Teagasc Food Research Centre, Moorepark, Fermoy, Co. Cork, Ireland P61C966; School of Food and Nutritional Sciences, University College Cork, Cork, Ireland T12YN60; Western Dairy Center, Department of Nutrition, Dietetics, and Food Sciences, Utah State University, Logan 84321.
School of Food and Nutritional Sciences, University College Cork, Cork, Ireland T12YN60.
J Dairy Sci. 2019 Jan;102(1):177-189. doi: 10.3168/jds.2018-15039. Epub 2018 Nov 15.
This study characterized the coagulation properties and defined the cutting window (CW; time between storage modulus values of 35 and 70 Pa) using rheometry for milk standardized to 4, 5, or 6% protein and set at 28, 32, or 36°C. Milks were standardized to a protein-to-fat ratio of approximately 1 by blending ultrafiltration retentate, skim milk, and whole milk. The internal curd microstructure for selected curd samples was analyzed with transmission electron microscopy and scanning electron microscopy. Lowering the coagulation temperature caused longer rennet coagulation time and time to reach storage modulus of 35 Pa, translating into a wider CW. It also led to a lower maximum curd-firming rate (MCFR) with lower firmness at 40 min at a given protein level. Increasing protein levels resulted in the opposite effect, although without an effect on rennet coagulation time at a given temperature. On coagulation at 28°C, milk with 5% protein resulted in a similar MCFR (∼4 Pa/min) and CW (∼8.25 min) compared with milk with 4% protein at 32°C, which reflects more standard conditions, whereas increasing milk to 6% protein resulted in more than doubling of the curd-firming rate (MCFR = 9.20 Pa/min) and a shorter CW (4.60 min). Gels set at 28°C had lower levels of rearrangement of protein network after 40 min compared with those set at 36°C. Protein levels, on the other hand, had no influence on the levels of protein network rearrangement, as indicated by loss tangent values. The internal structure of curd particles, as investigated by both scanning electron microscopy and transmission electron microscopy, appeared to have less cross-linking and smaller casein aggregates when coagulated at 28°C compared with 36°C, whereas varying protein levels did not show a marked effect on aggregate formation. Overall, this study showed a marked interactive effect between coagulation temperature and protein standardization of milk on coagulation properties, which subsequently requires adjustment of the CW during cheesemaking. Lowering of the coagulation temperature greatly altered the curd microstructure, with a tendency for less syneresis during cutting. Further research is required to quantify the changes in syneresis and in fat and protein losses to whey due to changes in the microstructure of curd particles arising from the different coagulation conditions applied to the protein-fortified milk.
本研究使用流变仪对标准化至 4%、5%或 6%蛋白质且设定在 28、32 或 36°C 的牛奶进行了凝固特性表征，并定义了切割窗口（CW；储能模量值为 35 和 70 Pa 之间的时间）。通过将超滤截留物、脱脂牛奶和全脂牛奶混合，将牛奶标准化为蛋白质与脂肪的比例约为 1。选择凝块样品的内部微观结构用透射电子显微镜和扫描电子显微镜进行了分析。降低凝固温度会导致更长的凝乳酶凝固时间和达到储能模量 35 Pa 的时间，从而导致更宽的 CW。这也导致在给定蛋白质水平下，最大凝乳强度（MCFR）较低，在 40 分钟时硬度较低。增加蛋白质水平会产生相反的效果，尽管在给定温度下对凝乳酶凝固时间没有影响。在 28°C 下凝固时，与在更标准条件下的 32°C 下 4%蛋白质的牛奶相比，5%蛋白质的牛奶产生相似的 MCFR（约 4 Pa/min）和 CW（约 8.25 min），而将牛奶增加到 6%蛋白质会使凝乳速度增加一倍以上（MCFR=9.20 Pa/min），CW 缩短（4.60 min）。与在 36°C 下凝固的凝胶相比，在 40 分钟后，在 28°C 下凝固的凝胶中蛋白质网络的重排水平较低。另一方面，蛋白质水平对蛋白质网络重排水平没有影响，如损耗角值所示。通过扫描电子显微镜和透射电子显微镜研究凝块颗粒的内部结构，与在 36°C 下凝固相比，在 28°C 下凝固时，凝块颗粒的交联程度较低，酪蛋白聚集较小，而不同的蛋白质水平对聚集形成没有明显影响。总的来说，这项研究表明，牛奶凝固温度和蛋白质标准化之间存在显著的相互作用，这会影响到奶酪制作过程中的 CW。降低凝固温度会极大地改变凝乳微观结构，在切割时凝乳收缩的趋势较小。需要进一步研究来量化由于应用于蛋白质强化牛奶的不同凝固条件而导致凝乳颗粒微观结构变化引起的凝乳收缩和脂肪及蛋白质向乳清的损失变化。
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