Instant Coffee

Instant coffee is the dried water-extract of roasted, ground coffee. Although used in army rations during the Civil War, instant coffee did not become a popular consumer item until after World War II. Improvements in manufacturing methods and product quality as well as a demand for convenience foods accounted for its rise in popularity. Some of the soluble coffee consumed in the United States is now manufactured in coffee-growing countries. This may be part of the reason why only 41% as much instant coffee was made in the United States in 1997 as in 1977.

Most soluble coffee blends contain Brazilian, Central American, Colombian, and African robusta coffees. Beans for instant coffee are blended and roasted as for regular coffee but are ground more coarsely to avoid excessive pressure drop and flow maldistribution during subsequent extraction (16). The roasted, ground coffee is charged into columns called percolators through which hot water is pumped to produce a concentrated coffee extract. The extracted solubles are dried, usually by spray or freeze drying, and the final powder is packaged in glass jars at rates of up to 200 jars per minute.

Extraction

Commercial extraction equipment and conditions have been designed to obtain the maximum soluble yield and acceptable flavor. In most processes, the water-soluble components in roasted coffee are first extracted at temperatures slightly lower than 100°C and atmospheric pressure. Additional solubles are then created by hydrolytically converting hemicelluloses and other insoluble components of the roasted coffee into water-soluble materials by pressure extraction at higher temperatures.

The factors influencing extraction efficiency and product quality are (1) grind of coffee, (2) temperature of water fed to the extractors and temperature profile through the system, (3) percolation time, (4) ratio of coffee to water, (5) premoistening or wetting of the ground coffee, (6) design of extraction equipment, and (7) flow rate of extract through the percolation columns (16).

Cylindrical percolators with height-to-diameter ratios ranging from 7:1 to 4:1 are common. They are usually operated in series as semicontinuous units of 5 to 10 percolators, with the water flowing countercurrent to the coffee. The ground coffee may be steamed or wetted with water or coffee extract; this supposedly improves extraction (17). Feed water temperatures range from 154 to 182°C, and unless the columns are heated, the temperature drops so that the extract effluent will have cooled to 60 to 82°C. The effluent extract temperature is further reduced by water cooling in plate heat exchangers to minimize flavor and aroma loss prior to drying.

The extract is removed from the percolators and stored in insulated tanks until dried. The extract solubles yield is calculated from the extract weight and soluble solids concentration as measured by specific gravity or refractive index. Yield is controlled directly by adjusting the weight of soluble solids removed and depends primarily on the properties of the coffee, operating temperatures, and percolation time. Soluble yields of 24 to 48% on a roasted coffee basis are possible. Robusta coffees give yields about 10% higher than arabica coffees (16).

High solubles concentrations are desirable to reduce evaporative load in drying and provide good flavor retention during drying. Water inflows that provide extract concentrations in the 20 to 30% soluble solids range are frequently used. More dilute extracts are obtained if greater water inflow is used, but lipophilic flavors and aromas that reside largely in coffee oil in the beans are extracted more efficiently. Some processors concentrate solubles by vacuum evaporation prior to drying. The concentrated percolate may also be clarified by centrifuging prior to drying so that the dry product will be completely free of insoluble fine particles.

The flavor of instant coffee can be enhanced by recovering and returning some of the natural aroma lost during processing. Aroma constituents are liberated and can be collected subsequently by condensation when roasted coffee is ground, when roasted coffee is steamed in a vented extraction column, or when percolate is concentrated. Many patents have been issued dealing with the separation, collection, and transfer of aroma from roasted coffee to instant coffee.

Drying

The following factors are important criteria for good instant coffee drying processes: (1) minimum loss or degradation of flavor and aroma, (2) free-flowing particles of desired uniform size and shape, (3) suitable bulk density for packaging requirements, (4) desirable product color, and (5) moisture content below 4.5%. Operating costs, product losses, capital investment, and other economic aspects must be considered in selecting the drying process.

Spray Drying

Most instant coffee is spray dried. Concentrated coffee extract is atomized, usually by pumping it through pressure nozzles. Nozzles and nozzle combinations are used that provide the desired particle size, product bulk density, and flow capacity based on the properties of the extract. The resulting stream of drops, joined by concurrently flowing heated air, flows downward into a tall cylindrical drying chamber. The air transfers heat to the drops, causing water evaporation and progressive drying of the drops. The air progressively cools and becomes more and more humid as drying progresses. Most processors prefer to use low inlet-air temperatures (200-260°C) for best flavor quality, and use enough air to provide outlet-air temperatures between 107 and 121°C. Spray dryers are usually constructed of stainless steel and must be provided with adequate dust collection systems, such as cyclones or bag filters (16).

The particles of dried instant coffee are collected from the conical bottom of the spray dryer and from cyclones through rotary valves and are conveyed to bulk storage or packaging bins. Processors may screen the dry product to obtain a uniform particle-size distribution.

Agglomeration

At one time most instant coffee was marketed as small, spherically shaped particles. Now it is marketed mostly in a granular form. Instant coffee granules are produced by moistening small spray-dried particles with steam, thereby fusing clusters of particles together. These clusters are then dried in a tower similar to a spray-drying tower using a low level of heat. Particle fusion and drying may also be carried out on a continuous belt.

Freeze Drying

Freeze-dried instant coffee is made by freezing coffee extract, grinding the frozen extract into granules and then converting the ice and bound-water content of the granules into water vapor by sublimation under high vacuum. Absolute pressures of 67 Pa (500 /¿mHg), or lower, are used. The heat used to induce sublimation is radiantly transferred from hot plates to the outer surface of beds of granules and then conductively transferred in the bed and granules to sublimation sites. Freeze drying occurs at much lower temperatures than spray drying. Heat input is controlled to give maximum end-point temperatures between 38 and 49°C (18). Drying times are much longer than for spray drying (18). Freeze-dried coffees use high-quality coffee and have better retention of volatile aromatics than do spray-dried coffees and are thus considered quality instant coffees.

Packaging

In the United States, instant coffee for the consumer market is usually packaged in glass jars containing from 56 to 340 g of coffee. Larger units for institutional, hotel, restaurant, and vending-machine use are packaged in bags and pouches of plastic or paper. In Europe, instant coffee is packaged in glass jars and, frequently, plug-closure metal containers with foil liners.

Protective packaging is primarily required to prevent moisture pickup. The flavor quality of regular instant coffee changes very little during storage. However, the powder is hygroscopic, and moisture pickup can cause caking and flavor impairment. Moisture content should be kept below 5%.

Many instant coffee producers in the United States incorporate natural coffee aroma fixed in expelled coffee oil in the powder. These highly volatile and chemically unstable flavor components necessitate inert-gas packing to prevent aroma deterioration and staling from exposure to oxygen.

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