Description of the internal agitator
C.B. O'Connor, S. Mezgebu and Z. Zewdie
The authors' address is: International Livestock Centre for Africa (ILCA), PO Box 5689, Addis Ababa, Ethiopia.
Traditionally, Ethiopian butter has always been made from sour milk. Small quantities of milk are collected in a clay pot over a period of a few days and allowed to sour naturally. When a sufficient amount of milk has been collected, it is churned by shaking the pot until butter granules are formed. Depending on the quantity of milk, the churn is shaken backwards and forwards either on the lap or on the ground.
This traditional method of churning is time-consuming, perhaps taking more than two hours. In making butter from sour milk, the objective is to extract the maximum amount of fat from the milk. The liquid that remains after the butter is made - buttermilk - is used to produce a type of cottage cheese, called ayib in Ethiopia. Since there is a big difference between the price of butter and that of cottage cheese, any butterfat remaining in the buttermilk can only be considered an economic loss to the smallholder. Consequently, efficient butter-making can be measured by the length of time it takes to churn the milk and, more importantly, by the amount of fat remaining in the buttermilk or the amount of fat extracted from the milk.
Observations made of traditional butter-making by smallholders have indicated that the efficiency should be improved to save time, thereby improving the economic return. Toward this end, the dairy technology personnel at the International Livestock Centre for Africa (ILCA) have developed a device that fits inside the traditional clay pot and rapidly and consistently agitates the milk. This internal agitator has passed through several phases of development over the years. Details on the design and construction of the agitator are given further on in the article. The dimensions may be amended to suit the size of the container used.
Before recommending the agitator to smallholders, carefully controlled trials were carried out to compare the traditional method with the new one. These trials took place at ILCA's milk-processing laboratories in Debre Zeit and involved the churning of sour whole milk with different levels of fat and at different churning temperatures.
The results of the trials clearly indicated that churning efficiency was considerably improved by using the agitator. At a churning temperature of 18 °C, the average fat content of the buttermilk was about 1.4 percent using the traditional method, compared with 1.1 percent when using the agitator. These losses of buttermilk fat led to significant differences in the percentages of fat recovered. In the traditional method of churning, the average percentage of fat recovered was about 67 percent, compared with 76 percent when using the agitator. In these controlled experiments, the length of time required for churning was 65 minutes for the traditional method and 31 minutes for the agitator method.
In separate on-farm trials in the Debre Birhan area, an average churning time of 57 minutes was obtained with the agitator fitted into the clay pot, while a churning time of 139 minutes was the average when using the clay pot only. The average fat content of the buttermilk was 0.36 percent using the agitator method and 1.1 percent using the traditional method. The longer churning times and the lower fat losses in the buttermilk could be attributed to the difference in altitude between Debre Birhan (2 800 masl) and Debre Zeit (1800 masl), the latter having lower churning temperatures.
Detailed look at the internal agitator system - Détail du système d'agitateur intérieur - Detalle del sistema del agitador interno
A - Wooden stopper
B - Clay pot; C - Padded pot seat
D - Pole
E - Brace
F - Agitator
G - Rope
One of the main factors affecting churning efficiency and, in particular, butterfat recovery is the temperature of the milk at churning. At a churning temperature of 18°C, 67 percent of the fat was recovered using the traditional method and 76 percent using the agitator. When the churning temperature was increased to 25 °C, the percentage of fat recovered decreased considerably: 44 percent for the traditional method and 55 percent using the agitator. These results highlight the advantages of using the agitator at a low churning temperature.
In order to increase efficiency, it is recommended that smallholders use the agitator and cool the milk to at least 18 °C when churning sour whole milk.
Details of the equipment required for churning butter using the internal agitator system (H) are given in the Figure. The equipment consists of:
· Wooden stopper (A). This fits into the neck of the clay pot. It holds the agitator in position and prevents the milk from spilling.
· Clay pot (B). A gourd may also be used. The size of the clay pot or gourd may vary with the amount of milk available. The container should not be filled more than halfway.
· Padded pot seat (C). The pot seat may be made of cloth or woven hay or straw. It reduces the risk of damage to the clay pot.
· Wooden pole and pegs (D). The wooden pole is made from a single piece of wood. The base of the pole is placed in the ground. In the upper part of the pole there are a number of holes through which the wooden pegs are inserted to keep the braces (E) in place.
· Wooden brace (E). Two wooden braces, placed about 20 cm apart, are used to connect the wooden pole (D) to the internal agitator (F). Four wooden pegs may be used, one on either side of each brace, to give added rigidity to the installation (H).
· Internal agitator with paddle blades (F). The base of the agitator is grooved or U-shaped to hold the paddle blades. There are two paddle blades, and these are kept in position by means of a wooden plug (dowel) inserted at the base of the agitator. When the agitator is not in use the paddle blades hang vertically, thus enabling the agitator to be inserted into the clay pot.
· Nylon rope and handles (G). Nylon rope is inserted through a hole in the upper part of the internal agitator. About half of the rope is wound around the agitator and the ends are attached to wooden handles that, when pulled, give the agitator a circular motion. This rapid circulation causes the paddle blades to rotate in a horizontal position.