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2014-R1 QC in chain - SCC & UHT

2018-8-14 12:20:16 Comments:0 Views:290 category:Project Introduction

Impacts of somatic cell counts and storage temperature on UHT Milk Proteolysis and the milk fat globule during Storage

1.1 About Principle Investigator



Main research area(s):

1.      The molecular mechanisms of milk source active substances’ function

Aiming at lactoferrin, milk fat globule membrane protein and other milk source active substances as the research objects, elucidated their interactions with the receptor and the start-up process of intracellular signal transduction and clarified their promotion of bone growth and the molecular mechanisms of neural development.

2.      Investigate the association between human milk composition and infant gut health

Breast milk and infant feces samples are collected and analyzed the composition of immune proteins and oligosaccharides in the breast milk by means of proteomics and glycomics. High-through put sequencing technology was used to study the structure and evolution characteristics of infants’ intestinal flora, then elucidate the relationship between breast milk composition and infant intestinal health.

 

1.2 Background

UHT treatment of milk is a heating process at very high temperatures for short holding times, which renders the milk commercially sterile and gives a product with along shelf life at ambient temperatures. UHT treatments can cause sufficient reduction of micro-organisms, while doing minimal teat damage to milk constituens, such as vitamins and whey proteins.

Although reached the standard of the commercial asepsis, UHT milk is not absolutely safe still. During the process, most bacteria are inactivated but heat-stable enzymes of native or bacterial origin can survive and cause serious defects during storage of the milk. The quality of raw milk, processing technology and storage conditions are some of the major limiting factors for the shelf life of UHT milk, which can affect the product quality during storage. Milk somatic cell count (SCC) and stage of lactation have been shown to affect the composition of raw milk. As far as we know, mastitis milk (milk with high SCC) subjected to UHT treatment is more susceptible to some quality problems than normal milk. Besides, the storage condition is also a key factor for keeping the good quality. Many reactions in UHT milk such as lipid oxidation may be affected by the temperature during storage.

Proteolysis in UHT milk can cause the development of bitter flavor and leads to an increase in viscosity, with eventual formation of a gel during storage, which is a major factor limiting its shelf-life and market potential (Datta & Deeth, 2003).

In this study, the influence of raw milk quality of somatic cell counts(SCC) and UHT storage temperature on the proteolysis and fat hydrolysis degree of UHT milk were investigated during storage at 20,30,40temperature.

 

1.3. Milk samples and UHT processing

Bulk milk samples with different somatic cell count were obtained from a local dairy enterprise. Grading samples according to SCC, all milk was processed by indirect heating at 147 °C for 4s in UHT plant, and packaged in sterile 225mL tetra container. All experimental batches were prepared on the same day and sampled for analysis after 0, 30, 60, 90, 120, 150 and 180d storage at 20,30,40°C,respectively.



1.4 Results and conclusions
1.4.1 Enzyme activity
1.4.1.1 Plasmin and plasminogen activity

The changes in plasmin and plasminogen activity of different SCC UHT milk during storage period in shown in Fig.1-1. Milk plasmin and plasminogen activity of both SCC level milk was constant between 0.0 and 0.0700U/mL during the first 6 mouths of storage. We can speculate that raw milk after UHT sterilization, milk PL has basically complete deactivation. 

1.4.1.2 Lipase activity

There’s no pronounced change of lipase activity was found during storage period for 6 months. Besides, no pronounced rule was found with the raw milk SCC and the storage temperature.


1.4.2 Viscosity

The changes in viscosity of the six UHT milk types during storage are represented in Table 1-3. All samples initially underwent a slight decrease in viscosity, followed by a long delay, during which little change occurred. Milk of both SCC level milk did not change significantly during the applied storage period which was constant between 2.0 and 2.4 mPa s., thus there is no significant different between different somatic cell count and storage temperature. 



An observed increase in viscosity with the onset of gelation has been reported previously by Kohlmann et al. (1991) and Kelly and Foley (1997) who proved that Gel aging phenomenon is associated with PL activity in milk. The results of this experiment also confirmed this view, may be due to UHT milk PL substantially completely inactivated, all UHT milk during storage have not gelled aging phenomenon.


1.4.3 pH and Titratable acidity (°T)

pH values of all UHT milk have always been decreasing in the 6 months’ storage period. Storage temperature has caused significant influence (P>0.05), which means the higher storage temperature could result in the lower pH value. Raw milk SCC made obvious difference. In the chart we can see that the LSCC group’ pH is lower than HSCC ones in the same month.



 

Titratable acidity of all UHT milk have always been increasing as the month went on. Storage temperature has caused significant influence (P>0.05), showing that UHT milk storaged at higher temperature had higher titratable acidity. However, the raw milk SCC made no obvious difference to the titratable acidity (P>0.05).




1.4.4 Heat stability
Heat stability (Heat coagulation time, HCT) is described by the start coagulation time milk at 140 °C. The development in Heat stability of the six UHT milk types during storage is shown in Fig. 1-2. Heat stability of all milk samples decreased during storage. The decrease was more significant in the milk store in 40 °C, then 30 °C, and HSCC milk shown more pronounced decrease than LSCC milk.

1.4.5 Nitrogen distribution of milk
Changes in the degree of hydrolysis true protein and casein of different somatic scores in different storage temperature during the applied storage peiod is shown in Table 1-6 and 1-7.

NPN/TN increase in general, the change is most obvious under 40 ° C. Initial values of HSCC was slightly lower than those of LSCC, but from storage for two months, under different storage temperature, NPN/TN are higher than that of LSCC, suggesting HSCC samples protein hydrolyzed more seriously.

The development in NCN/TN of all milk sample (Table 1-7) showed a rapid increase between 0 and 3 months’ storage and more slightly increase between 3 and 6 mouths storage. The greatest increase in NCN/TN was observed for the HSCC sample which store in 40°C.The increase was more significant in the milk store in 40 °C, then 30 °C, and HSCC milk shown more pronounced increase than LSCC milk.
There was no significance difference in NCN/TN between two SCC group under all storage temperature, indicating SCC has no impact on casein hydrolyzation for the moment.

1.4.6 RP-HPLC results of milk
In Fig.1-3 (a) a comparison of the protein profiles of LSCC under different storage temperature after 2 months. Higher storage temperature leads to greater level of casein hydrolyzation
A comparison of different SCC group under 30°C after 2 month is shown in Fig.1-3(b),At the same storage temperature, high somatic cell group degree of hydrolysis of casein is more serious than the low somatic cell group.

The caseins that have been hydrolyzed are mainly β and κ-casein,αS1, αS2-casein also hydrolyzed slightly under 40 °C,LSCC showed lower level of β and κ-casein hydrolyzation than HSCC,SCC has a significant effect on β and κ-casein hydrolyzation.
After 6 months’ storage, the b-casein peak decreased the most, followed by a slight decrease in aS1-casein and b-casein A2. The peak of b-casein A1 disappeared almost completely after 4 mouths storage, along with a large decrease in the aS1-caseinpeak. After 5 mouths storage, no intact aS1-casein with 9 phosphorylations(P) and b-casein A1 was present in the UV chromatogram (data not shown). Over the storage period, the k-casein peaks seemed to disappear, but developed similarly in both milk types, ruling out proteolysis as a cause for this change in k-casein profile. Instead, the disappearance can be attributed to a broadening of the peaks due to modifications of the proteins, likely by the Maillard reactions induced by the heat treatment (Gaucher, Mollé, Gagnaire, & Gaucheron, 2008).

1.4.7 Color difference and Diameter
Color difference and diameter results are shown in Fig.1-5.
The lightness of all UHT milk have been rise first and then fall along with the storage period. The value reached peak in month 4, and then decreased slowly.
Storage temperature has caused significant influence (P>0.05), which means the higher storage temperature could result in the lower L value. According to the result, those UHT milk storaged under 40°C had the darkest color. However, the raw milk SCC made no obvious difference to the lightness of UHT milk (P>0.05).
The color difference value in green/red axis of all UHT milk have always been increasing during the 6 month, meaning all groups of milk seemed more red from month to month. The raw milk SCC made no obvious difference to the lightness of UHT milk (P>0.05).
Storage temperature has caused significant influence (P>0.05), which means the higher storage temperature could result in the higher a value.
Storage temperature has caused significant influence on the color in blue/yellow axis (P>0.05). The storage condition of 40°Cmay cause the color changed to yellow significantly. This value of UHT milk under 30°Crises slowly, whereas those under 20°C almost kept at a constant level. Overall, the higher storage temperature could result in the higher b-value.
Nevertheless, raw milk SCC made no obvious difference to the lightness of UHT milk (P>0.05).
Fat globule size of UHT milk shad obvious increased from the second month of storage period (P<0.05) and no significant change appeared from then on. As to the difference caused by storage temperature, no pronounced rule was found corresponding to it. Consequently, the UHT milk fat globule diameter didn’t increased dramatically as the temperature raised to 40°C. The particle size of LSCC group is larger than HSCC ones in the same month (P<0.05).

The Nile red stained fat appears red and the fast green FCF stained protein appears green in these images. AFG means the aggregates of fat globules and the scale bars are 10μm in length. The CLSM images show that the majority of fat globule in the UHT milk before storage are separate and most of these MFG appear to contact the protein tightly. While after 10 months of storage, the fat globules have aggregate and the reduction of protein could also be observed.

1.4.8 ζ-potential
Apparent zeta-potential of all UHT milk fat globule have been rise first and then fall along with the storage period. The value reached peak in month 3, and dropped to its lowest level. 
Neither the raw milk SCC nor the storage temperature caused significant influence (P<0.05).



1.4.9 Free fatty acid
There’s no pronounced change of the FFA content was found during storage period for 6 month. Besides, no pronounced rule was found with the raw milk SCC and the storage temperature.

1.4.10 Conclusion
Under different storage temperature, 20,30,40°C, samples were stored for 180 days and tested every 30 days for proteolysis and fat hydrolysis indexes. Our results show that milk plasmin and plasminogen activity of both SCC level milk has basically complete deactivation. All samples underwent a slight decrease in viscosity, both temperature and body cells caused no significant difference in the change. Lipase activity trend similar to this, no significant changes was found during the entire storage. From proteolytic conditions and casein RP-HPLC results, we found that during storage, protein and casein hydrolysis are serious, predominate the hydrolysis of casein areβ- casein, κ- casein, indicate that involved in protein hydrolysis enzymes include plasmin and enzymes produced by psychrophile. In addition, each group UHT milk have been increased acidity, increased fat globule diameter, darken color and fat floating phenomenon, these problems may be in close relationship with proteolysis and Maillard reaction. On the contrary, the impact of SCC of raw milk on the indicators are not significant, no pronounced change of lipase and plasmin activity and free fatty acid (FFA) was found. Overall, the low storage temperature for maintaining the quality of UHT milk is more beneficial, whereas raw milk SCC had relatively less influence. This conclusion may be a reference from aspects of raw milk SCC and storage condition to UHT milk quality control.


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