Can people tell how appetizingly fresh a cabbage is, just by looking at photos of the leaves?
In 2010, a team of investigators from four prestigious Japanese research institutes (along with  The Japan Society for the Promotion of Science) set up a series of experiments to find out. Using a set of 32hr. time-lapse of photos of cabbage leaves (see picture below) they asked experimental participants to rate the featured cabbages according to how fresh (or not-so-fresh) they looked. When the results had been analysed, a clearcut pattern emerged from the data . . .
“…participants could quantitatively estimate the degradation in freshness of the cabbage simply by looking at the presented images.”
  All nine judges consistently rated the older leaves as less fresh. And the results showed:
“…a strong relationship between freshness perception and features of the luminance distribution of the stimulus image.”
- that's to say, depending how shiny they looked. The research team point out that:
“The current work is another good example of a fusion between psychological and food sciences.”
And one which confirms that:
“…humans can estimate the quality of food, such as the deterioration or the freshness of vegetables, using optical cues.”
But with the caveat that, even now :
“…it remains unknown how humans perform this quality assessment.”
The study is published in the journal Appetite Volume 54, Issue 2, April 2010, Pages 363-368, and can be read in full here: “Influence of luminance distribution on the appetizingly fresh appearance of cabbage“