Lactic acid (2-hydroxypropanoic acid), CH3CHOHCOOH [CAS 50-21-5], is the most widely occurring hydroxycarboxylic acid. It was first discovered in 1780 by the Swedish chemist Scheele. There are two isomers of lactic acid that are present in nature, L(+) and D(-) forms. L(+)-lacticacid is biodegradable and can be metabolized by the body and this property leads the application of lactic acid in biomaterial applications. It can be used wide range of industrial areas. In food industry, it is used as acidulant, preservative and antimicrobial agent. It has been also utilized in leather, textile, pharmaceutical and cosmetic industries for many years. Lactic acid is a naturally occurring organic acid that can be produced via fermentation or chemical synthesis. By the chemical synthesis method, racemic (DL) mixture of lactic acid is produced. By microbial production method L(+) and D(-)-lactic acid can be produced according to the type of microorganism which may be homofermentative or heterofermentative.

It is present in many foods both naturally or as a product of in situ microbial fermentation, as in sauerkraut, yogurt, buttermilk, sourdough breads and many other fermented foods. The lactic acid bacteria possess a large number of metabolic properties that are responsible for their successful use as starter cultures in the commercial production of fermented dairy, meat, and vegetable products and beverages. The genus Lactobacillus represents the largest group of rod shaped organisms with in the lactic acid bacteria. Some members of this group of organisms are important in the generation of particular flavor sand in other ripening processes associated with specific cheeses.

Experiment 1:

Plating techniques

Principles of the Procedure Nutrient Agar consists of peptone, beef extract, and agar. This relatively simple formulation provides   the nutrients necessary for the growth of a large number of microorganisms that are not excessively   fastidious. The beef extract contains water-soluble substances including carbohydrates, vitamins, organic nitrogen compounds, and salts.  Peptones are the principle sources of organic nitrogen,   particularly amino acids and long-chained peptides. Agar is the solidifying agent.
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Kaynak

Gravimetric analysis : As the name suggests, gravimetric analysis involves very accurate weighing of a chemical as a means of working out something about that chemical. A common type of gravimetric analysis involves precipitation of a single component (usually an ion) from a known amount of a compound or mixture. This is accomplished by reacting a solution of the test compound with a solution containing an ion that forms an insoluble salt with the component you wish to test for.
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In this experiment the sample containing an alkali sulfate is dried, weighed and dissolved in dilute HCl.The precipitation was occur. Barium chloride solution is added in excess to precipitate barium sulfate, and the precipitate is digested in the hot solution. After the precipitate is filtered through a paper filter which is then ignited and completely ashed. From the weight of the sample and weight of the precipitate, the percentage of sulfate in the sample is calculated. The precipitation reaction is the following in below:

Ba2+ (aq) + SO42- (aq) —> BaSO4 (s)

Variations in the acidity, temperature, manner of addition of the precipitant and time of digestion markedly affect the filterability of the barium sulfate precipitate and the extent to which various foreign ions are coprecipitated. Foreign anions such as nitrate, chlorate and chloride are coprecipitated as their barium salts, and the ignited precipitate contains the salt or oxide as an additive impurity. The coprecipitation of chloride can be decreased by slow addition of the precipitant. Since nitrate and chlorate interfere even at low concentrations, they should be removed from the solution before precipitation. Foreign cations such as ferric iron, calcium and, to a lesser extent, the alkali metals are coprecipitated as the sulfates. These are substitutional impurities, and the magnitude of the error depends upon the differences between the weight of the foreign sulfate or oxide and the weight of an equivalent amount of barium sulfate. The presence of ferric iron can produce errors as high as 2% in the determination.

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