Evaporative Cooling and Solar Pasteurisation Technologies for Value Addition of Camel (Camelus dromedarius) Milk in Marsabit and Isiolo Counties of Northern Kenya
The potential for evaporative cooling and solar pasteurisation technologies for value addition of camel milk in Marsabit and Isiolo counties of northern Kenya was investigated. To find out existing postharvest handling and preservation practices, a survey was conducted using a semi-structured questionnaire and focus group discussion on 167 camel milk producers, 50 primary and 50 secondary milk traders. Results showed that the camel milk chain was characterised by poor milk handling infrastructure, including poor roads and lack of cooling facilities. Camel milk was marketed raw under unhygienic conditions with minimal value addition, and spoilage was a major problem. Milk traders occasionally boiled milk using firewood as a means of temporary preservation during times when transport was unavailable. Provision of appropriate cooling facilities and utilisation of renewable energy technologies such as solar energy for milk processing were identified as possible intervention strategies to enhance marketing. Therefore, a low-cost charcoal evaporative cooler was developed and tested for the storage of camel milk. The cooler, 0.75 m3 in capacity, was made of galvanised angle iron (25 mm x 25 mm x 4 mm) frame with 10 cm wide charcoal walls which were moistened through a drip system. Temperature of camel milk inside the cooler did not significantly (p>0.05) change after storage for 10 hours. However, temperature of control milk at ambient conditions significantly increased (p=0.05) over the same period, from 22.6 ± 0.08°C to 28.1 ± 0.08°C.Milk inside the cooler was also significantly cooler (p=0.05) than control milk in the evening, with a net temperature reduction of 27.0%. Total bacterial count changed from 31.4±2.1 x 104 colony forming units per ml (cfu.ml–1) to 43.1±1.9 x 104 and 1638±81 x 104 cfu.ml–1 for milk inside the cooler and that at ambient conditions, respectively, after storage for 10 hours. The cooler’s performance was modelled using artificial neural networks (ANN), with inputs being ambient dry bulb temperature, wet bulb temperature, wind speed and temperature of drip water. The outputs were cooled milk temperature and cooling efficiency. The ANN predictions agreed well with experimental values with mean squa red error (MSE) of 10.2, mean relative error (MRE) of 4.02% and correlation coefficients (R2) in the range of 0.86-0.93. The development of the solar milk pasteuriser started with thermal performance testing of four water heating flat plate solar collectors available in Kenya with the objective of selecting a suitable one to be used to provide process heat for batch pasteurisation. The collectors included three commercial solar collectors purchased from local shops in Nairobi, Kenya and one prototype collector designed and fabricated by the author. The three commercial solar collectors had effective areas of 1.67, 1.87 and 1.83 m2 while the self-made collector had an effective area of 1.60 m2. Thermal performance of the collectors was determined in terms of the Hottel-Whillier-Bliss equation. The FR(τa)e values, obtained using the effective collector areas and the inlet water temperature, were 0.76, 0.75, 0.73, and 0.82, respectively, for the commercial collectors and the self -made collector. The FRUL values were 8.33, 12.01, 9.80 and 13.77 W.m–2.°C–1, respectively. The solar collector with the lowest FRUL value had a black chrome selective absorber surface and was the most cost effective for delivering temperatures of about 80°C at an efficiency of 15%. It was used to develop a low -cost batch solar milk pasteuriser consisting of the collector and a cylindrical milk vat. The milk vat had a 50 mm-wide hot water jacket and an outer layer of 38 mm thick fibre glass insulation. The water jacket held approximately 30 litres of water, whereas the milk tank had a capacity of 80 litres. The hot water produced by the collector was used for pasteurising milk. The optimum quantity of milk which could be pasteurised by this device under the study conditions was 40 litres, which was pasteurised in approximately 1. 3±0. 5 hours at an average insolation and ambient temperature of 22.5±0.9 MJ.m–2.day–1 and 29.8±0.1°C, respectively. The average temperature difference between hot water and milk being pasteurised was 8.1±0.6°C. Total bacterial counts in pasteurised milk were less than 10 cfu.ml–1 while coliform counts were negative. The solar milk pasteuriser was modelled using ANN as described for the cooler. The inputs of the model were ambient air temperature, solar radiation, wind speed, temperature of hot water, and water flow rate through the collector, whereas the output was temperature of milk being pasteurised. The ANN predictions agreed well with experimental values, with MSE, MRE and R2 of 5.22°C, 3.71% and 0.89, respectively. It has thus been established that there is both the need and potential for evaporative cooling and solar pasteurisation along the camel milk value chain in Kenya. The two technologies augment each other in increasing the quantity and quality of marketed camel milk from scattered pastoral production sites in Kenya. The devices are of low cost and can be locally fabricated by village artisans using locally available materials, and their performance can be successfully modelled using ANNs, which helps to design an appropriate system for any application.