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Modern baby food are commonly made using various strengths of blender, but what was used before then? I assume something like a potato masher, but that would only work for a few foods. So what was predominately used for baby food in the past?

Many babies were indeed fed mashed food, typically of cooked vegetables and fruits. While it's true that not all foods can be prepared like this, keep in mind that pre-modern families rarely have access to the kind of dietary diversity as modern developed economies anyway. So this was likely not a realistic concern for most.

Nonetheless, there is a variety of other historical baby foods. A common method of preparation is to soften food with liquid. For example, since antiquity European babies have been fed bread soaked in honey water, milk, soup, or even wine. Other, probably more familiar examples include what's basically oatmeal or porridge.

At around six months the child would begin a mixed diet of breast milk and cereal [that has been] soaked in milk or hydromel, soup or eggs. At six months the doctors ordered that the child should be given sweet wine or wine sweetened with honey, or water . . . or else bread soaked in wine.

_{Rousselle, Aline. "The Bodies of Children", in Porneia: on Desire and the Body in Antiquity. Wipf and Stock Publishers, 2013.}

Likewise, in Asia, infants and toddler were - and continues to widely be - fed congee. This dish is cooked simply by boiling rice in too much water, and has been prepared since time immemorial. No blender or other advanced kitchen appliances required.

One of the first meals fed to a baby is “congee", a broth and rice mixture of the consistency of oatmeal. This dish is usually prepared with meat, fish, or vegetables, but foods other than rice might be removed and not served to the child.

_{Morris, Heather M., et al. "Cultural brokering in community health." The Canadian Nurse 95.6 (1999): 28.}

Finally, there's pre-chewed food. Humans actually come equipped with a kind low powered biological blenders: their own teeth, with which most foods can be rendered viably pureed for feeding babies.

Another acceptable method suitable for meat was for the parents or wet nurse to pre-chew some food and then feed it to baby with their fingers. One text describes the individual servings of pre-chewed food as being morsels the size of an acorn.

_{Newman, Paul B. Growing up in the Middle Ages. McFarland, 2007.}

Premastication has been documented throughout human history and likely dates from the depths of prehistory - it is even observed in our biological cousins such as the orangutans.

Vegetables, fruits can be pureed. Meat is very easy to grind, or cooked meat can be chopped fine pieces. Carbohydrate sources like noodles, potatoes or rice are generally very easy to cook to a soft mash. Blender is not a must, even nowadays.