Commonly found in every corner of India, dahi - or “doi” in Bengali, dadhi in Sanskrit, yogurt in America and curd in Europe -- is revered for its many qualities as a food, a food ingredient, and as a medicine, with its real ability to treat a variety of diseases through its many variations of preparation.
Ayurveda begins the conversation about dadhi as part of the pentad of milk products that each have strong medicinal uses. Milk (dugdha in Sanskrt) is made into curd (dahi). Curd is churned into buttermilk (takra) and butter (navanita). The butter is heated into ghee (ghrtam).
Ayurveda also mentions the medicinal panchamrita, of which dadhi is also a member. These five food items when mixed together become a medicinal nectar. They are dadhi (curd), dugdha (milk), ghreeta (ghee), madhu (honey), sarkara (sugar). For sanctified purposes and puja, dadhi is an essential element and has a multitude of uses during every Pooja and in line with the nature-centric worldview of Hindu customs.
दध्युष्णं दीपनं स्निग्धं कषायानुरसं गुरु |
पाकेऽम्लं श्वासपित्तास्रशोथमेदःकफप्रदम् ||१||
मूत्रकृच्छ्रे प्रतिश्याये शीतगे विषमज्वरे |
अतीसारेऽरुचौ कार्श्ये शस्यते बलशुक्रकृत् ||२||
Transliteration - dadhiguṇa
dadhyuṣṇaṁ dīpanaṁ snigdhaṁ kaṣāyānurasaṁ guru |
pākē'mlaṁ śvāsapittāsraśōthamēdaḥkaphapradam ||1||
mūtrakr̥cchrē pratiśyāyē śītagē viṣamajvarē |
atīsārē'rucau kārśyē śasyatē balaśukrakr̥t ||2||
Source: Bhāva Prakāśa, Pūrvakhaṇḍa, Miśraprakaraṇa, chapter 15, dadhivarga, sloka 1 -2
Translation - Dadhi has usna guna , dipana (increases appetite), snigdha, kashaya anurasa (partly astringent), guru(heaviness); it increases problems in the respiratory system, pittaja roga, shotha rogas (swelling), medo roga, and increases kapha too.
Dadhi is very beneficial in mutrakriccha (urinary infections), pratisyaya (rhinitis), sheeta (feeling very cold), visamajwara (chronic undulating fever), atisara (diarrhea), aruchi (lack of appetite and decreased sense of taste), karsya (emaciation), decreased bala(strength) and poor sukra(sperm quality/quantity) too.
Throughout the many cultures of ancient Bharat, from present-day Afghanistan to Indonesia, and all the conquestors and invaders that stole from Indian wisdom, there are multiple variations on the making of dadhi. Some use earthen pots. Some use ceramic. Some use inert metals. Some use oven heat. Some use stove heat. Some use the heat of nature's hot climate. All use a form of bacteria, but some start with the cow's own Lactobacillus, and some start with green chili peppers. Most use cow's milk, but some use goat, buffalo, or sheep milk.
The process requires several essential steps. Milk is heated on low heat until it forms bubbles and is not quite boiling. It is allowed to cool naturally to 110o-120oF. About 10-15 grams of yogurt is added to a liter of this warm milk and stirred constantly until the yogurt is completely dissolved into the milk. This is carefully poured into an earthen pot or jar and placed with a thin cloth over the top, in a space warmer than 85oF, for 6-7 hours. When inspected it should be thick and formed, and not runny or semi-liquid. The container is then placed into a larger bowl of cold water to keep, or it is placed in modern day into the refrigerator.
Using other methods are part of culinary practices but they do not yield strong medicinal yogurt, which is highly probiotic and heals gut conditions due to dysbiosis. Use of yogurt starters or highly processed yogurt does not yield medicinal yogurt but is a common food practice.
Dadhi is a very common, cheap, and available food across India and also across the world. People irrespective of climate, season, age or socioeconomic status use yogurt as a staple food, or as a side dish in their daily meal. Dadhi is also a traditional way to utilize extra milk to prevent wastage or rotting.
Bhava Mishra, who belonged to the southern part of Bengal, a tropical region, was a great scholar in Ayurveda and mentioned many uses of yogurt in his compendium of medicinal foods. He cautioned that amidst it many qualities for treating diseases, it is also quite harmful for various diseases such as chronic respiratory infections, those having burning sensations indicative of acid reflux or ulcer, any type of swelling in the body, and obesity. This is due to its distinct property of clogging the channels of the body, known as abhishyendi, creating obfuscation and not allowing flow. The conditions mentioned all have phlegm or murky inflammation as part of the disease and are worsened by additional obstruction.