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Studies on Physical and Chemical Properties of Yogurt Prepared from Raw Milk and Reconstituted Milk
本研究分別以生乳（M：raw milk ; 12.0﹪total milk solids ）、還原乳（克寧）（P：reconstituted milk；12.0﹪total milk solids）、乳固形物強化生乳（生乳添加2.0﹪脫脂乳粉, 安佳）（MS：raw milk added 2.0﹪skim milk powder；14.0﹪total milk solids）、乳固形物強化還原乳（還原乳添加2.0﹪脫脂乳粉, 安佳）（PS：reconstituted milk added 2.0﹪skim milk powder；14.0﹪total milk solids）製成酸酪乳，並探討乳固形物含量、添加洋菜（0.2﹪）等對其物理及化學性質影響。試驗共分成三個部分：
接種0.1﹪商用菌株YC350（Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus）於生乳組（12.0﹪乳固形物：M組；14.0﹪乳固形物：MS組）及還原乳組（12.0﹪乳固形物：P組；14.0﹪乳固形物：PS組）基質中於42℃培養12小時，並分別於0、3、6、9及12小時測定其pH值、滴定酸度、黏度、剪切應力、色差（L, a, b值）、乳酸菌數及游離酪胺酸含量之變化情形。結果顯示各組隨發酵時間之增加，其酸度、黏度、剪切應力、L值、b值、乳酸菌數及游離酪胺酸含量均會增加（P<0.05）。各組間之黏度、剪切應力、L值、b值，還原乳組顯著較生乳組為高（P<0.05），而pH值及a值則反之（P<0.05）。且商業菌株YC350於12.0﹪乳固形物（M組或P組）中發酵只需5.5小時；而14.0﹪乳固形物者（MS組或PS組）則需發酵6小時。
分別以生乳（M組和MS組）及還原乳（P組和PS組）經添加洋菜（0.2﹪）或不添加洋菜後，再接種0.1﹪YC350菌元，以42℃分別培養至pH4.5或4.2後，分析各組的物理性質（黏度、剪切應力、凝乳強度、凝乳硬度、破斷強度、凝析、色差（L, a, b值）官能品評及顯微結構）及化學性質（游離胺基酸、游離酪胺酸及β-半乳糖苷酶活性）。物理性質方面顯示：各組酸酪乳未添加洋菜者之黏度、剪切應力、凝乳強度、凝乳硬度、破斷強度及色差，均以還原乳組（P組或PS組）會較生乳組（M組或MS組）為高（P<0.05），但經添加洋菜後，各組之黏度、剪切應力、凝乳強度、凝乳硬度、破斷強度均有增加（P<0.05），且於生乳組與還原乳組間差異不大。產品品評顯示總接受性方面以MS組最佳， P組最差（P<0.05）。顯微結構顯示以生乳製作酸酪乳其酪蛋白顆粒較還原乳者為細緻，且其孔洞亦較還原乳製作者為小。此外，各組隨固形物之增加，其質地會變得較粗糙且不規則。化學性質方面，游離酪胺酸及β-半乳糖苷酶活性則呈現添加洋菜與否對各組影響不大。游離胺基酸總量以PS組有較其他組為高之趨勢，並發現以生乳（M組、MS組）製作之酸酪乳可合成較多的磷絲胺酸，有較還原乳（P組、PS組）高之趨勢。此外，還原乳組（P組、PS組）比生乳組（M組、MS組）合成較多與甲硫胺酸產生拮抗作用之乙基硫胺酸（ethionine）。
添加洋菜製成pH 4.5之酸酪乳後，進行貯藏20天之物理性質（黏度、剪切應力、色差（L, a, b值）、凝析、凝乳結實度、凝膠強度、及破斷強度）及化學性質（pH、滴定酸度、乳酸菌數、β-半乳糖苷酶活性、游離胺酸含量）測試。結果顯示：生乳組（M組或MS組）之pH值、乳酸菌數、β-半乳糖苷酶活性、L和a值較還原組（P組或PS組）為高。而滴定酸度、黏度、剪切應力、b值、凝乳結實度、游離酪胺酸含量皆以還原乳組高於生乳組（P<0.05）。
This research is to study the difference of the qualities of yogurt prepared from local raw milk and reconstituted milk in order to understand whether the qualities of yogurt prepared from local raw milk is better than that of imported reconstituted milk or not. In this research, raw milk(M group: 12﹪total milk solids ), reconstituted milk(Klim) (P group: 12.0﹪total milk solids), milk solid fortified raw milk (MS group: raw milk added 2.0﹪skim milk powder;14.0﹪total milk solids) and milk solid fortified reconstituted milk(PS group: 12.0﹪total milk solids of reconstituted milk added 2.0﹪skim milk powder(Anchor);14.0﹪total milk solids) were used to prepare yogurt and to study how the content of milk solid and added agar (0.2﹪)affect the physical and chemical properties of yogurt. This trial was divided into 3 parts: Experiment 1: Fermentation of characteristic of different milk substrate by commercial strain YC 350 The commercial strain YC350 (Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus)0.1﹪was used to fermentation in raw milk group (12.0﹪milk solid: M group; 14.0﹪milk solider: MS group)and reconstituted milk group (12.0﹪milk solid: P group; 14.0% milk solider: PS group) at 42℃ for 12 hours, the changes of titrable acidity, viscosity, shear stress, colored( L-value, a-value, b-value), lactic acid bacteria counts and the content of free tyrosine were determined. The results showed that the fermentation time increased in each group, the titrable acidity, viscosity, shear stress, L-value, b-value, lactic acid bacteria counts and the content of free tyrosine of yogurt were significant increase(P<0.05). Moreover, that the viscosity, shear stress, L-value, b-value between in each group of yogurt made by reconstituted milk were significantly higher then that made by raw milk group. Otherwise, the pH value and a-value were not (P<0.05). The commercial strain YC350 fermented in 12.0﹪milk solid group(M group or P group)could produce yogurt affer 5.5 hours fermentation. However, the 14.0﹪milk solid group(M group or P group)that would be needed for 6 hours. Experiment 2: Effect of agar on the physical and chemical properties of yogurt prepared from raw milk and reconstituted milk This study was using raw milk (M group and MS group) and reconstituted milk(P group and PS group) after adding agar (0.2﹪) or not , and then to ferment and incubate by YC350(Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus) 0.1﹪ at 42℃ until they reach pH4.5 or pH4.2, then to analyzed each group's physical properties of “viscosity, shear stress, gel strength, curd hardess, breaking intension, the quantity of syneresis, colored(L-value, a-value, b-value), sensory properties, and microstructure” and the chemical properties of “ free amino acid content, free tyrosine counts, and β-galactosidase”. In physical properties, it showed that, before adding agar, the viscosity, shear stress, gel strength, curd hardess, breaking intension, color in each group of reconstituted milk (P group or PS group) were higher than those of raw milk(M group or MS group)(P<0.05). However, after adding agar, the viscosity, shear stress, gel strength, curd hardess and breaking intension in each group increased(P<0.05). The sensory properties showed that, in total acceptance, the group of raw milk which was added 2.0% skim milk powder (MS group) was the best; the group of 12.0% reconstituted milk (P group)was the worst (P<0.05). Microstructures showed that casein complex of yogurt prepared from raw milk was more delicate and smooth than reconstituted milk, and its holes were smaller than that was produced by reconstituted milk. Further more, with the increasing of solid, the properties of each group would become rough and irregular. In chemical properties, free tyrosine and β-galactosidase activity would not affect much by whether adding agar or not. The total quantity of free tyrosine of PS group with 14.0% milk solid was higher than other groups. It was also found out that the yogurt prepared from raw milk (M group or MS group)could compose much posphoserine, and it was higher than reconstituted milk groups. Furthermore, ethionine produced from reconstituted milk group(P group and PS group)is more than raw milk group(M group and MS group)that resist the synthesize of methionine. Experiment 3: Changes in physical and chemical properties of yogurt prepared from raw milk and reconstituted milk during storage period After adding agar to produce pH 4.5 yogurt, samples were stored for 20days to determineits physical properties(viscosity, shear stress, colored( L-value, a-value, b-value), curd hardess, gel strength and breaking intension) and chemical properties(pH value, titrable acidity, lactic acid bacteria counts, β-galactosidase, and free amino acid content). The result showed that the pH value, lactic acid bacteria counts, β-galactosidase and L-value and a-value of color of raw milk group (M group or MS group) were higher than those of reconstituted milk group (P Group or PS group). Regarding to titrable acidity, viscosity, shear stress, curd hardess, gel strength , breaking intension, b-value of color, and free tyrosine content, the reconstituted milk group were higher than those of raw milk group (P<0.05). To summarize all above points, the results showed that even though the viscosity, shear stress, gel strength and breaking intension of yogurt prepared from raw milk substrate are lower than these produced from reconstituted milk, the quality is more delicate, without graininess, and filled with wonderful milk smell. Besides, its lactic acid bacteria counts, β-galactosidase activity, posphoserine , and essential amino acid contents are higher than those of reconstituted milk group(P group), but its bitter material- free tyrosine content is less than that of reconstituted milk group. Therefore, the quality of yogurt prepared from raw milk is better than that of reconstituted milk.
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