def _init_worker(model, variable_name, coefficient):
    """
    Initialize a global model object for multiprocessing.

    Parameters
    ----------
    model : cobra.Model
        The metabolic model under investigation.
    variable_name: str
        The name of the variable representing the metabolite exchange.
    coefficient: int
        The value of the metabolite's stoichiometric coefficient: -1 to test
        if the model can produce the metabolite and 1 to test if it can be
        consumed.

    """
    global _model
    global _sink
    global _coefficient
    _model = model
    _sink = model.variables[variable_name]
    _model.objective = _sink
    _coefficient = coefficient
