This Halacha is an excerpt from our Sefer
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It is a renowned Chabad custom to avoid eating all processed foods on Pesach, as much as possible.
Matzah and wine: The above custom is with exception to Matzah and wine, which due to inability to self produce, is widely purchased from a store or company. Nevertheless, many families are stringent to produce their own wines and even bake their own Matzahs.
Oil: The Rebbe writes that “Anash use Natala margarine on Pesach”. This refers to a congealed cottonseed oil that was processed by a company under a local Mehadrin Hashgacha. Thus, one may purchase any Kosher for Pesach oil that contains a most reliable Hashgacha. Nevertheless, there are those who are stringent to only use melted chicken fat [Shmaltz] as their oil base product for Pesach food and cooking.
Dairy: The Rebbe ate dairy products on Pesach.
Chocolate: There is no source for the statement that the Rebbe ate chocolate on Pesach. He would however, eat sugarless chocolate during the year.
Sugar: See C!
From what time are those who are accustomed to not eat processed foods to begin their stringency? From the night of Pesach or from 5th hour of Erev Pesach?
One who is stringent to avoid eating processed foods is to avoid doing so starting from the 5th hour of Erev Pesach. However, many are lenient to eat processed foods up until the night of Pesach.
 The source of the custom: This custom is not recorded in the Poskim, or compilations of Chabad Pesach customs, but is the widely accepted Minhag.
 The reason: Seemingly, this custom is based on the custom of the Alter Rebbe not to eat at other people’s houses on Pesach due to that we are not aware of their level of stringency. [See Halacha 11] Thus, we avoid eating processed foods being that it is equivalent to foods made in other people’s homes. [Nitei Gavriel 40 p. 218] Alternatively, the reason behind this custom is because we suspect that perhaps in the production line, which involves hundreds of people of various backgrounds, Chametz fell into the food and has not disintegrated. [This can easily occur if a worker had a sandwich and did not wash his hands properly afterwards. Having worked as a Mashgiach in a Kosher for Pesach food production, I can attest that it is almost impossible to ascertain that all the workers wash their hands from all Chametz leftovers prior to beginning work on the line.] According to this reason, one is to avoid processed foods beginning from the time of prohibition to eat Chametz from the 5th hour of Erev Pesach, as Chametz Beiyn which is found in a food is at times not nullified even before the night of Pesach. [See Admur 442 Kuntrus Acharon 15; 466:9-11; Shut Rabbeinu 18; See Piskeiy Admur Yoreh Deah chapter 100; Shach 109:8; “A Semicha Aid for learning the Laws of Taaruvos” Chapter 104:4] Alternatively, perhaps we suspect for even disintegrated Chametz, as we suspect for the opinion in 447:22 who holds that we say Chozer Veniur by all foods that contain Chametz of any amount, even taste, even if it was nullified before Pesach. According to this reason one is only to be stringent in avoiding eating processed foods beginning form the night of Pesach.
 Igros Kodesh Vol. 21 p. 96. This was a written telegram to Anash of some part of America or Canada in 1949.
 Heard from the Rebbe’s secretary, Rav Binyamin Klein and Rav Leibel Groner; Some say this was likewise the custom of the Rebbe Rayatz and Rebbe Rashab
 Response of Rabbi Yehuda Leib Groner [secretary of the Rebbe] to a query of the author; Custom of many of Anash from vintage Chabad homes as heard by the author
The reason: The reason for not eating processed foods is due to a suspicion of Taaruvos Chametz. Now, before the night arrives, although Chametz can be nullified, this only applies if it is a mixture of Yaveish Beyaveish or Lach Belach. However, by a mixture of Yaveish Belach it is not nullified even before Pesach. [Admur 442 Kuntrus Acharon 15; 466:9-11; Shut Rabbeinu 18; See Piskeiy Admur Yoreh Deah chapter 100; “A Semicha Aid for learning the Laws of Taaruvos” Chapter 104:4] Thus, if there is a crumb of Chametz that fell into the processed food and is still intact it is not nullified and one transgresses eating Chametz on Pesach.
 Custom of many families of Anash.
The reason: Seemingly the reason behind the leniency on Erev Pesach until the night, despite that which is explained above, is because on the night of Pesach begin the severe stringencies of Issur Mashehu, and Issur Kareis for eating Chametz. Thus, although from a Halachic perspective there can be suspicion of Chametz in processed foods even before the night of Pesach in a way that it is not nullified, nevertheless one only begins to suspect for this starting from the night. Vetzaruch Iyun. Alternatively, perhaps it is because we suspect for the opinion in 447:22 who holds that starting from the night of Pesach we say Chozer Veniur by all foods that contain Chametz of any amount, even taste, even if it was nullified before Pesach.