Stability and Modes of Degradation Folate Stability
Folic acid exhibits excellent retention during the processing and storage of fortified foods and premixes [49]. As shown in Tables 2 and 3, little degradation of this form of the vitamin occurs during extended low-moisture storage. Similar good retention of added folic acid has been observed during the retorting of fortified infant formulas and medical formulas.
Many studies have shown the potential for extensive losses of folate during processing and home preparation of foods. In addition to susceptibility to oxidative degradation, folates are readily extracted from foods by aqueous media (Table 26). By either means, large losses of naturally occurring folate can occur during food processing and preparation. The overall loss of folate from a food depends on the extent of extraction, forms of folate present, and the nature of
TABLE 26 Effect of Cooking on Folate Content of Selected Vegetables
Total folate3 (mg/100 g fresh weight)
TABLE 26 Effect of Cooking on Folate Content of Selected Vegetables
Total folate3 (mg/100 g fresh weight)
|
Vegetable (boiled 10 min in water) |
Raw |
Cooked |
Folate in cooking water |
|
Asparagus |
175 ± 25 |
146 ± 16 |
39 ± 10 |
|
Broccoli |
169 ± 24 |
65 ± 7 |
116 ± 35 |
|
Brussels sprouts |
88 ± 15 |
16 ± 4 |
17 ± 4 |
|
Cabbage |
30 ± 12 |
16 ± 8 |
17 ± 4 |
|
Cauliflower |
56 ± 18 |
42 ± 7 |
47 ± 20 |
|
Spinach |
143 ± 50 |
31 ± 10 |
92 ± 12 |
Source .Adapted from Ref. 82
the chemical environment (catalysts, oxidants, pH, buffer ions, etc.) Thus, folate retention is difficult to predict for a given food. Degradation Mechanisms
The mechanism of folate degradation depends on the form of the vitamin and the chemical environment. As mentioned previously, folate degradation generally involves changes at the C9-N10 bond, the pteridine ring system, or both. Folic acid, H4folate, and Kfolate can undergo C9-N10 cleavage and resulting inactivation in the presence of either oxidants or reductants [91]. Dissolved SO2 has been found to cause cleavage of certain folates, although few other reducing agents relevant to foods can induce such cleavage. There is little direct oxidative conversion of ^folate or folic acid.
It is well known that oxidative cleavage of H4folate, Kfolate and, to a lesser extent, folic acid yields nutritionally inactive products (p-aminobenzoylglutamate and a pterin). The mechanism of oxidation and the exact nature of the pterin produced during oxidative cleavage of ^folate vary with pH, as shown in Figure 37.
The major naturally occurring form of folate in many foods is 5-methyl-H4folate. Oxidative degradation of 5-methyl-H4folate occurs by conversion to at least two products (Fig. 38). The first has been identified tentatively as 5-methyl-5,6-dihydrofolate (5-methyl-H2folate), which retains vitamin activity since it can be readily reduced back to 5-methyl-H4folate by weak reductants such as thiols or ascorbate. 5-Methyl-H2folate undergoes cleavage of the C9-N10 bond in acidic medium, which causes losses of vitamin activity. The alternate product of 5-methyl-H4folate degradation was originally identified as 4a-hydroxy-5-methyl-H4folate, although spectral data are more consistent with a pyrazino-s-triazine structure formed by rearrangement of the pteridine ring (Fig. 38) [68]. Whether 5-methyl-H2folate is an intermediate in the formation of the pyrazino-s-triazine has not been determined.
Blair et al. [10] reported that the pH dependence of 5-methyl-H4folate oxidation is pronounced. Stability (as monitored by oxygen uptake) increases as pH is reduced from 6 to 4, this range corresponding to the range of protonation of the N5 position. Contrary results have been reported [95], and factors responsible for this contradiction have not been determined.
In certain animal products, such as liver, 10-formyl-H4folate may account for as much as one-third of the total folate. Oxidative degradation of 10-formyl-H4folate can occur either by oxidation of the pteridine moiety to yield 10-formyl-folic acid or by oxidative cleavage to form a pterin and N-formyl-p-aminobenzoylglutamate (Fig. 39). 10-Formyl-folic acid has nutritional acitivity while the cleavage products do not. Factors that influence the relative importance of these oxidative patheways in foods have not been determined. In contrast to 10-formyl-H4folate, 5-formyl-H4folate exhibits excellent thermal and oxidative stability.
Factors Affecting Folate Stability
Many studies have been conducted to compare the relative stability of folates in buffered solution as a function of pH, oxygen concentration, and temperature. Stability of folates in complex foods is less well understood.
Folic acid is generally the most stable form. It is resistant to oxidation, although reduced stability occurs in acidic media. H4folate is the least stable form of the vitamin. Maximal stability of ^folate is observed between pH 8 and 12, and 1 and 2, while the stability is minimal between pH 4 and 6. However, even in the favorable pH zones, ^folate is extremely unstable. ^folates having a substituent at the N5 position exhibit much greater stability than does unsubstituted ^folate. This suggests that the stabilizing effect of the N5 methyl group is due, at lest in part, to steric hindrance in restricting access of oxygen or other oxidants to the pteridine ring. The stabilizing effect of the N5 -substituent is more pronounced with 5-formyl-H4folate than with
FIGURE 37
Oxidative degradation of H4folate. (Adapted from Ref. 107)
FIGURE 37
Oxidative degradation of H4folate. (Adapted from Ref. 107)
5-methyl-H4folate, and both exhibit much greater stability than H4folate or 10-formyl-H4folate. Under conditions of low oxygen concentration, 5-methyl-H4folate and folic acid exhibit similar stability during thermal processing.
The influence of oxygen concentration on the stability of folates in foods, buffer solutions, and model food systems has been widely studied. As menthioned previously, the rate of oxidation
of 5-methyl-H4folate is dependent on the concentration of dissolved oxygen in accord with a pseudo-first-order relationship. In relatively anaerobic conditions, the presence of added components such as ascorbate, ferrous iron, and reducing sugar tends to improve the oxidative stability of folic acid and 5-methyl-H4folate. These components apparently function by reducing the concentration of dissolved oxygen through their own oxidation reactions (Fig. 40). These findings indicate that complex foods can contain components that influence folate stability by consuming oxygen, acting as reducing agents, or both.
Barrett and Lund [6] studied thermal degradation of 5-methyl-H4folate in neutral buffer solution and observed both aerobic and anaerobic degradation. Surprisingly, rate constants for aerobic and anaerobic degradation reactions are of similar magnitude (Table 27). The extent to which other folates conform to this behavior has not been determined.
The ionic composition of the medium also significantly influences the stability of most folates. Phosphate buffers have been reported to accelerate oxidative degradation of folates, while this effect can be overcome by addition of citrate ions. The frequent presence of Cu(II) as a contaminant in phosphate buffer salts may explain this effect because metal catalysts are known oh ÇH3 „ ,,
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