# Folic acid fortification of double fortified salt

26 Oct.,2022

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In the process described by Li et al.15 for the double fortification of salt, iodine as a solution of potassium iodate was sprayed onto the salt, and iron was added as extruded and microencapsulated ferrous fumarate. The addition of folic acid through the iodine solution was thought to be the easiest path for adding folic acid to salt. Hence, in a first step to making salt fortified with folic acid, a solution of folic acid and iodine was added to salt22. However, the salt's high moisture content (2.9%) due to the low concentration of folic acid and iodine (0.35%, each) in the spray solution accelerated iodine loss in the salt. Therefore, a higher concentration of folic acid and iodine (at least 1%, each) was formulated. With this spray solution, less solution was added to salt, resulting in low moisture content (0.06% or less).

### Preliminary formulation of spray solution

The impact of sodium carbonate buffer concentration on folic acid solubility in the spray solution was evaluated. The solubility of folic acid in the solution increased as the carbonate buffer concentration increased; at least 0.2 M sodium carbonate buffer was required to dissolve 3% folic acid. The MS spectra of folic acid in the solution showed the formation of mono- and disodium salts of folic acid. The relative abundance of the disodium salt was higher in the solution that contained potassium iodate (Fig. 1). The formation of sodium salt was responsible for folic acid solubility and explained why a higher concentration of sodium carbonate buffer is required to dissolve 3% folic acid.

Figure 1

Mass spectrum of folic acid in spray solution (a) folic acid in sodium carbonate solution (b) folic acid and potassium iodate in sodium carbonate solution.

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The stability studies carried out on the second set of solutions (that contained 1% folic acid + 1% iodine, 2% folic acid + 2% iodine, and 1.8% folic acid + 3% iodine) showed that both iodine and folic acid were very stable in the solution. Less than 20% of the added micronutrients were lost after 2-month storage, even at 45 °C. This result was consistent with those reported by McGee et al.18,22. While increasing the iodine concentration resulted in a significant increase in iodine and folic acid stability, increasing folic acid concentration did not impact iodine and folic acid stability in any particular pattern. The relatively higher folic acid stability observed in the solution that contained higher iodine (3%) may be due to the formation of more disodium salt of folic acid. At the point of preparation, 0.2 M sodium carbonate buffer dissolved folic acid, irrespective of the amount of folic acid added (1, 1.8, or 2% folic acid). However, in the solutions that contained more than 1% w/v folic acid, some of the folic acids precipitated out from the solution, which can be a significant problem for small salt plants that store their spray solutions for up to a month.

### Stability of iodine and folic acid in the optimized spray solution

To prevent folic acid precipitation, the spray solution was reformulated, reducing the folic acid concentration to 0.5–1%, and the buffer solution used in the previous method was replaced with a sodium carbonate solution (0.1 M). The solution of sodium carbonate itself is a buffer as it dissociates to sodium and bicarbonate ions. The iodine concentration was maintained at 2% in the spray solution to be consistent with industrial practice. The change from the buffer (prepared with sodium carbonate and sodium bicarbonate) reduced the number of steps required for making the spray solution. In all the samples, 70–100% of folic acid and iodine were retained after two months of storage (Table 2).

Table 2 Stability of folic acid and iodine in the optimized spray solution.

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The concentration of folic acid in the spray solution did not significantly affect folic acid and iodine stability in the spray solutions, except at 25 °C (Table 2). At this temperature, the percentage of folic acid retained in the spray solution, containing only 0.5% folic acid, was significantly lower than other solutions. This trend was also was reported by McGee et al.18. As shown earlier, potassium iodate accelerated the formation of sodium salts of folic acid in the solution. This may be responsible for the improved stability of folic acid in solutions that contained potassium iodate.

pH had a significant effect on the solubility of folic acid in the KIO3 solutions. After a few weeks, folic acid precipitated out of the solutions at pH 7 and 8. This observation was consistent with the study of Taub and Lieberman23, who found that folic acid solution at pH 6 turned cloudy after a few days. Folic acid in solutions adjusted to pH 9 and 10 did not precipitate even after a few months. Hence, the pH of subsequent solutions was maintained at ≥ pH 9. In order to obtain pH 9, 0.742 g sodium carbonate, 3.37 g potassium iodate, and 1 g folic acid were dissolved in 100 mL of water.

### Stability of iodine and folic acid in triple fortified salt (TFS) after 6-months storage

The first attempt on triple fortification of salt was reported by McGee22, who formulated TFS by spraying 0.35% iodine and folic acid on salt and adding iron premix prepared by a one-step agglomeration and encapsulation using spray drying24. The idea was that the relatively smaller iron premix particles would adhere to the salt particles' surface. The lower concentration of folic acid and iodine in the spray solution led to adding more liquid to the salt (30 mL/kg); hence, increasing the salt's moisture content. The high moisture content of the salt accelerated the rate of loss of iodine. The iron premix colour was not acceptable as it formed grey spots in the salt24.

In contrast to the technology developed by McGee22, a higher concentration of folic acid and iodine (05–1% and 2%, respectively) was used. The spray solution volume was drastically reduced from 30 to 2.5 mL/kg salt. The newly fortified salt's moisture content was 0.06% compared to 2.9% of the fortified salt formulated by McGee22. The premix, prepared by forming extrusion and microencapsulation similar to that developed by19,20, was used in place of the premix formulated by spray drying. The similar particle size and density of the premix and salt were produced ensured that the iron premix would not segregate from the salt.

Table salt stays in the distribution channels for an average of 2 months25. Also, the target population buys a small amount of salt, typically consumed within two months. Hence, the goal is to have at least 70% of the micronutrients retained in the fortified salt after 6-month storage. This was achieved; after 6-month storage, 70–85% of folic acid and 85–95% of iodine were retained in all the samples, even at 45 °C and 60–70% RH (Table 3). This result confirmed that the technology could deliver iron, iodine, and folic acid simultaneously through salt. Given the traditional distribution channels of salt, Triple Fortified Salt has the potential of reaching millions of vulnerable households that otherwise may not have access to diets with sufficient iron, iodine, and folic acid.

Table 3 Stability of folic acid and iodine in T after 6-month storage.

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The direct addition of folic acid on the surface of the salt resulted in bright yellow salt. Although the reduction in folic acid concentration in the salt (12.5 ppm) significantly reduced the salt's yellow colour, it was still visibly yellow. The colour may reduce the end-users' acceptance of the salt since colour is an essential factor for selecting food products by consumers26. Hence, technology to eliminate this colour change was investigated.

### Optimizing the process of triple fortification of salt

Folic acid was added to the iron premix to form a Fe-FA premix. There were two designs for Fe-FA premix—either iron and folic acid were added to the core of the premix (Fe + FA) or folic acid was separated from the iron in the core by a thin layer of TiO2 (Feextrudate + FA), as illustrated in Fig. 2B. Iron and folic were coextruded to have both micronutrients in the core of the premix. Coextruding iron and folic acid was a straight forward process; it only involved adding folic acid to ferrous fumarate before extrusion. Separating folic acid from the core was achieved by two different routes:

1. a.

adding folic acid as a uniform suspension in water to colour masked iron extrudate, or

2. b.

spraying a folic acid suspension in 2.5% HPMC (in a 1:1 ethanol and dichloromethane solvent system) on color-masked iron extrudate tumbled in a pan coater.

Figure 2

(A) Stability of iodine and folic acid in TFS formulated with Fe-FA premix. (B) Schematic of the two designs for iron-folic acid premix and how one of the premix samples (Fe extrude + FA) was made (Fe + FA) has both iron and folic acid in the core of the premix; (Feextrude + FA) has folic acid separated by a tiny layer of TiO2 in the premix; (Fe) has only iron in the premix, and folic acid and iodine was added as a solution.

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The solvents' ratio that made up the solvent system is vital to having a uniform suspension of folic acid that is volatile enough for pan coating. A 1:1 dichloromethane: ethanol solvent system was used. More dichloromethane caused folic acid to settle at the bottom of the spray flask, while more ethanol caused the colour masked iron extrudate to clump inside the pan coater. Spraying folic acid suspension in 2.5% HPMC (Route b) resulted in a more uniform distribution of folic acid than the other method (Route a).

### Calculation and validation of the kinetic parameters for the stability of iodine and folic acid in TFS

Kinetic tools, such as degradation constant and Gibb free energy, are vital to predicting micronutrients' stability. The data obtained from the 6-month stability study were used to calculate the kinetic parameters for micronutrient stability in salt (Fig. 3A). The linear regression (R2) of the different rate laws of degradation of the micronutrients was used to predict the order of degradation.

Figure 3

(A) Sample of the zero- and first-order degradation kinetics of iodine in a fortified salt; (B) Sample of the Arrhenius plot for the zero- and first-order degradation kinetics of iodine in a fortified salt.

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The correlation coefficient (R2) of the first-order rate was slightly higher than that of the zero-order (Fig. 3A) but very much higher than that of the second-order rate. The slight difference between zero and first-order rate may be due to the two likely mechanisms of degradation of the micronutrient in the salt- diffusion through the coat of the premix and chemical interaction among the micronutrients. While the diffusion is a zero-order rate, the chemical interaction is a first-order rate. The first-order rate of degradation was chosen because chemical interaction is likely prominent of the two mechanisms. Although the activation energy (obtained from Fig. 3, Arrhenius plot, and Eq. 3) for the first-order degradation was higher than that of the zero-order, they showed similar trends (Table 4).

$$E_{a} = - \left( {slope~\;of\;Arrhenius\;~plot~ \times R~} \right)$$

(3)

Table 4 Kinetic Parameters of the Degradation of Folic Acid and Iodine in DFS* and TFS.

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where Ea = activation energy (J/mole), R = gas constant (8.314 J/K⋅mole).

The activation energies of the C salts were higher than those of the D salts. The iron premix's presence increased the activation energies for folic acid in the salt but reduced iodine's activation energies in the salt (C vs. CA and D vs. DA, Table 1). These observations imply that folic acid and iodine were more stable in the C salts than in D salts and that iron seems to improve folic acid stability while decreasing the stability of iodine in the salts. As stated earlier, the reducing power of the ferrous fumarate may have played a role in this. While it reduces the iodate to iodine, which is then lost by sublimation, the iron may have reduced folic acid's oxidative degradation. These deductions are consistent with the trend of folic acid and iodine stability in TFS samples.

The kinetic parameters of the degradation of micronutrients in the TFS were derived based on a 6-month was validated with the stability of micronutrients in the salt in a 12-month study (Table 1). Equation 4 was derived given that the degradation of iodine and folic acid in the TFS primarily obeyed the first-order rate law. Value t(R, T) being approximately equal to 12 months, obtained when R(T) from twelve-month stability study and corresponding k(T) from six-month stability was put into Eq. (4), validates TFS's degradation constants and Eq. (4). Using Eq. (4), the time it will take to lose 25% of the iodine and folic acid (R(T) = 75%) in TFS samples was calculated. From the calculation, it will take 15, 9, and 7 months to lose 25% folic acid in TFS with the best outcome at 25, 35, and 45 °C, respectively. For iodine, it will take 26, 16, and 13 months, respectively. In all samples, the micronutrients can be projected to retain at least 75% of the added micronutrients for more than 6 months.

$$t_{{\left( {R,T} \right)}} = \frac{{\ln \left( {\frac{1}{{R_{{\left( T \right)}} }}} \right)}}{{k_{{\left( T \right)}} }}$$

(4)

where; t(R, T)= time (in months) required to retain micronutrient (%) in fortified salt at a given temperature, R(T)= retention of micronutrients in fortified salts (%) for a given temperature; values were obtained from the 12-month stability study, k(T)= degradation constant obtained from the 6-month stability study.

### Impact of cooking on the stability of folic acid

The impact of boiling and fermentation on the folic acid was evaluated in cooked rice and Bondi raita. There were several failed attempts for cooked rice because of the difficulty in extracting the micronutrients from cooked rice. The extraction involves sieving, and the high content of amylopectin in rice made sieving impossible. The use of sodium carbonate as a flocculant did not solve the problem. After several failed attempts, cooking rice with excess water (1:9, rice: water) helped resolve this problem. Even then, the filtration was nearly impossible. The extraction of folic acid from Bondi raita did not cause much of a problem. The fortified salt contributed significantly to the folic acid content of the two foods. Given the amount of salt added to the foods and concentration of folic acid in the unfortified and fortified cooked foods, over 70% of the folic acid due to added fortified salt was retained in the cooked rice. There was no observed sensory difference between the cooked foods with or without the fortified salt.

### Cost analysis

Since the poor and vulnerable populations, who cannot afford or do not have access to processed fortified foods, are the primary targets of this technology, there was a need to evaluate the technology's cost implication to assess if the targeted population can afford the fortified salt. The formulation of premix is the additional unit operation to the traditional process of making iodized salt. Based on the average daily consumption of 10 g salt31,32, the additional cost of triple fortification is about 27¢/person per year. It is assumed that this cost can be reduced further by large-scale production and be covered by government or philanthropic contributions for the needy.