After fertilisation at flowering, the berries start to grow very quickly. The acid concentration in the unripe berries rises to a peak, then the acid concentration is reduced and the sugar content increases rapidly. At this point, the ripening phase, known as véraison within the vine's annual vegetative cycle, begins, at which point the berries soften and begin to change colour. The end of the ripening phase or peak of maturation is characterised by an optimal ratio of sugar to acid (see also under physiological ripeness). The length of time between flowering and ripening varies greatly from grape variety to grape variety, which is why we speak of early to late ripening grape varieties. Soil type, climate, weather and the work of the winemaker also have an influence. The time of ripening also depends on the microclimate in the vineyard and can also differ between individual clones of the same grape variety. So even in smaller vineyards, the same grape variety can give different results.