Discussion
Apple juice of 10% total solids (TS) is being concentrated in a single effect evaporator working with a surface condenser to 40% TS under a vacuum of 20 kPa. After some time the vacuum pump stops but the evaporation process continued. Choose the combination of possible implications from the following.
(P) Product quality is affected
(Q) Substantial increase in thermal energy requirement
(R) Decrease in the rate of evaporation
Answer: C
The implications are R & P: a decrease in the rate of evaporation and an effect on product quality.
The vacuum pump keeps the evaporator at low pressure, so the juice boils at a low temperature. When it stops, non-condensable gases accumulate and the pressure rises, and the boiling point rises with it. The temperature difference between the heating steam and the boiling juice shrinks, so the heat transfer rate and the rate of evaporation fall (R). The juice is also held at a higher temperature, which degrades heat-sensitive flavour, colour and nutrients, for example through browning (P).
The heat needed per kilogram of water evaporated is essentially the latent heat, which changes little with the boiling temperature (it is slightly smaller at higher temperature), so there is no substantial increase in thermal energy requirement (Q).
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