In wine tasting, a wine is considered "clear" when there are no visible particles suspended in the liquid and, especially in the case of white wines, when there is some degree of transparency. A wine with too much suspended matter will appear cloudy and dull, even if its aroma and flavor are unaffected; wines therefore generally undergo some kind of clarification.
Before fermentation, pectin-splitting enzymes and, for white wine, fining agents such as bentonite may be added to the must in order to promote the agglomeration and settling of colloids later. Pectins are structural molecules in the cell walls of fruits which have the important function of 'gumming' plant cells together. The pectin content of grapes increases steadily throughout ripening, reaching levels of about 1 g/l, although it varies by varietal and pre-fermentation handling processes. Large pectin molecules can affect the amount of juice yielded at pressing, ease of filtration and clarification, and extraction of tannins. Grapes contain natural pectolytic enzymes responsible for softening the grape berries during ripening, but these are not active under wine-making conditions (due to pH level, SO2, and alcohol.) Therefore, fungal pectolytic enzymes are often added to white must to break up pectins, decrease the viscosity of the juice, and speed up settling. In red musts, this increases color and tannin extraction.
Observational studies of ecosystems use constancy to describe living systems that can remain unchanged.
Resistance and inertia (persistence)
Resistance and inertia deal with a system's inherent response to some perturbation.
A perturbation is any externally imposed change in conditions, usually happening in a short time period. Resistance is a measure of how little the variable of interest changes in response to external pressures. Inertia (or persistence) implies that the living system is able to resist external fluctuations. In the context of changing ecosystems in post-glacial North America, E.C. Pielou remarked at the outset of her overview,