In most of our minds lemons are yellow (and while we are on the subject limes are green) however this is not always the case but just what we are used to, (some varieties of limes turn yellow when ripe). We have even taken the popular names for these two fruit and attached them to colours, lemon and lime mean yellow and green to us all.
why are these lemons green and the next fruit, the Primofiore, more yellow, well it all comes down to temperature. As summer gives way to autumn the days and nights get colder but more importantly the temperature differential between day and night also gets greater. This temperature differential essentially shocks the fruit into turning yellow (there are of course all sorts of clever chemical processes taking place including chlorophyll in the fruit). Many people believe that a green lemon is not ripe but this is not necessarily true, often there is little difference in terms of juice and acidity levels (taste) of green and yellow fruit.
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