Sorry for the delay in answering I think there is no problem with your technique Because the lyophilization process does not alter the metabolic composition of the sample

In general, it should be okay to freeze an aqueous plant extract at -20°C for 24 hours before freeze-drying, as long as the extract is properly stored and handled to avoid degradation or contamination. However, whether the freeze-dried extract will retain its laxative properties depends on several factors, including the specific plant species, the extraction method used, and the chemical composition of the extract.

Some plant compounds with laxative properties, such as anthraquinones found in senna or aloe vera, can be sensitive to heat, light, and other environmental factors that may cause them to degrade or lose their activity. Therefore, it is important to optimize the extraction and freeze-drying conditions to minimize any potential loss of activity. For example, using lower temperatures during the extraction process or adding antioxidants or stabilizing agents to the extract may help preserve the activity of the laxative compounds.

It is also important to note that the quality and consistency of the extract can vary depending on the plant material used, the extraction method, and the analytical techniques used to quantify the active compounds. Therefore, it may be useful to perform quality control tests on the freeze-dried extract, such as determining the total anthraquinone content or performing a bioassay to confirm the laxative activity.

Overall, freezing an aqueous plant extract at -20°C for 24 hours before freeze-drying can be a useful step in the sample preparation process, but it is important to carefully consider the specific properties and requirements of the extract to ensure the desired activity is preserved.