Yes, folks, there is a right way to do it, and it does make sense. Here is how, and why:
First, let's look at the most common wrong way to do it. Remember the old "anticipation" commercials from the 1970s? The bottle is held upside down, then the hungry diner waits... and waits. Most people I know attempt to improve on this by tapping the bottom (that is, the upper end when the bottle is upside-down). That may help, but not necessarily for the reasons you imagine.
Ketchup can be regarded as a highly viscous liquid, or a thixotropic (flows under pressure) solid. Neither term is exactly correct, but the problem is not what to call it. The problem is how to get the ketchup out of the bottle, in measured quantities, without making a mess.
In order for the ketchup to emerge, air must enter the bottle. With an ordinary liquid such as water, a very narrow (dropper) neck would prevent the displacement of water by, air, but even a ketchup bottle has a neck that is much to thick to prevent water escaping, and air rising in, when the bottle is inverted. In the case of ketchup, however, the sauce is thick enough that the gravitational pull on the ketchup does not suffice.
Ketchup will certainly fall, of its own accord. In the case of a partly empty bottle, tapping the base can expel some ketchup without the need for air to enter. This is because the air that is already inside the bottle can slightly de-pressurize. Tapping the inverted base may slightly liquify the ketchup, enough for some to drop out even though no air is taken in. But in a full bottle, the ketchup must move to the side so that the air can rise through the neck at the same time that the ketchup escapes. The weight of ketchup is not great enough for this to happen in most circumstances.
So, in the case of the full ketchup bottle, the problem can be divided into two issues: (1) How can I get the ketchup to move aside, so that air can enter on the opposite side? (2) How can I give the ketchup "extra weight," so that it will be pulled out of the bottle faster?
The first issue is solved by holding the bottle sideways, with a slight downward tilt, rather than upside-down. In this position, the ketchup naturally is pulled to the lower side of the neck, and the air naturally will channel along the higher side of the neck. Anyone who pours an ordinary liquid from a bottle knows this. Yet it is amazing how the experience is forgotten when it comes to ketchup.
Merely holding the bottle in the correct position is not very effective. It is necessary to "increase the weight" of the ketchup by applying some G-force. This can be done by making a fist, and tapping the bottle downwards against the fist, to bring the bottle to an abrupt halt. Don't hurt yourself! If your hands are delicate, you may try some other method of applying an abrupt stop to the bottle, provided that the stop is not rigid or fragile, and that you mind where the ketchup is going to emerge. Striking the bottle at the upper side of the neck is much less effective, since it applies the G-force in the wrong direction.
Tapping the inverted bottom of a full bottle – the customary way – is counterproductive. If you do that, most of the G-force will tend to keep the ketchup in the bottle. But now you know the right way, thanks to the amazing power of the Internet to unleash this kind of information. Of course, it would be possible to print instructions on the side of the ketchup bottle, but you wouldn't read them if they were there, now would you?
