Papers

¹University ''St. Kliment Ohridski", Faculty of Biotechnical Science, Bitola Macedonia

²Unversity :St.Ciryl and Methodius" Narural Mathematics faculty, Institute for Chemistry,Skpje, Macedonia

³METAROM " Food Flavouring and Colouring Industry", Sydney Australia

The presence of lactic ions in milk and milk products is an indicator of microbial growth and their metabolic activity. Lactic acid can contribute to the flavor of the yogurt, and has multiple health benefits, such as boosting the immune system, better food digestion and general well-being. Lactic acid also contributes to the flavor of fermented milk, and is a key factor in regulating calcium metabolism in bones. The method for determining lactic acid is comprised of treating the yogurt with barium chloride, sodium hydroxide and zinc sulfate. Iron(III) chloride is used as a color reagent. The absorbance is measured using "Spectro Quant Pharo 300" spectrophotometer by "Merck" at 400 nm wavelength. Citric acid is metabolized by the microorganisms thus forming acetic and propionic acid. The method for determining the citric acid is based on the color forming complex with the reaction between pyridine and acetic anhydride. The absorbance is measured at 428 nm, and is calculated using specific formulas. Yogurt made from milk with different fat content is used as a material for this research. The milk is inoculated using different concentrations of microbiological culture CH-1 (Lactobacillus bulgaricus and streptococcus thermophilus) by CHR.Hansen Denmark. The aim of this research is to monitor the development of lactic and citric acid during the fermentation i.e. since the start of fermentation till end of shelf life at 4^oC. Quantity of lactic acid is between 0.1% in fresh milk to 2.0% at the end of fermentation at pH 4.59.