I have been making homemade pesto and have been checking a lot of basil leaves. I use a very high powered blender to make the pesto and its totally liquid after blending.
Do I need to check the basil for bugs before blending it?
I cannot offer you a definitive answer regarding your home-made pesto. Basil leaves may contain insects in your locale. the degree of infestation will vary depending on location, season and agricultural attention by the farmer. A specific investigation need be made for the Basil leaves available in your area.
Some kosher vegetable companies market insect free leafy vegetables to avoid any concerns.
Nevertheless, I shall offer you some guidelines that might be helpful to you.
- Leafy vegetables and certain other kinds of produce are presumed to be infested with insects. Infested vegetables and fruit may only be eaten after all of the insects have been removed. If this is impossible, then that food item may not be eaten. Although when one cannot find a forbidden item in a mixture, one need not be concerned about it provided the volume of permissible food is at least sixty times that of the non-kosher food, insect infestation cannot benefit from that halochoh. A whole insect (a beryoh) is never considered insignificant enough to be considered non-existent. Thus, for example, if an insect falls into a pot of cholent and cannot be found, none of the food in the pot may be eaten. Pieces of meat and potatoes that have been inspected and do not have the insect on them may be eaten.
- However, if the insects are not whole, e.g., if they have been puréed, they are considered non-existent (batel) as long as there is a majority of permissible food in the mixture. (The rule of bitul beshishim allowing one part forbidden food to sixty parts permissible food applies to non-kosher food that enhances the flavor of the food, not undesirable insects in food.)
- One is permitted to purposely grind food that may be infested with bugs if all of the following criteria are met:
- Therefore, infested wheat may be ground into flour and used.
- Any insects that were in the wheat may have left the wheat as it entered the grinder due to the noise, so it is not certain that the insects are still in the wheat.
- The purpose of grinding the wheat is to make flour, not to crush the insects.
- It is impossible to separate the insects from the wheat.
The same principle is used for wine and fruit juice production.
- Even puréeing food in order to crush any insects in it is permissible as long as it is unlikely that there are insects in the mixture. A food item that is not likely to have insects in it but has not been checked well may be puréed or ground in order to remove any possibility that there is a whole insect. Still, you should glance at the food first to determine whether there are any obvious insects on it. In other words, if a food item is practically impossible to check so thoroughly that you can be unequivocally certain that there are no insects in it, and you have taken steps to minimize the insect population in the food and may have succeeded in removing all the insects but cannot be absolutely sure, you may grind the rest in order to be able to eat the food.
- Thus, in places where broccoli, cauliflower or parsley are infested and it is impractical to find and remove the insects, you may prepare the vegetables in a way that can remove the insects and then purée them. For example, you may scrub parsley leaves in soap and water and then place them in a blender in order to make them permissible to eat.
- In order for an insect to be forbidden, it must be large enough for a person with excellent eyesight to determine that it is in fact an insect and not a speck of dirt. If it is necessary to rely on a magnifying glass or other instrument to make the determination, the insect may be eaten. Only the small size serves as grounds for deeming the insect kosher. A well-camouflaged insect that is large enough to be seen against a different background may not be ingested. Furthermore, even if the insect is too small to be identified but is noticeable as a spot that moves, one can be certain that it is an insect and must not ingest it despite its small size. Only movement of a tiny insect indicates a forbidden creature. The location, shape and coloring are insufficient evidence if the insect itself is too small to see with the naked eye. Thus, certain marks on citrus fruit peels that are indicators that there were insects on the peel would not be enough to forbid the fruit. However, because in Israel those spots have been seen to move, one must be concerned that the fruit is infested with insects even if yours don’t move.
 The only concern is that one might end up eating the whole insect; halochoh is not concerned with the flavor of the insect being absorbed by the kosher food, if the general consensus is that eating insects or their flavor is unappetizing.
 א”צ ששים בזה כיון שאינו נותן טעם לשבח, ולכן בטל האיסור ברוב. ואע”ג דמין במינו עכ”פ צריך ששים מדרבנן אע”פ דאינו נותן טעם לשבח, י”ל דשאני דברים מאוסים שאינם נותנים טעם לשבח בשום דבר משא”כ מין במינו דחז”ל גזרו מינו אטו אינו מינו שלא יבואו להקל ברוב אילו נתערב מין במינו היכא דנותן טעם לשבח.
ודע, דביצי תולעים ושרצים אף דלדעת רוב הפוסקים אסורים באכילה מה”ת, מ”מ אם נתערבו בדבר אחר ולא ניכרים בטילים ברוב ואין להם דין בריה.
 בפמ”ג יו”ד סימן צ”ט במ”ז סק”י דאם נטחן שלא כדין בכוונה כדי לבטל את האיסור שבו, כל המאכל נאסר עבור מי שטחן וגם למי שנטחן עבורו. אכן אם מטרת הטחינה לא היתה כדי לבטל את האיסור, המאכל מותר גם עבור הטוחן עצמו וכ”ש עבור למי שנטחן עבורו למרות שלא היה לו רשות לבטל את האיסור לכתחילה.
 עיין ט”ז סימן צ”ט סק”ז ופת”ש שם סק”ד.
 עיין יו”ד סימן פ”ד סעיף י”ד וש”ך שם סק”מ בשם תרה”ד.
 עיין בינת אדם לח:לד, עה”ש יו”ד פד:לו ושו”ת שבט הלוי ז:קכב, שמירת שבת כהלכתה פ”ג הערה לז,