Coffee processing

 

Coffee Post Harvest Handling and processing in Kenya

Introduction

Kenya grows the Arabica coffee species almost exclusively, which is processed by the wet method. Small-scale farmers produce the majority of the coffee. These farmers have formed co-operatives, which have put up pulping units referred to as Factories. These co-operatives have management committees that oversee the running of the Factories. Coffee is pulped and dried as parchment coffee in these factories. The dried parchment is stored temporarily before being transported to centrally located companies for hulling. The hulling companies are privately owned and hull the coffee for the farmers at a fee.

Harvesting

Coffee harvesting in Kenya is done by selective picking of the ripe berries. This is a labor intensive exercise and involves most of the members of a family and hired labor. Transportation to the Factories is by Ox-drawn carts, pick-up vehicles, and sometimes lorries. This is done immediately after harvesting.

 

Cherry Sorting

The cherry is sorted out before pulping. This helps to remove the immature, diseased, insect damaged and dry berries as well as the leaves, twigs and other foreign matter. The sorted out berries are processed by the dry method.

Pulping

Pulping is the mechanical removal of the pulp from the cherry to have parchment coffee. After pulping, the coffee is graded into three grades 1,2 and lights. This is done by density and size of the coffee. Parchment 1 is conveyed to the fermentation tanks while grade 2 and lights are further processed again through another smaller pulper called a re-passer.

Mucilage removal

i) Biochemical removal: Fermentation

Fermentation allows the mucilage layer on the parchment to be washed off easily. Completion of fermentation is determined by washing a bit of the parchment with clean water and then feeling the coffee with the hand. A gritty feel is an indication of the completion of fermentation. This stage takes 1 to 4 days depending on the prevailing weather conditions, faster on warm days and slow on cold days.

ii) Chemical Removal

Several chemical products are used for removal of the mucilage, mainly lime, which precipitates the pectins in the form of insoluble pectates, which are then easily removed by washing. Alkaline carbonates have also been used. This method is not common in Kenya.

Underwater Soaking

Soaking is a complete immersion of the parchment under water. Studies in Kenya have shown that soaking of coffee parchment after fermentation for about 12 hours improves the coffee quality both in color and taste. The parchment is thoroughly washed to remove the degraded mucilage and acids completely before soaking.

Final Washing and grading

After fermentation and soaking, the parchment coffee is thoroughly washed with clean water to remove any dirt or remains of mucilage or sugars. Final washing is done in concrete channels by pushing the parchment with wooden paddles against a stream of water. The washing channels are painted like the fermentation tanks with acid resistant black paint. The paint allows heat retention during fermentation and reduces friction between the coffee and the concrete surfaces during washing.

During final washing, the coffee is graded again by weight into different grades.

Drying

Freshly pulped coffee has a moisture content (mc) of about 55%, which has to be reduced by drying to 11%. This is the ideal level of moisture content required for proper storage, hulling and roasting. In Kenya, sun drying is predominantly used and mainly by the co-operatives and the coffee is spread on wire mesh tables for several days (normally about 14 days), until fully dry. When it rains, the coffee is covered by a polythene sheets to avoid re-wetting. Some big commercial estates use mechanical drying.

The following are the stages of parchment drying that are observed:

 

Skin Drying (55 - 45% mc)

This stage involves the removal of surface water and that between the parchment and the bean. The parchment is spread on layers not exceeding 0.5 inches on wire mesh tables and turned frequently to encourage rapid evaporation and at the same time it is fully exposed to the sun. This stage is normally completed on the same day of final washing. While stirring the parchment to ensure uniform drying, discoloured and broken beans are sorted.

This stage of drying can be mechanised.

 

White Drying Stage (44-35% mc)

At this stage, the parchment is white and it is easy to sort out the defective beans. Drying at this stage is made slow and controlled, and during very hot days, the coffee is covered during the hottest part of the day, (from 10.30 a.m. to 3.00 p.m.) in order to avoid cracking of the parchment cover. This stage can be mechanised with well controlled temperatures to avoid cracking of the parchment.

Soft Black Stage (35 -25 % mc)

At this stage the parchment attains is final black colour. In Kenya, it is recommended to only sun dry in this stage, for the coffee is said to be photosensitive and the sun light makes the coffee to acquire some preferred quality characteristics. The coffee is fully exposed to the sunlight for a period of 48 -50 hours. Mechanical drying is discouraged at this stage.

Hard Black Stage ( 25 -12 % mc)

At this stage the parchment is hard dark in colour and can be done rapidly without any loss of quality.

Fully Dry and Conditioning (12 -11 % mc)

This is done in ventilated stores or bins in order to even out the moisture of the coffee. At this moisture content, the coffee can be stored in well controlled environment without any effect on quality.

Other processing techniques:

Roasting:

Once the green beans are treated, they undergo roasting.

It is a time-temperature dependent process, whereby chemical changes are induced by pyrolysis within the coffee beans, together with marked physical changes in their internal structure.

The required change takes place with a bean temperature from 190 ֯C upwards; bean temperature up to 240 ֯ C may be reached in less than 12 minutes.

Batch operated horizontal rotating drum roaster with either solid or perforated walls, in which hot air from a furnace/burner passes through the tumbling green coffee beans. Green coffee beans under movement are subjected to heat by conduction from hot metal surfaces, or convection from hot air, or more generally a mixture of both methods of heat transfer, together with contribution by radiation.

Other roasters include:

•              Vertical static drum with blades

•              Vertical rotating bowl

•              Fluidized bed

•              Pressure roasting

Fast-roasted coffee is advantageous because of lower bulk density and high yield on brewing.

Physico-chemical changes in coffee

Chemical changes

The chemical changes include Maillard type reactions and caramelization of sucrose. Volatile complex comprising of furan derivatives, pyrazines, pyridines, benzenoid aromatics, aliphatics, alicyclics and various sulphur compounds are important for the flavour/aroma in medium-roast Arabica coffee.

Some compounds are generated by straight pyrolysis of single compounds e.g. chlorogenic acids in generating phenols; there is overall 40% residual content for a medium roast. The change in chlorogenic acid content is used as analytical measure of degree of roast.

Similarly, coffee oil leads to formation of small amounts of aldehydes and hydrocarbons. The coffee oil is practically unaffected, as is the caffeine content.

Newly formed residuum of ~ 25% by weight of roasted coffee is melanoidins/humic acids. The loss of mass is 2-3% on dry basis for light roast, whereas it is up to 12% on dry basis for very dark roast. The beans lose 15-20% of their weight, but increases up to 25% in size.

 

Physical changes

The physical changes that occur include:

•              Change in colour.

•              Formation of cavities/cracking of surface.

•              Void volume is 47% of medium roast bean vs. 0% in green bean.

 

Cooling

In batch operation, the roasted beans have to be quickly discharged at the end of required roasting period into a cooling car, or vessel, allowing upward passage of cold air.

In addition, water may be sprayed from within the rotating drum, just before the end of the roast so called water quenching.

 

Advantages of water quenching

•              Assists in necessary cooling.

•              Adds a small percentage of water by weight to roasted beans, thereby assists uniformity of particle size in subsequent grinding.

 

Grinding

Multistage twin horizontal rollers up to 4 stages may be used to ensure more uniform particle size distribution.

1st and 2nd stages: Essentially performs cracking or crushing the beans into smaller units.

3rd and 4th stages: Leads to progressively finer grinding.

The grind size required is related to subsequent method of brewing to be adopted and whether for home use or subsequent large scale extraction i.e. coarse, medium, fine, very fine.

 

Packaging

Roasted and ground (R & G) coffee releases substantial quantities of entrapped CO2 gas which develops high internal pressure, leading to bursting of package.

The usual packaging material is laminates.

o             Packaging under vacuum

It allows a low percentage of oxygen content in headspace to be established within the package and accommodate release of CO2. Alternatively CO2 scavenger may be used.

o             Degas over a sufficient time period

The R & G coffee is allowed in bulk to degas over a sufficient time period to a low level, followed by gas purging whilst individual packages are being filled.

Gas purging is used to ensure that the residual oxygen in headspace is below 1.0%.

 

 

**Aromatization of Coffee

Process whereby the headspace coffee aroma volatiles are made available from roast coffee, or other sources onto the soluble coffee, usually at the packing stage.

This is a treatment imparted to improve the flavor and aroma.

The powder lacks full flavor and aroma of freshly brewed coffee. The flavor and aroma constituents are trapped and recovered during roasting, grinding and extraction and from oils pressed from coffee bean.