Feasibility of Sub-Zero Peltier Cooler
You did not tell us the mass, which is absolutely crucial.
Assuming

Tofu and water have same latent heat and same latent heat of fusion.

Perfect insulation.


Water takes 4.2 Joule/gram/degree C. Adjust to suit for Tofu.
The transition to frozen (assuming that Tofu freezes to ice at about 0 degrees C - which it may not, is 334 J/gram for water.
For the cooling, with water, you need 4.2J x 30C = 126 J/gram.
For say 1000 g that's 126,000 J.
In 2 hours that's 126,000/(2 x 3600) = 35 Watts.
5 Watts cools about 140 gram.
BUT that's at 100% efficient with perfect insulation.
If freezing to Tofu-ice add 334,000J/kg or about another 46 Watts per kg.

Added:

Assume 50% water and 1 kg total mass.

For 1kg of Tofu assuming 50% water and minimal other thermal material (a bad assumption probably) = 500 ml water equivalent.
Cooling 20 to -5 = 63 kJ. Freezing = 117 kJ. = 180 kJ  total
180,000 / 3600 / 2 hours = 25  Watts mean over 2 hours.
BUT you cannot freeze it until you have reached 0 C OR whatever the freezing point of Tofu is. (eg saturated salt (NaCl) solution freezes at  -20.5 C so freezing point depression MAY happen.
Fortunately, Tofu appears to freeze in the 0 to -5 C region (you probably already know that).
See this paper for this graph. This shows Tofu being cooled to final temperatures of -10, -30

So at 25 Watts

you cool it to 0C in 120 x 63/180 = about  40 minutes

and then freeze it over the next 80 minutes.


This assumes good heat transfer throughout the mass.
Note that this is the refrigeration power - NOT the electrical input power.
At say 5% efficiency that would need about 500 Watts of electrical input (!). Or 250 Watts at 10% efficiency,... .
Less if you have higher efficiency.  Peltier effect devices tend to have lower efficiencies for higher cooling differentials. You need to carefully check all parameters. Also, you specify 20C as the hot temperature - presumably this is ambient temperature.
For say 250 Watts thermal output and 5 degrees C temperature rise you need a 5/250 = 0.02 C/watt heatsink. To get the hot sink anywhere near ambient at these sorts of power levels you will need a very good heat sink indeed  - usually only accomplished with forced air cooling (fan). This need not take a vast amount of extra power, but needs to be allowed for.  Even with fan cooling the heatsink will still be impressive&quot;.