There are instances where I have thrown away ketchup bottles with a little content left inside, simply because I could not get it out.
Some bottles have a narrow neck & opening and others have complicated designs preventing me from inserting anything. I have tried inserting a knife and spoon to get the sauce, but it is not effective.
Is there a hack for it? I do not want to dilute the taste of sauce with water.
You can usually turn the bottle upside down and stand it on its lid. So once you get to the point where there's a little remaining ketchup/mayonnaise and it won't come out, just store the bottle upside down. After a few hours, any remaining ketchup/mayonnaise will have fallen down to the lid. Then the next time you need ketchup/mayonnaise, you can carefully open the lid and ketchup/mayonnaise will easily come out.
If for some reason the ketchup/mayonnaise doesn't fall down to the lid using this method, then I would recommend that you get a chopstick and insert that into the bottle. It will be thin enough to fit in the thinnest bottle necks, and should be long enough for you to reach the bottom of the bottle. Then hold the bottle almost upside down and use the chopstick to scrape the ketchup/mayonnaise from the bottom and have it fall down near the lid.
Ketchup and similar "pasty sauces" are thixotropic. They are non-Neutonian liquids whose viscosity changes with different external factors. (Water and other "normal" Neutonian liquids have a constant viscosity.)
Thixotropic materials appear to be thick fluid which is difficult to get flowing. Once you do get it flowing, however, it sometimes flows too easily.
Inverting the bottle will allow gravity to work on the bulk of the contents over time. Smaller blobs are not affected due to another property of the stuff referred to as "shear thinning." Sometimes, swinging a bottle around in a circle to increase the "gravity" by centrifugal force is tried. If you do it hard enough, long enough, sometimes it works.
There's a more effective way.
Ketchup becomes more liquid with a minimum amount of stress applied to it in order to start. It is referred to as "yield stress." If less force is applied, nothing happens. At the yield stress point, however, the viscosity can drop as much as 1000 times! That's why you get a sudden glug of the stuff after hitting the base of a freshly opened bottle of it. You have found the yield stress point. This is the key to removing almost all of the contents.
Now: to extract the optimal amount of pasty stuff, try this (be patient): Tip the bottle and tap it to drive the stuff to one side. Gradually increase the force of the taps until you find the amount of force (yield stress point) that works. Increasing the strength of the taps after that, won't work any better or faster. (I strike the bottle against the heel of the palm of my hand.) Larger blobs are easier to get flowing than small blobs. Maneuver large blobs toward smaller blobs to join them. Gradually, you can get practically all of the contents out this way.
Alternate method: If I can't get the contents out of the bottle, sometimes, I'll put other ingredients into it, shake to distribute the stuff and to coat everything before emptying the bottle.
And for the very frugal among us: To get the very last bit of precious material from its container, use a bit of something to dilute it. This won't affect the taste very much and the texture only slightly.
Use a bottle scraper.
Full acknowledgements here.
Almost every household in the Netherlands seems to have one, I have grown up with one in the kitchen drawer and there is at least one in my kitchen at all times.
The bit at the end is very flexible, you just push it in, use it to scrape the left over content of the bottle and pull it out as easily. Just a small word of warning, often the stem of the scraper gets rather covered in the stuff you try to get out.
The 'easy' way would be to buy a bottle scraper. (They should be available online from the Netherlands by now, but as I reside in the Netherlands, it is hard to test it for you.)
The Life-Hack solution is to make your own. A piece of rubber cut to shape screwed onto the end of a chopstick might work.
For those who prefer to buy and can get someone in the Netherlands to shop for it, they are sold in all main stores for household goods, like Hema, Blokker, Marskramer but also supermarkets, like Albert Hein.
For those not in the Netherlands, I have seen them on Amazon but I am sure other sites will also sell them.
