Purpose This study aims to concentrate on quality improvement in plain yogurt production process at company A through adjusting the factors affecting the acidity of the yogurt and determining the optimal level of these factors. Design/methodology/approach Six Sigma-based framework using define-measure-analyze-improve-control (DMAIC) methodology is adopted through the application of design of experiments tool to focus on customer’s requirements to improve the quality characteristic of plain yogurt production process in dairy products manufacturing company (company A) in Iran. Findings The results showed that incubation time and fat percentage were significant factors on pH values of yogurt and the optimum settings for these factors were defined as 12 h for the incubation time and 1.5 per cent for the fat percentage. Research limitations/implications This study focused solely on the plain yogurt production process in dairy products manufacturing company. Practical implications Simplicity of Six Sigma plays a leading role for enabling any dairy manufacturer to determine the problem and minimize its cause through a systematic approach. Social implications Six Sigma has been considered to be a systematic, powerful technique to continuously improve the processes and develop the new products by using effective analytical and statistical tools and methods. This paper presents a Six Sigma-based framework using DMAIC methodology to improve the quality characteristic of plain yogurt production process in dairy products manufacturing company. Originality/value This study contributes to show a potential area in which Six Sigma DMAIC approach can promote to improve the quality of yogurt production process. This case can prompt managers of the company to apply Six Sigma method to address complicated problems in other processes, where causes particularly are not clear.

Design of Experiments (DOE) is a necessary component of product development and improvement, and prepares a scientific approach to evaluation and optimization of experimental factors. This study presents the application of DOE to homemade yogurt production process in order to detect the critical process factors which influence pH values of homemade yogurt and identify the optimal setting for these effective factors. Homemade yogurt were made by changing two different levels of skim milk powder, inoculation temperature, incubation temperature, incubation time, and fat to investigate main and interactions effects on pH of the fermented milk and achieve the optimal pH value from customer prospective in the range of 4.2 to 4.6. The results demonstrated that incubation time and fat are the most effective factors on pH development and the optimal settings for these factors should be 43200 seconds (12 hours) for the incubation time and 17.83 (g/kg) for the fat.