Buying a "blender" that can purée a potato

Your requirements appear to be for a "raw food" blender, the motor on those units are typically well above 500 watts (1000+ Watts). This enables harder/tough foods (such as ice, kale, nuts) to be broken down smooth enough that no sieving is required, with less risk of overheating the motor. Note: when a blender has a nut bag, or a nut bag option, it alludes to potentially not able to break down harder food without leaving noticeable particles.

There are a few blenders suitable for raw food enthusiasts, depending on each model they may have a:

• Tamper
• Higher rpm (20,000+ rpm)
• Higher power (500 Watts — 1000+ Watts/2–3 horsepower) Compared to a standard blender (typically 200–400 Watts)
• Different jug design/dimensions (width/height)
• Different speeds and/or cycles/programmed settings (e.g. smoothie, 60 seconds, 90 seconds, clean, etc.)
• Different Blade designs
• Smaller portion containers (capsule/bullet shaped)

A tamper is usually provided to allow the user to push food items back towards the blade. When a tamper is not included on some of the other blenders it may be due to a different blade design along with a different power motor. This may cause all of the food in the blender to be pulled back to the blade by the vortex it creates, thus alleviating the need for human intervention.

The price variance between blenders is wide, and quite expensive for some of the higher performance models. Intrinsically some research into refurbished models.

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In direct answer to the questions from OP.

What is it about a blender that allows it to make a good purée rather than just mixing things easily? Does anybody know what makes a machine capable of making a good purée as opposed to just blending, especially when both kinds are sold as “blenders”.

Mainly power of the motor, and the blade design enables the blender to breakdown hard foods completely. Regular blenders do not have the power, nor the cooling ability for the motor to operate at long enough periods - even some models that market suitability for crushing ice can be known to overheat/fail.

Should I be looking for ones with different options for blades like a food processor, to make it well suited to different purposes? E.g. I have seen blades just for milkshakes. In general, how do you shop for one when the only information a seems to be total power and the number of presets?

Blenders that have a high power motor, high rpm, and/or market its suitability for ice/nut butters/hard spices (e.g. nutmeg), indicate potential. Where possible, check reviews (where blending nuts has been tested) or better still, when the blender has demonstrations. You may be able to request a demo on nuts, if so you could taste or strain this into a sieve or paper towel/kitchen towel — to check for any particles that have not broken down.

If you have access to one, try a food processor - they vary, but some (left running long enough) achieve a pretty convincing puree. You could also try pre-grating the potato (and apple, if you like), using a different blade, to speed the process along.

I just "upgraded" to a 30+? year old cuisinart (from the made in Japan years), which is both quieter and more prone to achieving a puree than the presto I'd been using for 20+ years. Sticker-shock (which would have been profound when new) should be blissfully low at a yard sale, estate sale, second-hand shop or similar source, and I'm pretty sure there's no teflon to come off.

In general, how do you shop for one when the only information a seems to be total power and the number of presets?

Product reviews from real users is your best tool. Problem: it is time consuming to read many reviews and weed it down to the helpful ones, and the comments are not always accurate.

I heavily rely on Amazon reviews, even if I don't end up purchasing there. What I like about Amazon reviews is the "ask a question" feature. For example, you could ask "can this puree an apple, not leaving a gritty feel?". When I've asked questions on a popular product, I've gotten same day answers.

The one number which can give you some real insight of whether a blender can pack a hard food is the motor torque. Others like watt numbers are not really relevant - while you probably cannot blend a potato well with a 5 watt blender, it is perfectly possible to make a 1000 watt blender that's not up to the task, and many producers on the market do it.

Since I have never seen a blender manufacturer wihch publishes torque, you have to rely on reviews.

Also be aware that a blender is for making liquids. If you intend to throw in chunks of something hard without it being suspended in a liquid, it is quite likely that it will have trouble, unless you get one of the extra powerful top-tier machines.