Development of decentralized solar-assisted yogurt processing unit for the production of on-farm high-quality yogurt

In Pakistan, the dairy sector represents a significant part of agricultural production and is an important source of income, especially for small farmers. Yogurt production at the farm level is important for adding value to the milk. In this study, a solar-assisted yogurt processing unit capable of performing three processes of heating, fermentation, and cooling in a single unit was developed. It consisted of a circular chamber surrounded by a coil for heating by a solar vacuum tube collector and a pillow plate for cooling by a solar PV-powered chiller unit. Experiments were performed using 50, 40, and 30 L of raw milk under a constant water circulation rate of 50 L per minute for heating followed by a cooling process under 36, 18, and 6 rpm of stirrer speeds. The total energy consumed was calculated to be 6.732, 5.559, and 4.207 kWh for 50, 40, and 30 L batch capacity respectively. Energy and exergy-based thermal analysis showed that 40% of the total energy was consumed during the heating process of raw milk in all cases. The specific product energy was calculated to be lower (485 kJ/kg) for the higher volume of milk (50 L). The exergy recovered during the refrigeration process was found in the range of 0.48-4.54 kJ/kg, 1.35-3.96 kJ/kg, 0.84-6.18 kJ/kg for 50, 40, and 30 L of batches respectively. Out of the total available power (2218W) at the evacuated tube collector, 69.70% of energy was available for milk heating. The comparative quality analysis of yogurt processed in a solar-assisted yogurt processing unit with the existing milk value chain and its techno-economic feasibility was evaluated. The quality attributes for processed yogurt like fat (5.5%), solid-not-fat (8.683%), acidity (0.93%), lactose (4.73%), total solids (14.183%), pH (4.3433), density (1.039 kg/L) syneresis (9.87 mL/100g), S. thermophilus count range (10.18-10.30 log cfu/mL) and L. bulgaricus count range (10.26-10.34 log cfu/mL) were found within the standardized ranges. Moreover, no detection of coliform count in solar-processed yogurt endorsed the current idea of performing three processes of heating, fermentation, and cooling in a single unit. Based on the energy sources utilized, the payback period was calculated to be 1.3 to 9 years with an expected lifespan of 15 years while in terms of product profit, the payback period was predicted to be 1.78 years. The processing cost per liter of milk for yogurt production was calculated to be 0.0189 USD. Considering CO₂ emission savings, it is anticipated that a solar-powered yogurt processing unit can generate 107.73 MWh of useful energy during its operating life with zero CO₂ emission. The study offers a sustainable energy solution for the decentralized processing of raw milk, particularly in remote areas of developing countries where access to electricity is limited.