Effect of Storage Conditions and Cooking on the Stability of Micronutrients and the Potential of FT-NIR Spectroscopy in Predicting Retinol in Commercial Fortified Maize Flour in Kenya

Abstract

Food fortification is one of the strategies that has been used to overcome micronutrient deficiencies among vulnerable populations. Maize, a common staple food in Kenya, is used as a suitable fortification vehicle. However, several factors including storage conditions and consumer preparation methods involving heat processing impact micronutrient stability in fortified maize flour. Additionally, non-compliance with fortification standards hinders the success of fortification programs due to lack of techniques to rapidly check the amounts of the added fortificants. Fourier Transform Near-Infrared (FT-NIR) spectroscopy has been proposed as a fast and reliable analytical technique for vitamin determination. The objectives of this study, therefore, were to assess the influence of storage condition on the retention of retinol and B-vitamins in selected commercial fortified maize flour, to assess the impact of heat processing on retinol, B-vitamins, iron, and zinc in fortified maize flour and to develop an FT-NIRS-based model for determination of retinol in a fortified maize flour sample. Fortified maize flours from two brands (coded XX1 and XY2) were sampled from two manufacturers at the point of production. The stability of retinol and B-vitamins in the two brands (XX1 and XY2) was determined periodically over a storage period of 6 months at 25°C/ 75% relative humidity and 35°C/ 83% relative humidity. Cooking stability was assessed by comparing the amounts of micronutrients in sample XX1 before (uncooked maize flour samples) and after (ugali) cooking. For FT-NIR studies, 150 fortified maize flour samples were randomly collected from 10 counties in Kenya. Retinol reference values obtained by high performance liquid chromatography (HPLC) and NIR spectra of the fortified maize flour samples were used to develop calibration models using partial least squares regression (PLS-R). In storage stability studies, retinol was the least stable vitamin for brand XXI at both 25 °C/75% RH and 35 °C/83% RH, followed by thiamine, riboflavin, folate, and niacin. However, brand XY2 showed that under both storage conditions, thiamine was the least stable vitamin, followed by retinol, riboflavin, folate, and niacin. Vitamin retention was higher in samples stored at a lower temperature and relative humidity (25°C/ 75% RH) than in samples stored at higher temperature and relative humidity (35°C/ 83% RH) for both brands. In cooking stability studies, retinol was the least stable vitamin (43.2% retention) while niacin was the most stable vitamin (61.1% retention). As expected, iron (91.4% retention) and zinc (95.6% retention) were the most heat-stable among the micronutrients analysed. In FT-NIR model development studies, two calibration models were developed to predict retinol above and below 1.0 mg/kg. The performance metrics of model one developed to predict retinol < 1.0 mg/kg were: R2c = 0.81, RMSEE = 0.08, RPD = 2.29 and R2v = 0.82, RMSEP = 0.09, RPD = 2.07 for the calibration and validation, respectively. The second model developed to predict retinol ≥ 1.0 mg/kg had the following performance metrics: R2c = 0.93, RMSEE = 0.16, RPD = 3.58 and R2v = 0.81, RMSEP = 0.22, RPD = 2.43 for the calibration and validation, respectively. In conclusion, the stability of vitamins for both brands XX1 and XY2 progressively declined over the six-month storage period. There was a significant difference (p < 0.05) in the amounts of retinol, thiamine, riboflavin, folate, and niacin in the flour samples after 6 months of storage at 25 ℃/ 75% RH, and 35 ℃/ 83% RH for both brands XX1 and XY2. The amounts of all the vitamins assessed differed significantly (p < 0.05) between cooked and uncooked fortified maize flour, while the amounts of iron and zinc in cooked and uncooked fortified maize flour were not significantly different (p > 0.05). FT-NIRS model development studies results demonstrated that NIR spectroscopy can be used to adequately predict retinol in fortified maize flour. NIR spectroscopy, by replacing time-consuming and laborious wet chemistry laboratory procedures, has the potential to be used for rapid regulatory monitoring of fortification compliance for a large number of samples.