Instant Coffee

Abstract

This chapter begins with discussions on various operations, i.e., extraction, concentration, and drying, using manufacturing process of instant coffee as an example. Solid-liquid extraction, the same as that employed for obtaining coffee extract from coffee beans, is widely used in food processing. First, we elaborate on how to describe extraction efficiency and extraction rate; we then turn to calculation method of extraction time. Next, we introduce the most widely used equipment in food processing, the evaporative concentrator, and also discuss the calculations involved. Finally, the chapter presents the principles and characteristics of spray-drying and freeze-drying prior to discussing the expression of amount of water vapor contained in air and the design of dryers.

Exercise

• 4.1 In an extraction process, an extraction efficiency of 98 % was achieved after the second extraction operation. If the total amount of extractant used for the two extraction operations is to be used for one single extraction operation or to be divided equally into three extraction operations, what will the extraction efficiency be?

• 4.2 A cheese wheel of 15 cm in diameter and 5 cm in thickness is immersed in a NaCl solution, allowing NaCl to diffuse into the cheese. What would the NaCl concentration be inside the cheese after 5 days of immersion? Assume that the concentration of the NaCl solution is constant throughout the immersion and that at the surface of the cheese wheel immediately reaches an equilibrium concentration of 60 kg/m³ cheese and stays constant thereafter. Let the diffusion coefficient of NaCl inside the cheese be 3 × 10⁻¹⁰ m²/s.

• 4.3 A sugarcane solution of 38 % (w/w) sugar concentration is concentrated using an evaporator at 400 kg/h to 74 % (w/w) sugar concentration. Determine the amount of water evaporated by the evaporator and the amount of concentrate obtained.

• 4.4 A single-effect evaporator with a heat transfer area of 30 m² is used to concentrate a juice of 12 % (w/w) solid concentration to 20 %. The juice enters the evaporator at 50 °C and evaporation occurs at 60 °C. Saturated steam of 100 °C is employed as the heat source. Let the overall heat transfer coefficient of the heat transfer surface (of heat exchanger) be 1000 W/m² and the specific heat capacity of the juice be 4.18 − 2.51C kJ/(kg · K), where C denotes the solid concentration (mass fraction) of the juice. What are the feed rate of raw juice into the evaporator and the amount of saturated steam required for heating? Let the latent heat of evaporation of water be 2500 − 2.24 t, where t denotes temperature in the unit of °C.

• 4.5 Thirty thousand cubic meter of air of 70 % RH (at 30 °C) is mixed with 1.0 × 10⁴ m³ of air of 50 % RH (at 40 °C). Determine the absolute humidity, H, of the mixture air. The respective saturated water vapor pressures at 30 and 40 °C are 4.243 and 7.376 kPa.

• 4.6 A solid food that contains 15 % (w/w) moisture on wet basis is subjected to hot air-drying by which the moisture content is reduced to 7 %. Part of the outlet hot air is recycled and mixed with the inlet hot air for drying the solid. The humidity of the fresh inlet hot air, the recycled outlet hot air, and the mixture hot air that enters the dryer is 0.01, 0.1, and 0.03 kg-water vapor/kg-dry air, respectively. When the process raw material is fed at 100 kg/h to the dryer, what are the flow rates of the fresh inlet hot air and the recycled outlet hot air, and what is the discharge rate of the finished product?