Sprouting

A newly lifted potato tuber or a newly harvested onion or garlic bulb is dormant. During that period the organ will not initiate growth, and the store is, in sprouting terms, working with a considerable margin. Dormancy is not indefinite. It has a characteristic duration that varies by crop and, within a crop, by cultivar; it is also influenced by the conditions the crop grew under and by its maturity at harvest. When it ends, the organ becomes capable of sprouting, and from that point the store is no longer preventing sprouting so much as slowing it.

This is why two apparently identical stores can produce different outcomes, and why the same store can perform differently between seasons or between cultivars. Storage temperature and the store environment influence the rate at which dormancy runs down and, once it has ended, the speed at which sprouts extend. They do not reset dormancy, and they do not give a dormant organ the ability to sprout. A store cannot be judged on sprouting alone without knowing which cultivar it was holding and how far through its dormancy that cultivar had come.

Sprouting is one of the very few post-harvest defects that is genuinely unambiguous on sight. Where most disorders on this site present as symptoms that several different processes could have produced, a sprout is a sprout: nothing else looks like one, and no laboratory is needed to say that a tuber has sprouted. That much is a real and unusual advantage, and it should be stated plainly rather than hedged.

The honest difficulty runs the other way. The physiological change that accompanies sprouting does not begin when the sprout becomes visible — it begins earlier, inside the organ. Reserves are mobilised to supply the growing point, and the texture and composition of the tissue change as that proceeds. By the time a sprout has emerged far enough to be seen, that internal transition is already under way and has been for some time. A lot inspected and recorded as sprout-free is therefore not necessarily a lot in which nothing has started; it is a lot in which nothing has yet emerged. The absence of visible sprouts is evidence about the surface, not assurance about the physiology.

In potato especially, sprouting and greening are routinely collapsed into a single complaint — that the potatoes "have gone off" — and they are not the same thing at all. Sprouting is a developmental event driven by the end of dormancy. Greening is a response to light exposure. They have different causes, they are prevented by different means, and a lot can readily show one without the other: a tuber kept dark can sprout, and a tuber well within its dormancy can green.

• Sprouting follows the end of natural dormancy; it is a growth event on the crop's own timetable, and darkness does not prevent it.
• Greening follows exposure to light; it is not a dormancy phenomenon and does not indicate that dormancy has ended.
• The two frequently appear together simply because a lot that has been held long enough to sprout has often also had opportunities for light exposure — co-occurrence is not a shared cause.
• Because they are conflated in casual reporting, a complaint of "sprouting" may describe greening, and a complaint of "greening" may describe sprouting; the underlying observation is worth establishing before the cause is investigated.

Management of sprouting begins before the store, with the choice of cultivar against the intended storage duration and market. A cultivar with a short natural dormancy will reach the end of its dormancy sooner regardless of how the store is run, and no storage regime recovers that. Where long holding is required, the crop selected for it has to be capable of it.

• Match cultivar and intended storage duration: dormancy length is a crop characteristic and sets the outer limit of what the store can achieve.
• Cure the crop appropriately before storage so that surfaces are sound, then hold it under conditions matched to the commodity and end use, per sourced guidance for that crop.
• Hold at the temperature appropriate to the commodity and its market, recognising that in potato the settings that most slow sprouting interact with other quality outcomes and the balance is end-use-specific.
• Sort and grade to remove units that have already begun to sprout, so that the lot presented is uniform — noting that this addresses the lot's appearance and uniformity, not the dormancy status of the units that remain.
• Anticipate the end of dormancy rather than react to the first visible sprout, since by then the internal change has already begun.