I’ve long heard this, but given the fact that lettuce is just fiber and moisture, even if this is true, is this same sort of difference between the weight of a shirt when it was hanging in my closet and after I tried it on but decided not to wear it?
I’d heard that it wasn’t so much about nutritional value as it was about preventing the lettuce from turning brown too soon. A metal knife supposedly will react with the lettuce and cause it to brown at the cut edges. Tearing it prevents this.
(No cite…this is from reading my mother’s Household Hints books…give it that level of credibility, I guess…)
According to Alton Brown, IIRC, if you tear lettuce it doesn’t wilt as quickly. Tearing goes down the path of least resistance, and leaves the veins mostly intact, so it doesn’t lose a lot of moisture. Cutting severs all the veins in it’s path, and moisture escapes the lettuce leaf more quickly. I think he did a neato demonstration with garden hoses & green bubble wrap.
Too late to edit:
Here’s the Good Eats I was referring to. About 6:30 in. I was kinda right - it’s cell walls that cutting ruptures, not veins. He says it’s not necessary for sturdy greens like romaine though. And I guess the garden hoses were a different episode.
He used them to explain why cutting on the bias across the grain results in steak slices that are tender but not falling apart.
Yeah, er…I watched that one a few times. :o
I can’t conceive of any reason why the nutritional value of lettuce would be affected based on how it’s rendered into bite-sized pieces.
And since (in my house at least) lettuce is prepared for salads shortly before serving (or, if made up beforehand, is refrigerated till needed) there’s never been any significant wilting observed if it’s cut rather than torn.
I would observe, though, that if you cut lettuce rather than tearing it, you’re less likely to excite any resident guinea pigs and thus avoid loud rodent shrieks when making a salad.*
*For the same reason, it’s probably better to buy heads of lettuce that aren’t encased in plastic, as GPs quickly learn to associate the crinkling of plastic with suppertime.
Moderator’s Note: Moving from Great Debates to General Questions.
I’m pretty sure you’d need a knife blade sharpened to the micron level to cut through a cell wall… 
Yours aren’t?
I knap my own flints before approaching a rogue head of iceberg.
Neither can I, but isn’t it fun trying to figure out how it could?
drop “No, I’m NOT stimulated enough in everyday life. Why do you ask?” zone
AFAIK, surgical scalpels are, only to the ~100 micron level.
I do not believe that even a new domestic knife set is sharpened to that sort of tolerance, but I could be wrong.
It doesn’t affect the nutritional value if you tear or chop lettuce right before serving, but, as said, chopping will make it wilt more quickly, affecting the long term storage and nutritional value.
At a fine restaurant I worked at, the lettuce prep was to tear off pieces for romaine, and separate the baby greens from the heads whole leaf, put them in a industrial size spinner, wash, and spin dry. Then they stored well in bins for a couple of day’s service. If they had been chopped, they wouldn’t have lasted the night. They’d be mush, and I’d suppose the more surface exposed to oxidation would affect the nutritional value, for long-term storage.
This type of prep meant that half the lettuce was less than perfect for fine dining, usually just thrown out. I’d go home with huge cardboard boxes of primo california baby greens , pick out the best to eat, and haul the rest out to my goats. Them were some spoiled goaties!
I must say, the first time reading through this, I somehow drew the most grisly interpretation possible, and only with the postscript figured out what you meant. I was thinking “God, this guy is really a sicko.”
That may be true, but the process of cutting the leaves might still cause more cells to be broken/crushed/torn open than tearing them.