Brahmin is a Hindu caste, where the majority is of scholars and priests. It is believed that Brahmins are bound to their traditions and close to God by performing ceremonies and vratas. This caste is further divided into three categories: Vaishnava who worships Lord Vishnu, Sri Vaishnava who follows Lord Laxmi Narayana, and Samartha who are the devotees of both Lord Shiva and Lord Vishnu. Despite their strict culture, they also follow a very strict and certain different diet. Some things are prohibited from their kitchens, that list also includes onion and garlic.
It is also believed that in ancient times people didn’t eat onion and garlic. Some of them got changed with the passage of time but some Brahmin families are still there who do not consume onion and garlic. Many theories are given on this. As according to the Ayurveda food is kept into three different categories: Satva, Rajas, and Tamas. Satva refers to the satvic food that provides peace mentally keeps mind cool and calm, and help us to always keep our mind under control. This could be the main reason that Brahmins always prefer satvic food.
The second is the type Rajas, the food under this category is known to develop worldly pleasures. Foods that are very hot, sour, bitter, or salty are Rajasic. Too much consumption of Rajasic food makes the mind restless and uncontrollable. Eating food in a hurry is also regarded as Rajasic. This is one of the reasons why onions were restricted in ancient times.
The last category is Tamas. Tasmic food includes poultry, fish, eggs, meat, and alcohol that are difficult to digest, prepared unconsciously, or in anger. After consuming these our mind becomes evil, out of control and we tend to get angry. Overeating is considered as Tasmic and it is the unhealthiest of all.
These could be the reasons why people avoided onion and garlic. Another reason states that it is not suitable for meditation and causes acidity. We humans do not have control over ourselves as well as over our minds. So, by avoiding such things like onion and garlic Brahmins believed that this was their step towards attaining peace.