| Abstract: |
5. SUMMARY AND CONCLUSIONMany investigators reported that cultured milk products containing bifidobacteria has beneficial and therapeutic effects on human body. Also, it was suggested that, the use of bifidobacteria as possible dietary adjuncts in culture dairy products.In some cases bifidobacteria can be used to control enteric infections, lower serum cholesterol levels, improve infant formula and make milk products more nutritious and more easily digestable by lactose intolerant people.The aim of the present work was some trails to produce and examine stirred yoghurt which manufacture by replacing traditional yoghurt starter culture by ABT-3 or ABT-5 culture consisting of mixed culture of Lactobacillus acidophilus, bifidobacteria and Streptococcus thermophilus.This study included two parts :Part I : Survey of some stirred yoghurt obtained from local market.The first part dealt with surveying study on some local fermented products Zabado Juhayna, Zabado Juhayna with mango, Zabado Juhayna with apricot, Zabado Juhayna with strawberry, Zabado El-Masrien and plain stirred yoghurt from small local dairy laboratories). Immediately after purchasing these samples, they were subjected to microbiological analysis. The microbiological analysis included total bacterial, lactic acid bacterial, coliform bacteria, B.cereus, Staph. aureus bacteria and yeast & mould counts. The fat and acidity percentages were estimated on the day of samples collection and then after 3 and 7 days of storage at 5±1ºC.The results revealed that, there was a difference in the total count of bacteria which depends on the type of product, the ingredients used, the processing method and the handling procedures. The lactic acid bacterial count increased with the progress of storage period up to 7 days.The studied samples were free from B.cereus, Staph. aureus bacteria, and coliform bacteria either it was newely purchased or after storage. The count of yeasts and moulds varied between the different products and increased with storage.The fat percentage ranged from 1.5 up to 3.2%. These percentages were agree with the fat % written on the label claim of each type of stirred yoghurt. The acidity percentage ranged from 0.9 to 1.5% ( as lactic acid%).Part II : Manufacture and some properties of stirred yoghurt.This part included five section.1. The first section dealt with the manufacture of stirred yoghurt from cow’s milk of different fat percentages.2. The second part included the manufacture of stirred yoghurt with different starter cultures.3. The third section dealt with the manufacture of stirred yoghurt with the addition of different percentages of ABT-3 starter.4. The forth section dealt with the determination of cholesterol in stirred yoghurt.5. The fifth section dealt with the study of some rheological properties of the stirred yoghurt.Section 1: Effect of different percentage of fat in cow’s milk on stirred yoghurt quality.In this section the skimmed cow’s milk and cow’s milk with different fat percentages 1, 2 and 3% were used in the manufacture of stirred yoghurt with the addition of 0.5% traditional yoghurt starter culture (YC-281). The microbiological, chemical and organoleptic analysis on the fresh and stored product were conducted.The results of microbiological analyses showed the increase in the total bacterial count during storage. It is observed that B.cereus, Staph. aureus and coliform bacteria were not present in the final product. The yeasts & moulds appeared in some stored samples at 5±1°C after 3 and 7 days of storage. The lactic acid bacteria increased when the storage increased up to 7 days.When the fat percent of the stirred yoghurt samples was estimated, it was found to be 0.3 in the final product when skim milk was used. It was found to be 1.5, 2.5 and 3.5% when cow’s milk with different fat percentages i.e. 1, 2 and 3% was used respectively in fresh and stored samples.The total solid in the final product manufactured from skim milk was found to be lowered than that made from cow’s milk with different fat percentages. When the samples were stored for 7 days at 5±1°C, slight increase in the total solids were observed.The total nitrogen did not differ in all fresh samples studied altough there was a slight increase during the storage. The percentages of soluble nitrogen showed slight increase during storage.The acidity percentage (as lactic acid %) ranged from 0.83 % to 0.98% in the fresh stirred yoghurt samples. The acidity percentage in the final product manufactured from the skim milk was 0.94%. The acidity of the final products from cow’s milk contained 1, 2, 3% fat were 1.1, 1.0, 0.78% respectively.The acetaldhyde & di-acetyle which are responsible for the flavor compenents were estimated. Stirred yoghurt containing 3% fat had got the highest acetaldhyde content followed by the product made from 2% fat, then the product made from 1% fat. When the skim milk was used, it had got the lowest values. After storage of these products for 7 days, the acetaldehyde was increases gradually.As for the mentioned for acetaldehyde it was similar to diacetyl but diacetyl was decreased with the advance of the storage period.Organoleptic properties of the stirred yoghurt which contained different fat percentage showed that the product which was made from cow’s milk containing 3% fat was the best when it was fresh and after storage. It can be concluded from the mentioned resultus that the cow’s milk contained 3% fat can be selected for manufacturing of stirred yoghurt in the next parts in the study.Section 2 : Effect of different type of starter culture on stirred yoghurt quality.In this section the stirred yoghurt was manufactured with the use of cow’s milk containing 3% fat with the addition of different starter cultures.The added starter cultures were:YC-281 consisting of mixed culture of Lactobacillus delbruckii subsp bulgaricus and Streptococcus salivarius subsp thermophilus. This treatment was considered to be as a control.ABT-3 & ABT-5 cultures consisting of mixed cultured of Lactobacillus acidophilus, bifidobacteria and Streptococcus thermophilus was concedered to be a probiotic starter cultures.The starter culture was added at level of 0.5% to the cow’s milk and manufacturing process was completed according to the traditional method. The final product was stored for 7 days at 5±1°C.The product were subjected to the microbiological, chemical analysis and sensory evaluation when it was fresh and after 3 and 7 days of storage.The results of this experiment revealed that:1. Total bacterial count increased during storage and this was clear in the stirred yoghurt more than the probiotic stirred yoghurt.2. The yeasts and moulds were absence in all samples.3. The count of lactic acid bacteria was increased during storage.4. The B.cereus, Staph. aureus and coliform bacteria were not detected in all samples when fresh or during storage.The chemical analysis decleared that:1. The total solids, fat % and total nitrogen % in all samples were similar and increased slightly during storage.2. The percent of soluble nitrogen showed an increase in the product manufactured with YC-281 and followed by ABT-5 then the product contained ABT-3. The soluble nitrogen % increased slightly during storage which indicated slight proteolysis.3. The acetaldhyde % was increased during storage but diacetyle was decreased during storage.4. The organoleptic properties revealed that the stirred yoghurt containing ABT-3 had the highest records then followed by the stirred yoghurt containing ABT-5 and finally the stirred yoghurt containing YC-281 (control).Due to that, starter culture containing ABT-3 was selected to conduct the future experiments.Section 3 : Effect of different percentage of starter culture on probiotic stirred yoghurt quality.In this section the probiotic stirred yoghurt was manufactured with ABT-3 from cow’s milk (3% fat) and different percentages of ABT-3 starter culture (0.5, 0.1, 1.5 and 2%). The product was stored for 7 days at 5±1°C. The obtained results could be summarized as follows:1. The total count was increased in the stirred yoghurt with the increase of the starter percentage. This increase was clear in the probiotic stirred yoghurt. The total count was increased with the progress of storage.2. Yeast and mould did not appear in the fresh and store probiotic stirred yoghurt made with ABT-3.3. Lactic acid bacteria increased with the increase of ABT-3 percentage. Also, the count was increased with the increase of the storage period.4. All the samples were free from B.cereus, Staph. aureus and coliform bacteria.5. Total solids, fat and total nitrogen were similar in all samples. A slight difference in the soluble nitrogen percent was observed with the increase with of the storage period.6. The acidity percent increased when ABT-3 starter increased up to 2%.7. The increase of the starter culture percent and the increase of storage period caused a higher quantity of acetaldhyde. However, Diacetyl increase with the increase of starter culture % and decrease with the progress of the storage period.8. The sensory evaluation of the product revealed that the use of 0.5% ABT-3 had the highest scores for fresh product and after storage up to 7 days.Section 4: Total cholesterol content in the stirred yoghurt and the probiotic stirred yoghurt.In this section the total cholesterol content was determined in the stirred yoghurt and the probiotic stirred yoghurt which was manufactured from cow’s milk containing 3% fat. ABT-3 or ABT-5 was added at a level of 0.5 and 1 % during processing of probiotic stirred yoghurt. Also, traditional starter culture (YC-281) was used in the manufacture of stirred yoghurt as a control.The results revealed that the total cholesterol content was decreased with the prolongation of storage.The decrease of cholesterol in fresh probiotic stirred yoghurt samples ranged from 51.51 up to 57.53% when compared with control. The results revealed that there is no difference in cholesterol content when the quantity of probiotic starter has been increased.The presence of probiotic bacteria reduced the total cholesterol content in the final product. This is consider to be a benificial effect in some cases which require the reduction of cholesterol in diet.Section 5 : Some rheological properties of stirred yoghurt comparing with probiotic stirred yoghurt.When the apparent viscosity and shear stress of stirred yoghurt and probiotic stirred yoghurt was estimated, it was found to be high in the beging of the experiment then decreased with the increase of shear rate.The stirred yoghurt contained ABT-3 had lower appearent viscosity than the stirred yoghurt contained YC-281.The obtained results showed that, apparent viscosity and shear stress were high at low shear rate then a sharp decrease was observed with the increase of shear rate. This effect differs according the fat percent in the milk and type of starter used in stirred yoghurt making.So, it can be concluded that stirred yoghurt and probiotic stirred yoghurt are generally a non-Newtonian system. This behavior may be attributed to the formation of a weak gel like structure as a result of intermolecular interaction between the stirred yoghurt constituents and the development of this structure during fermintation process. This structure existed only at low shear rate when disruptive forces were low. The increase of shear rate broken down this structure and resulted in sharp drop in the apparent viscosity as this structure was responsible for the high apparent viscosity of stirred yoghurt at low shear rateThe conclusion is the suggesion of manufacturing of probiotic stirred yoghurt containing 3% fat and ABT-3 starter culture at a level of 0.5% in order to obtained a product with high quality and good characteristics.
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