Modern baby foods 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.
Mother's milk, overlapping with more solid food, was typically a major part of a baby's diet for much longer than we now think of in many western countries, where starting weaning at a few months and completing within another few months has become common in the last few decades, and breastfeeding is by no means guaranteed. Substitutes for breastmilk weren't as good or as readily available as now until the 20th century.
Babies develop physically very rapidly around the time they're ready to start weaning, so from around 6 months they can start getting some nutrition from properly solid foods rather than mash, especially if milk is still available. If however you're trying to replace milk with other foods from 4 months, babies will struggle with even soft solids.
The modern version of this is called baby-led weaning, essentially providing foods that the baby can eat with their fingers (this is actually becoming increasingly recommended by health authorities . Raw fruit and veg cut into grabbable pieces, stewed veg/meat, even bread (though that's rather salty). None of these are new - in fact some of them are among the earliest foods known to humans. Of course other primate species don't have blenders and wean onto things like ripe fruit (see First molar eruption, weaning, and life history in living wild chimpanzees, TM Smith et al.)
Further interesting reading:
- Isotopic evidence of weaning in hunter-gatherers from the late holocene in Lake Salitroso, Patagonia, Argentina. (a long paper, search for "weaning foods" for some discussion relevant to my first paragraph)
- From the ape's dilemma to the weanling's dilemma: early weaning and its evolutionary context, G.E.Kennedy
- Isotopes and new norms: Investigating the emergence of early modern U.K. breastfeeding practices at St. Nicholas Kirk, Aberdeen, K. Britton et al shows that for about the last thousand years weaning in the UK has commenced at a few months to a year and completed by about two years.
- Why are babies weaned early? Data from a prospective population based cohort study, CM Wright et al discusses modern practice and recommnedations in the UK.
Methods for making fruit and vegetable purees existed long before the modern electric blender.
A mechanical food mill is usable on most cooked fruits and vegetables with very good results. I don't know about early historical times, but these things were very typical throughout the 20th century in locales where blenders were not common, for example in Eastern Europe.
Mothers would also use a special kind of grater, with four star-like pips on each hole, to create a slightly rough puree from either raw or cooked plants.
Then there are also meat grinders, also a post-industrial-revolution tool, but quite good at making a mash-like substance out of many foods.
A more affordable and broadly represented instrument is the simple masher you already mentioned.
And to make it very simple, there is always the humble fork. A star chef might turn their nose up at a mashed puree full of 3 mm pieces, a baby would swallow.
And if you are looking for truly ancient tools, a mortar and pestle have been used for tasks like flour making thousands of years before mills were invented. They are not only usable with raw grains, but with most other foods, and are the preferred preparation method for many traditional recipes, even outside of baby food.
Of course, all of these tools require a much longer preparation time than using a blender. But spending that kind of time on food preparation was the norm, and people just did it.
I cannot give you data on which society used which tool in what proportion, or what was the actual ratio of using pureed foods versus foods which are soft for other reasons. But as you see, humans have always been able to puree food, independently of electric appliances.
Also, an aside which is not really related to history: if you want to puree a food nowadays, be it for a baby or for some other purpose, some of these tools are still superior to a blender, which is very sensitive about the liquid-to-solid ratio. You also have the choice of further modern tools such as food processors, slow juicers, and electric mill/grinders.