Storage stability assessment of freeze-dried royal jelly by furosine determination

The effect of freeze-drying and the assessment of the storage stability of freeze-dried royal jelly were investigated by the determination of furosine and blocked lysine. The level of furosine in the royal jelly samples collected from cells at different times (1, 2 and 3 days after grafting) showed that the Maillard reaction had already occurred in the hive as indicated by the increase in furosine: from 9.6 to 20.8 mg/100 g protein. Freeze-drying increased the Maillard reaction slightly, since furosine content rose from 18.0 mg/100g protein of RJ harvested 3 days after grafting compared to 24.6 mg/100g protein of the lyophilised sample. Freeze-dried royal jelly was more prone to the early stage of the Maillard reaction than fresh RJ, as confirmed by the significantly higher furosine values found after 12 months, both at 4°C (253.4 versus 54.9 mg/100g protein) and at room temperature (884.3 versus 332.5 mg/100g protein). After 18 months at room temperature, the lyophilised samples reached a furosine level of 1440.4 mg/100 g protein, which corresponded to the blocked lysine levels, amounting to 24% of total lysine.