Roasting process Properties of coffee beans Coffee roasting method

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The roasting process is the main process in heating green beans to develop coffee aroma and taste. The aroma and taste emitted are determined by the original characteristics of each raw bean. The depth of fried beans depends on the type of raw beans, market demand or cooking method. Which baking machine controls the depth of the beans. Even though the chemical and physical components of the various raw beans themselves are different, these components are similar in principle, even if they vary slightly in the process of frying beans.

Roasting is the process of exposing raw beans saturated with moisture to rapid heating to 400 degrees Fahrenheit (200 degrees Celsius). At this temperature, the interior of the coffee bean undergoes pyrolysis. A brief exothermic process occurred. The temperature of the coffee beans increases to 392 to 410 degrees Fahrenheit (200 to 210 degrees Celsius), and the coffee beans lose 4 to 6 percent of their weight. Raw beans have 12% moisture, and if you count from there, the total loss of moisture during the frying process is 16 to 18%. The more moisture is lost during roasting, the darker the beans become. The color of coffee beans is also formed during the rapid loss of weight. Most of the sugar will change, and the beans will continue to swell (about twice the size of the original beans), and cracks will appear on the surface.

At the beginning of pyrolysis, about 7.5% of chlorophyll in raw beans was slightly reduced, and only about 4% was left in fried beans. The pH of raw beans is about 6.0, gradually decreasing to 5.5 at the beginning of pyrolysis, and then to 4.9 in lightly fried beans. Basically, the process is a dry distillation process that uses heat to decompose organic matter in raw beans. Stir-fried beans stop when they reach the desired taste, which is the same as when they reach the desired degree of pyrolysis and weight loss. The timing of stopping the roasting is determined by the depth of the beans during roasting, and can also be determined by the temperature of the coffee beans under the temperature control system of the roaster. The key point is that the high temperature decomposition of sugar and the degree of caramelization determine the ideal coffee taste and aroma.

There are two rates of weight loss during baking. The first is slow, caused by evaporation of water from the beans. The second is rapid pyrolysis. As the rate of water loss accelerates, the carbon dioxide in the beans increases rapidly. The timing of this transition occurs at the beginning of pyrolysis, i.e., at about 365 degrees Fahrenheit (185 degrees Celsius) or about 10% moisture loss from green beans. Although a 2% CO2 release rate is the standard calculation, a 1% CO2 release rate is possible in the delicious, lightly fried bean way. These common stir-fry phenomena are very similar among all types of coffee, even though they are very different from Robustas such as Java coffee. But these similarities should not be confused with differences in size, type, moisture content, and other factors that affect the speed at which each batch of beans develops the desired taste.

Factors affecting the degree of baking

Light fried beans contain more acid, so it is suitable for areas with alkaline water. Lower-grade coffee beans have poor taste characteristics that are easily revealed by light frying. So we usually fry the beans to drive out the bad volatiles. From the previous discussion, there are many benefits to mixing already baked beans. Habit and local people's favorite tastes usually determine the depth of fried beans. In cities, the beans are usually fried deeper than usual, as we call them "city fries." This saute-fry, lighter than French saute-fry and devoid of any over-burnt properties, is often used by coffee makers to prepare instant coffee. Because there are more carbohydrates in the filtration process in the factory than in brewing coffee at home, more carbohydrate flavors must be balanced by deep frying.

The color of roasted coffee beans has a certain relationship with the percentage of roasting weight loss. Light to medium-fried cinnamon colors, just into the delicious coffee range, weight loss of about 14%. A dark brown but not all black roast will lose about 15.5% of its weight. The darkest roast with a brown tinge loses about 17% weight and is the most common deep fry in the United States. However, single-serve fried coffee, such as French fried coffee, is very dark brown and has oil on the surface of the beans, weight loss of about 18%. Italian fried method is very dark brown, weight loss of about 20%. In all cases, we assume that raw beans have about 12% moisture.

The amount of dissolved matter in different coffees varies at 212 degrees Fahrenheit (100 degrees Celsius), but deep frying reduces the production of extracts. For example, medium frying, at a weight loss of 15%, produces 23% of the extract. However, when the same coffee beans are fried to 18% weight loss, only 21% of the extract is produced. Hydrolysates of interest to instant coffee manufacturers are also reduced in deep frying. Similarly, hydrolysed solubles can range from 25 to 22%. That is, the sum of all reduced extracts and hydrolyzed solubles is 48 to 43 percent. This has a huge impact on the true dissolved matter that can be obtained by cooking coffee. Cooking lightly fried beans produces 38% dissolved matter, while cooking lightly fried beans produces 35% dissolved matter. Deep-fried beans have greater damage to cell walls and strong structures, so they compress better. For this reason, deep-fried beans, compressed and under more pressure, leak more often from the bottom of the coffee filter than lightly fried beans. Deep fried beans are also fragile, and there are more fine powders when grinding. These finer powders also give more pressure to each drop of coffee or more resistance to extraction. Although the weight loss of coffee described above is important, it is less measurable. It is possible to distinguish eight colors from the lightest to the darkest with the naked eye. These recipes are:

very light baking

Cinnamon/Light Roast

medium baking

medium-deep baking

deep baking

deep bake

French Roast

Italian baking

When using different coffees or different green coffee beans for roasting, and trying to control the desired taste with the roasting depth, frequent testing is required. Blended light fried beans show more distinctive flavors than single deep fried beans. Deep fried beans release more fatty acids and their extracts, which can make coffee have more coffee flavor. When coffee beans come out of the roaster, cooling them with water increases the moisture content and deepens the color of the beans, so dry and wet coffee beans should not be compared together.

Three baking processes: drying, pyrolysis and cooling

Fresh coffee beans usually have about 12 percent moisture, and when the beans are roasted at high temperatures, from 500 to 800 degrees Fahrenheit, the moisture evaporates as the beans heat up. Most of the moisture is released in the first few minutes of baking, although the typical baking time in the United States is about 15 minutes. After 8 minutes, the temperature of the beans will rise to 300 degrees Fahrenheit and the beans will lose their green color, turn yellow and begin to turn brown. Between 260 and 370 degrees Fahrenheit, in 11 to 12 minutes, more moisture slowly dissipates, the beans reach 390 degrees Fahrenheit, and the coffee turns light brown. At 14 minutes, the temperature of the beans reaches 410 degrees Fahrenheit and the color turns medium brown. At 16 minutes, the beans reach a temperature of 440 degrees Fahrenheit and turn dark brown, almost equivalent to French roasting. If baked for another minute, the temperature will exceed 450 degrees Fahrenheit, and at 465 degrees Fahrenheit, the beans will turn Italian and dark brown at 20% weight loss.

The actual time control, temperature and color changes depend on the method of frying beans, cycle time, type of beans, original moisture content, heating speed, etc., rather than a general description of the entire process. In all cases of roasting, what really matters is that there is a lot of hot gas released from the coffee beans at 400 degrees Fahrenheit, which is called pyrolysis or chemical decomposition of caramel. Sivetz invented temperature and time measurements at the end of 1974, and in June 1975, presented at the A.S.I.C conference in Hamburg, demonstrated that heat release was measurable and directly related to the quality of raw beans baked, i.e., the most fragrant beans grown at the highest latitudes released the most heat, the mildest washed beans at the lower latitudes released less heat, and the lowest latitudes released no heat.

Pyrolysis produces acetaldehyde and other attractive coffee aromas, which are short-lived in appearance and persistence. Pyrolysis determines how long coffee tastes. That is, how deep can we allow coffee to roast? Some of the chemicals volatilized by coffee under high temperature decomposition include carbon dioxide, acetaldehyde, ketones, ether, acetic acid, methanol, vegetable oil, water vapor, and glycerin. Especially bad smell after baking has acid, and spicy choking smell. The coffee roasting process usually takes 7 minutes in a continuous roaster and 12 to 15 minutes in a 4-bag roaster. When roasted for more than half an hour, coffee beans will have a flat roast flavor rather than a rich aroma. The baking process is usually accompanied by a crisp popping sound and soot formed by mostly blue water vapor. As soon as the desired coffee color is obtained, the beans must be poured from an extremely hot fire source and cooled quickly and correctly with ambient air and spray. When the coffee beans are cooled, most of the water evaporates, and even if a small amount of water mist is sprayed, almost no water is absorbed by the coffee beans. Chilled beans stop pyrolysis. If the beans are kept hot, the flavor will decrease a lot.