NDC'01

QUALITY AND PROPERTIES ON HEAT PUMP DRIED INSTANT POTATO

Erlend Indergård, Odilio Alves-Filho, Ingvald Strømmen and Trond I. Bøhn
Department of Refrigeration and Air Conditioning
The Norwegian University of Science and Technology, NTNU
The Foundation of Industrial and Scientific Research at NTNU,
SINTEF Trondheim, Norway
E-mail: erlend.indergard@energy.sintef.no

Keywords: Instant potato, heat pump dryer, quality, rehydration, bulk density.

ABSTRACT

Potato was pre-treated (boiled and cut, or cold expanded) and dried in a heat pump fluidized bed dryer under atmospheric pressure and at different conditions to produce instant potato powder. The drying was done in two stages, i.e., starting at -5°C and finishing drying at +20°C. Properties and quality aspects of the heat pump dried instant potato were studied and compared to vacuum freeze dried and commercial instant potato. The experimental work was done with varying process conditions, such as varying drying-programs, boiling time and different potato varieties. Drying characteristics and sorption isotherms were measured as well as quality and properties parameters like bulk density, color and water adsorption and compared.
A boiling time of 15 minutes was found to be appropriate for the product quality. The best drying program according to drying time and quality was to change the temperature from -5°C to +20°C as soon as the moisture content of the product reached 50% wet base. Different potato varieties showed differences in optimal boiling and drying times. In a few seconds after warm water rehydration at 99°C the dried potato powder was ready to eat. The new instant potato powder had a higher rehydration index than both the vacuum freeze-dried and commercially dried instant products. The instant powder had a higher bulk density than vacuum freeze dried powder but significant lower than commercially dried instant potato. The measurement of color on the different products showed that both the vacuum freeze dried and the commercial dried potato powder were a slightly whiter than produced instant powder, but the commercially dried powder had a more yellow color than the other products. The sorption isotherm had the "S" shape, which is typical for most biological products. The equilibrium moisture content versus water activity was plotted for temperature of 23°C and shows that the moisture content drops quickly from 85.0 to 30% wet base, for the water activity range between 1.0 and 0.80. Afterwards the moisture drops gradually towards the final moisture content of 6.0% wet base.