Impact of Iodine Biofortification on Tomato Bioaccessibility through Cooking, Soaking and Simulated Digestion

The earth’s iodine (as iodide) is extensively spread but unevenly distributed because iodine is a rare element that is primarily found as a salt, it is referred to as iodide rather than iodine. Iodine is found in soils in both inorganic and organic forms [Iodate-(IO₃^-) and Iodide- (I^-)]. Iodine is an essential micronutrient for a person’s mental and physical development (Antonyak et al., 2018). Iodine is a component of thyroid hormone, which is essential for human health and plays an indispensable function in metabolism (Sorrenti et al., 2021). The recommended daily allowance (RDA) of iodine is 120 µg for children 6 to 12 years old, 150 µg for adults over 12 years old and 200 µg for pregnant and nursing women, according to the World Health Organization (WHO) and the United Nations Children’s Fund (UNICEF).
       
The thyroid hormone will stop acting if its iodine requirements are not met. Low thyroid hormone levels in the blood caused a slew of functional and developmental problems known as iodine deficiency disorder (IDDs) (Snart et al., 2019). The universal fortification of salt with iodine is one technique to treat iodine deficiency disorder (IDD). Over the last few decades, numerous countries have embraced this technique, which has drastically reduced the prevalence of iodine deficiency around the world (Abdurrahim et al., 2023). This strategy, however, will not be sufficient to treat iodine deficiency disorder. This is due to the fact that the iodine in table salt is unstable and exposed to increased volatilization (Dávila-Rangel et al., 2019). The concentration of iodine in salt, on the other hand, is highly variable and depends on the level of iodization at the time of manufacture, as well as future losses due to cooking or inappropriate storage (Desta et al., 2019).
       
Over the last few decades, fortification has helped to lower the prevalence of iodine deficiency around the world. Adding iodine-containing salts or iodine-rich organic materials (e.g., seaweed) to soils can help crops absorb and concentrate this trace element. Iodine deficiency can be avoided by biofortifying commonly consumed crops with iodine (Dijck-Brouwer et al., 2022). Potassium iodide (KI), Potassium iodate (KIO₃) and potassium per iodate (KIO₄) are some of the common salts used for biofortification study. Among them potassium iodate salt is more stable when compared to potassium iodide as it is oxidized readily and lost by evaporation (Iwan et al., 2019).
 
Chitosan (Cs) is a biological polymer that acts as a metal and trace metal complexing agent and it is biodegradable (Gamage et al., 2023). Chitin is a polysaccharide-rich fibrous substance found in the exoskeletons of shellfish such as shrimp, lobsters and crabs as well as the cell walls of fungi (Ali et al., 2022). If Iodine is applied as a chitosan-iodate complex, the absorption of the iodine will be boosted. Hence in the present study potassium iodate along with chitosan is preferred over potassium iodide. The objective of the present study was to assess the potential of biofortified iodine in retaining the iodine content in the biofortified crop through some bioavailability experiments. 

To know the effect of biofortified iodine in improving the bioavailability of iodine in tomato  two field experiment was carried out in summer and kharif season during the year 2021 in Viraliyur village of Thondamuthur block of Coimbatore district of Tamil Nadu (GPS value: 10°.9'99.284"N; 76.7' 82.652"E). Iodine was biofortified through soil, foliar and chitosan iodate complex mechanism for the main crop. The experiments were performed in randomized block design with three replications in palaviduthi soil series using hybrid tomato “Shivam”. The treatments were T₁- KIO₃-Soil Application (SA)- 5 kg ha^-1, T₂- KIO₃-SA- 10 kg ha^-1, T₃- Chitosan-SKIO₃ Complex-5 kg ha^-1,T₄- Chitosan-KIO₃ Complex-10 Kgha_-1, T₅- Foliar Application (FA)-KIO₃-0.2% @ 60 and 90 DAT, T₆- FA-KIO₃-0.3% @ 60 and 90 DAT, T₇- KIO₃- SA- 5 Kgha_-1 +FA-KIO₃- 0.2% @ 60 and 90 DAT, T₈- KIO₃-SA- 10 kg ha^-1 + FA-KIO­₃- 0.2% @ 60 and 90 DAT, T₉- Chitosan-KIO₃ Complex-5 Kgha^-1 +FA-KIO₃-0.2% @ 60 and 90 DAT, T₁₀-Chitosan-KIO₃ Complex-10 Kg ha_-1 + FA-KIO₃-0.2% @ 60 and 90 DAT, T₁₁- KIO₃- SA- 5 Kg ha^-1 + FA-KIO₃- 0.3% @ 60 and 90 DAT, T₁₂- KIO₃- SA- 10 Kg ha^-1 + FA-KIO₃- 0.3% @ 60 and 90 DAT. T₁₃- Chitosan-KIO₃ Complex-5 Kg ha^-1 + FA-KIO₃-0.3% @ 60 and 90 DAT, T₁₄- Chitosan-KIO₃ Complex-10 Kg ha^-1 + FA-KIO₃-0.3% @ 60 and 90 DAT, T₁₅- Chitosan Spraying (control) and T₁₆- Water spraying (Absolute Control). After two days of transplanting potassium iodate and the chitosan iodate complex solution was fertilized into the soil. The plants were grown in medium black clay loam soil which is low in nitrogen and medium in organic carbon, phosphorous and potassium. The soil is neutral in pH and non saline in EC. Fruit samples were collected during green; pink and red ripen harvest stages of summer (main) and kharif season tomato (residual) crop to conduct cooking, soaking and digestion bioavailability experiments (Table 1). The iodine concentration was measured using inductively coupled plasma-optical emission spectrometry by following (Knapp et al., 1998) procedure. The data obtained were subjected to one-way ANOVA. The programme IBM SPSS ® Statistics, version 25 was used to run all statistical tests.
 

Effect of potassium iodate and iodine chitosan complex on soaking
 
Cleaning of vegetables is a necessary provision before going for cooking. It is standard practice in the kitchen to always wash the vegetables initially and then slice them, as compared to the reverse. Food can lose important nutrients if it is chopped first and then washed. The soaking experiment has been conducted in three stages to determine the loss of iodine in the fruits of tomato. During the first stage (0-8 hours) the loss of iodine from the biofortified fruits accounted for about 8-24% in main crop and 16-35% in residual crop. During second stage (8-16 hours), the iodine loss increased significantly and reached a maximum of 47% in main crop and 78% in residual crop. During third stage (16-24 hours), the iodine loss was only 52% and 87% and it almost attained equilibrium in both the crops. Since the cleaning process for vegetables only takes a matter of minutes, very little iodine is lost during washing of biofortified vegetables. Irrespective of the treatments in all the stages the loss of iodine in biofortified fruits was higher in the KIO₃ foliar application alone and combined KIO₃ soil and foliar application treatments. The amount of iodine retained in the biofortified fruits was higher in combined Cs-KIO₃ and FA- KIO₃ treatments in all the soaking stages (Table 2 and 3). As chitosan tend to form electrostatic interaction with iodate and thereby preserves the loss of iodine from the fruits (Grande-Tovar et al., 2018). The loss of iodine for the treatments Chitosan-KIO₃ Complex-10 kg ha^-1 + FA-KIO₃-0.3% at 60 and 90 DAT and Chitosan-KIO₃ Complex-10 kg ha^-1 + FA-KIO₃-0.2% at 60 and 90 DAT at stage-I (8% and 10%), stage-II (34% and 38%) and stage-III (38% and 43) of main crop and at stage-I (16% and 19%), stage-II (38% and 40%) and stage-III (55% and 57) of residual crop further confirmed the above findings.
 

 

 
Effect of potassium iodate and iodine chitosan complex on cooking
 
The cooking experiment has been conducted to determine the volatilization loss of iodine in the fruits of tomato at three stages. The biofortified tomato was boiled with non iodized salt at 100°C and in contrary to that the non biofortified tomato was boiled with iodized salt at same temperature for 2,5 and 30 minutes. The initial iodine concentration after addition of iodized salt in non biofortified tomato was 0.0075 mg kg^-1 in main crop and 0.0068 mg kg^-1 in residual crop which decreased to 0.0052 mg kg^-1, 0.0038 mg kg^-1 and 0.0012 mg kg^-1 in main crop and 0.0045 mg kg^-1, 0.0030 mg kg^-1 and 0.0003 mg kg^-1 in residual crop after 2, 5 and 30 minutes of boiling respectively (Fig 1 and 2). The iodine concentration in the biofortified tomato was 0.904-0.099 mg kg^-1 and 0.683-0.047 mg kg^-1 in main and residual crop after boiling for 2 minutes. After boiling for 5 minutes, the concentration of iodine in the biofortified tomato of main and residual crop was reduced to 0.895-0.093 mg kg^-1 and 0.672-0.034 mg kg^-1. The concentration was further reduced to 0.857-0.075 mg kg^-1 and 0.624-0.011 mg kg^-1 in main and residual crop after 30 minutes of boiling. The concentration of iodine in the tomato decreases in both the crops with increasing boiling time, whether the tomato is biofortified or not. This is because only a very small amount of iodine seeped into tomato fruit after the iodine in the soup was volatilized with water. But the decrease rate was more in non biofortified tomato when compared to biofortified one. This is due to the fact that the chitosan-potassium iodate iodine is more stable when compared to the iodized salt inorganic iodine used during cooking (Li et al., 2017). The results clearly revealed that application of combined Cs-KIO₃ and FA- KIO₃ treatments has retained more iodine in fruits and less iodine is lost through the tomato soup in all the stages of cooking when compared to other treatments in main as well as residual crop. Additionally, the iodine loss rate was much lower accounting for about 2.79%, 3.76% and 7.84% in main crop and 3.8%, 5.35% and 12.11% in residual crop respectively after cooking for 2, 5 and 30 minutes. While the corresponding iodine loss rate for the non-biofortified tomato + iodized salt treatment was 30.6%, 49.3% and 84% in main crop and 33.8%, 55.8% and 95.5% in residual crop. During 2, 5 and 30 minutes of boiling with non-iodized salt, the biofortified tomato can retain 97.2%, 96.2% and 92.15% and 96.1%, 94.6% and 87.8% of its initial iodine in main and residual crop.
 

 

 
Effect of potassium iodate and iodine chitosan complex on digestion
 
Iodine in food forms complex compounds with other substances. After digestion, the iodine present in the solid component of the vegetable is regarded as inaccessible to the human body. Iodine bioaccesibility refers to the physiologically available iodine in the vegetable prior to digestion (Li et al., 2018). The bioaccessible iodine (BI) in the biofortified tomato increased with increasing duration of digestion from 30 minutes to 90 minutes in main and residual crop. Irrespective of the treatments in all the stages of digestion the lower bioaccessible iodine in biofortified fruits was found in the KIO₃ soil and foliar application alone and chitosan alone treatments. The amount of BI in the biofortified fruits was higher in combined Cs-KIO₃ and FA- KIO₃ treatments followed by SA-KIO₃ and FA- KIO₃ in all the stages of digestion in main and residual crop (Fig 3 and 4). The bioavailable iodine of the treatment Chitosan-KIO₃ Complex-10 kg ha^-1 + FA-KIO₃-0.3% at 60 and 90 DAT in the main crop reached 47.32%, 26.34% and 25.80% at 30, 60 and 90 minutes of digestion. Similarly for residual crop it reached 43.7%, 25.35% and 24.9% at 30, 60 and 90 minutes of digestion. When the digesting time exceeds 90 minutes, the BI becomes relatively constant, indicating that the iodine exchange between the tomato fruits and simulated stomach fluid has achieved a state of equilibrium.
 

 

       
Previous research also revealed that digestion and absorption are dynamic processes that occur concurrently (Thambiliyagodage et al., 2023). The digestion experiment clearly revealed that the most of the iodine was stored as soluble cellular substances in tomato.

The findings of bioavailability experiment showed that the absorbed iodine in tomato fruits is stable after harvest. The use of combined Cs-KIO₃ and FA-KIO₃ treatments preserved the iodine content and reduced iodine losses in the fruits. The iodine loss of the biofortified tomato during soaking was very low at 8% in the main crop and 15% in the residual crop. During cooking, the iodine in the biofortified tomato was more stable than the iodized salt. After 30 minutes of cooking, about 96% and 94% of the main and residual crop iodine are still maintained in the fruit. The majority of the iodine absorbed by the tomato is kept in the solid part of the fruits. After 30 minutes of digestion, the bioaccesibility of iodine in biofortified tomato accounted for approximately 48% and 44% in the main and residual crop, respectively. Because of the strong electrostatic connection between iodate and chitosan, the fruits do not loss iodine. Furthermore, iodine loss is greater in the residual crop and less in the main crop and also the iodine bioavailability is greater in the main crop and less in the residual crop. This is due to the lack of iodine application to the residual crop in comparison to the main crop.

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All datasets generated or analyzed during this study are included in the manuscript.

This article does not contain any studies with human participants or animals performed by any of the authors.