Question

A Newtonian liquid food with a viscosity of 2.5 cP and a density of 1020 kg/m3 is to be pasteurized in a HTST system that heats the liquid food to 80°C followed by holding it in a 0.02 m diameter pipe for a minimum time of 0.07 min.

a. Calculate maximum velocity and the length of the holding tube required, given that the flow rate is 35 L/min.

b. The temperature at the cold spot for a liquid food in a can is as follows (Table Q2). The sterilization process is being applied to a microbial population with D121 of 1.1 min and z = 11°C. Calculate the total lethality for the sterilization process.

Time (min) 0 1 2 3 4 5 6

Temperature (°C) 80 105 130 135 125 115 105

Answer 1

Flow Rate = Area of Pipe* Velocity of Fluid

Hence the maximum velocity achievable in a pipe= 2*v = 2*1.8568 = 3.7136 m/s

And the length of the pipe then shall be 2*8 = 16 m

Lethality :

Time(min)	Δt	T(^0C)	Lethality (L)	F0(min)
0	0	80	0.0001835	0
1	1	105	0.034384573	0.034384573
2	1	130	6.44304121	6.44304121
3	1	135	18.35001247	18.35001247
4	1	125	2.262275304	2.262275304
5	1	115	0.278903873	0.278903873
6	1	105	0.034384573	0.034384573
				Sum = 27.403003

Total F0 = 27.403003 minutes

Hence Lethality :

Please comment for any doubts :)