This study systematically investigated the effects of seasonal variation, parity, lactation stage, and production region on the plasmin (PL) and plasminogen (PG) activity in bovine raw milk. Raw milk samples were collected from an intensive dairy farm in Hohhot, Inner Mongolia to evaluate the impact of different seasons (summer and winter), lactation stages (initial, early and late) and parities (parities 1-4) on the plasmin activity and the transformation efficiency of PG. To examine the production region effect, another milk samples were collected from Ma’anshan, Anhui; Yinchuan, Ningxia; Harbin, Heilongjiang; and Changji, Xinjiang. Our results showed that season had no significant effect on the PL activity (P > 0.05) but significantly affected PG activation levels (P < 0.01). The PG activity of raw milk was 34.4% in summer than in winter, ((5.98 ± 0.25) versus (4.45 ± 0.43) U/L). Parity had no significant effect the activity of PL or PG (P > 0.05). Lactation stage had a significant impact on PG activation (P < 0.01); PG activity was 8.6% and 6.8% higher in the late stage than the initial and early stages, (5.67 ± 0.70) versus (5.22 ± 0.70) and (5.31 ± 0.67) U/L. Likewise, the geographical origin significantly impacted the conversion efficiency of PG (P < 0.05); PG activity was significantly higher in Harbin samples than in Yinchuan samples, (5.62 ± 0.88) versus (5.12 ± 1.19) U/L. Somatic cell count showed a weak positive correlation with both PL activity (r = 0.33) and PG activation (r = 0.21), suggesting that subclinical mastitis had a potential impact on them. This study revealed that both season and lactation stage affected milk stability by affecting the PG activation pathway, and the PL and PG activity in milk from high-latitude regions might be enhanced via the environment-physiology interaction. These findings provide a novel perspective for establishing a quality evaluation system for raw milk based on PL/PG regulation.

Plasmin is an important factor affecting the shelf-life quality of milk products, and its hydrolytic activity is regulated by many factors, such as activators, inhibitors, and activator inhibitors. Compared with protease secreted by psychrophilic bacteria, plasmin presents different thermal inactivation parameters and hydrolysis properties; the milk gelation properties induced by the two enzymes, the gelation mechanisms and the methods used to detect the milk hydrolysates are different. Plasmin activity in raw milk is affected by dairy cow breeds, lactation stage, parity, and the number of somatic cells, as well as protease secreted by bacteria. As whey protein, casein and Maillard reaction can affect plasmin structure and substrate recognition by it, heating, membrane filtration, and high pressure processing can affect plasmin activity. In this paper, we summarize the composition and regulatory mechanism of the plasmin system in milk, the mechanism of plasmin-induced age gelation of milk, the factors affecting plasmin activity and the methods used to assay plasmin activity, with the aim to provide a reference for further research on milk plasmin.

In this study, the effects of adding mixtures of mono- and di-fatty acid glycerides and Tween 80 with different hydrophilic-lipophilic balance (HLB) values on the stability and whipping characteristics of cream were studied, and the hardness, whipping rate, apparent viscosity, stability, and microstructure of the cream samples were detected. The results showed that with the increase in HLB value, the apparent viscosity of the emulsion decreased, and shear thinning occurred in all cream samples. The centrifugal creaming rate and turbiscan stability index (TSI) decreased first and then increased, and so did the absolute value of zeta potential. The results of whipping characteristics analysis showed that with the increase in HLB value, the whipping time and whipping rate of cream decreased, and the hardness after whipping increased first and then decreased, reaching a maximum of 1150.1 g when the HLB value was 9. At HLB values in the range of 9-11, cream exhibited a shorter whipping time, higher whipping hardness, appropriate whipping rate, and good mounting performance with no collapse or deformation. Meanwhile, the microstructure showed that the size of the cream bubbles was relatively uniform after whipping, and the fat globules around the bubbles were dense and close to each other, thus forming a better network structure to protect the bubbles and make the foam more stable. Therefore, the HLB value of mixed emulsifiers should be controlled in the range of 9-11 to obtain cream with good stability and whipping characteristics.