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Thought for the Day: Checking for Bugs in Fruits and Vegetables -- How Thoroughly?

My first job after graduate school was as a physicist in a radiation oncology department.  My job was mostly to check the charts to ensure that that the the techs had given each patient the correct dose -- as measured by time under treatment -- to each patient.  (Dose was measured by time because all patients were set up with the machine 80cm from their skin, so opening the shutter/turning on the machine for a certain time gives a certain dose.)  Not exciting.  One day, though, the physician came running (that's my memory) to me with a problem.  "Because of the size of the patient and tumor I need to move the machine back to 100cm, but I can't remember 's formula for how to adjust the time!"  I had, of course, never heard of  's formula -- and there was no Google in those days -- so I just worked out the time from my knowledge of (freshman) physics.  I showed him my calculation and he gasped, "That's 's formula!"  Not knowing 's formula, but being able to derive it as obvious increased the level of respect I got from the physicians.

I mention this because I just heard a shiur on checking bugs that showed that our poskim are well versed in Bayesian statistics.  While I am reasonably confident that none (that is to say, a מיעוט שאינו מצוי) of them know that term, the fact that it is part of their standard repertoire of handling halachic issues is grounds for increased respect from us עם הארצים who feel haughty because of our advanced degrees.

The need to check for bugs is mediated by how likely you are to find one of them.  There are basically three levels: מוחזק/certain that the bugs are there in the majority of cases; מיעוט המצוי/common, though only in a minority of samples; מיעוט שאינו מצוי/minority of samples and uncommon.  The Torah requires checking if the sample only if the bugs are מוחזק.  Chazal extended that to obligation to מיעוט המצוי.

So far, so good.  We are all used to needing to check certain fruits and vegetables.  However, the Shulchan Aruch (84:8) says that if a vegetable or fruit requires checking, then it is not sufficient to check only a sample; rather, each and every fruit/vegetable needs to be checked!  Really?  Do you check each leaf of your cabbage, every sprig of parsley?  The CRC instructions for checking vegetables includes many, many cases where only a sampling is needed.  The CRC is well known to be strict about actually following the Shulchan Aruch.  So what gives?

Let's review that halacha in Shulchan Aruch: only vegetables that have an infestation that is a מיעוט המצוי require an individual inspection.  The first thing I need to do, therefore, is determine if my vegetables have an infestation a that is מיעוט המצוי.  If they are, get to work; if not, bon appétit.

To take a specific example: broccoli grown in California is מוחזק with bugs, while broccoli grown in Guatemala is essentially bug free (it's the climate).  (Same goes for hydroponics vs outside, use of pesticides vs pesticide free; all examples where you need more information than just the species of vegetable.)  The problem is that "broccoli" is too broad of a category.  If I knew that my broccoli came from Guatemala, then I could just check a sampling because it does not fall into the category of an infestation that is a מיעוט המצוי.  Suppose I don't know from whence my broccoli hails, but I know it all comes from the same place.  When I check my sample, I am determining whether it comes from Guatemala -- and therefore does not actually require checking, or comes from California -- and therefore I would need to check each and every flowerette.

The reason for only checking the outer few leaves of cabbage is slightly different, but along the same lines.  It turns out that the bugs found in cabbage come from the outside, as opposed to other vegetables where the bug eggs are laid in the bud or plant stalk.  That being the case, there will be a distrubution of bugs throughout the head of cabbage: highest concentration near the outside, fewer toward the middle, very few in the center.  Checking the three outer leaves determines if there are any bugs likely deeper inside.  Checking three inner leaves would obvously not accomplish the same thing and would not exempt you from checking the outer leaves.

Back to my comment about Bayesian statistics.  Bayesian statistics is an approach to statistical analysis that takes into account statistical knowledge about the context of the problem under study.  That is, it allows one to include his experience from previous trials to predict the future.  It also makes heavy use of "conditional probability"; that is, how the probability of an outcome changes when I have some prior knowledge.

That's one of real joys for me of studying halacha; getting a glimpse of how much goes into even the most mundane activities.

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