def process_content(df, config, dataframes):
    """
    Applies a function to each value of the column
    "process_content": [
        {
            "function": lambda x: str(x),
            "columns": ["column1", "column2"]
        }
    ]
    """
    for function_dict in config.process_content:
        for column in function_dict['columns']:
            df[column] = df[column].map(function_dict['function'])
    return df