) in vitamin C-dependent animals. Thick arrowed lines indicate
directional pathways; [H] signifies enzymatic reduction. (From Ball, G.F.M., Vitamins. Their
Role in the Human Body, Blackwell Publishing Limited, Oxford, 2004, p. 393. With permission.)

absorb dehydroascorbic acid efficiently is important because, apart from
the indigenous dehydroascorbic acid content of the diet, additional oxidation of ascorbic acid occurs in the gastrointestinal tract as the vitamin
functions to maintain other nutrients such as iron in the reduced state.
The intestinal uptake and reduction of dehydroascorbic acid
explains why this compound, orally administered, maintains plasma
concentrations of ascorbic acid and prevents scurvy. The overall system
of intestinal transport and metabolism is designed to maximize the conservation of vitamin C and also to maintain the vitamin in its nontoxic reduced
state, whether it is derived from the diet or from the circulation.

15.4.2

Transport Mechanisms

15.4.2.1 Ascorbic Acid
Absorption of physiological intakes of ascorbic acid by guinea pigs takes
place mainly in the ileum and occurs as a result of specific carriermediated mechanisms in the brush-border and basolateral membranes
of enterocytes [41 –44]. Ascorbic acid is 99.9% ionized within the pH
range of intestinal chyme, and therefore it is the ascorbate anion
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