Optimization of Conditions for Production of Calcium Citrate from Egg Shells ^†

1. Introduction

Eggshells contain high amounts of CaCO₃ (94–97%), some proteins (3–4%), and minor traces of MgO, P₂O₅, K₂O [1,2]. Calcium carbonate has the highest concentration of elemental calcium (40%). Its bioavailability is low due to the fact that it is soluble only in strongly acidic environments, and as a result, a low absorption rate of Ca ions occurs in the gastrointestinal tract where the pH is neutral or alkaline [3]. The very high content of calcium carbonate allows the use of these eggshell powders as a raw material for obtaining organic calcium in the form of calcium citrate, which is water soluble, generally regarded as safe, tasteless and has high absorptive properties by the human body. Adequate calcium intake contributes to the prevention of calcium deficiency in the elderly and in children. This study aims to optimize the technological parameters for production of calcium citrate and establish the order of the factors’ influence on the yield.

2. Materials and Methods

Ten grams of eggshell powder are mixed with a series of citric acid concentrations (5–35%), at different solid: liquid ratio (1:10–1:20), temperatures (250–450 °C), and time intervals (2–4 h), with gentle stirring, according to the experimental design. The filtered solution containing calcium citrate is dried at a temperature of 550–600 °C and then grounded. Purification of product was made and the total yield was calculated. The Orthogonal Array Testing Strategy (OATS) was applied to optimize the calcium citrate technology. Four independent variables at three levels were adopted [L9 (3)⁴], namely citric acid concentration (A), solid ratio:liquid (B), temperature (C) and time (D), and the selected levels represent the maximum number of values for each factor determined in preliminary studies, based on experiments with a single factor-design. The yield of calcium citrate is the dependent variant (taken as an indicator). Statistical analysis was performed using Minitab 19 software. Analysis of variance (ANOVA) was performed in Minitab and tested at a significant difference level with p < 0.05.

3. Results

In this study, a total of 9 experimental runs were performed for optimizing the four individual variables and the results were expressed in yield. The yield of calcium citrate exceeded 50%. Range value index shows that the order of influence of the parameters in obtaining calcium citrate is RA > RD > RB > RC. Analysis of variance showed that the contributions of citric acid concentration and time were significant.

4. Conclusions

Based on the analysis of the experimental results, the optimal conditions are determined for a maximum yield as: 30% citric acid concentration, time 3 h, solid:liquid ratio 1:16, temperature 300 °C. The yield of obtaining calcium citrate prepared under optimized conditions is 88.64%.