Abstract: Increasing amounts of different types of UHT milk are now available in the Egyptian market.
Chemical and physical properties of UHT milk (Ultra high temperature or ultra heat temperature may be affected during storage period which extended to six months. Data of storage condition which cause change in UHT milk help to set standards for the shelf life of the local UHT milk and help in suggesting some guidance for prolonging its shelf life which is of great concern to the consumer.
Also, it was interesting to determine the chemical, physical and microbiological quality of UHT milk manufactured under Egyptian conditions during storage under different storage temperatures.
Four types of UHT milk produced by Dairy Companies in Egypt (Dolce, Sabeh, Juhina and Siclam) were available in local market. Twenty eight package 1 L were obtained for each type, all from the same processing batch. These samples were divided in four groups. The first was kept at 45 ± 1°C, the second at 25 ± 1 °C, the third at 5 ± 1°C and the fourth was kept at -20 ± 1 °C. One package was monthly taken from each group for analysis over a period of 6 months.
The milk samples were analysed for acidity, pH value, lactose % fat % total solids, total nitrogen, non-casein nitrogen, non-protein nitrogen, casein nitrogen, total volatile fatty acids, total calcium, total magnesium, total phosphorus contents, total bacterial count proteolytic bacterial count lipolytic bacterial count.
From the results it can .be concluded that:
1. Effect of milk type:
The milk type had no significant effect on acidity, pH value, while it had significant effect on lactose, fat total solids, total nitrogen, non-casein nitrogen, non-protein nitrogen, casein nitrogen, total volatile fatty acids, total calcium, total magnesium, total phosphorus, viscosity.
2. Effect of storage temperature:
Storage temperature had significant effect on acidity, lactose, fat, total solids content while it had no significant effect on its content of pH value, total nitrogen, non-casein nitrogen, non-protein nitrogen, casein nitrogen, total volatile fatty acids, total calcium, total magnesium, total phosphorus and viscosity.
1- Sample stored at 45°C had slightly higher acidity % The differences in acidity % between samples stored at 25°C, 5°Cand -20°C were not significant.
2- There were significant decrease in samples stored at 45°C and
25°C only in lactose content with the advancement of storage period. While samples stored at 5°C and -20°C showed no significant change as storage period advanced.
3- Total solids showed significant decrease at 45, 25 and 5°C only after 4, 5 and six months respectively.
3. Effect of storage period:
Storage period had significant effect on the following values:
1- Acidity: It slightly increased as storage period advanced in samples stored at 45°C only.
2- pH vlaue : The decrease in pH value was significant after 6 months of storage at 45, 25°C.
3- Lactose %: Lactose content slightly decrease after four months of
storage at 45 and 5°C. The decrease being significant after 6 months only.
4- Fat %: It slightly decrease in samples stored at 45 and 25°C only.
5- Total solids: It slightly decrease in samples stored at 45 and 25°C and
5°C only.
5-Non-casein nitrogen: It slightly increased in samples stored at all storage
temperatures.
6-Not-protein nitrogen: It increased ,in samples at all storage
temperatures.
7-Casein nitrogen: It decrease in samples at all storage temperatures.
8- Total volatile fatty acids: It increased in samples at all storage temperatures.
In general, the quality of UHT milk stored under hot climatic conditions might not be acceptable after 6 months. Therefor storage in air conditions stored or reduction of Egyptian UHT milk storage period to four months is recommended.
PART II
The use of UHT milk in dairy products was manufacture was studied in
this study in order to utilize UHT milk at its expire time.
Also to facilate the production of dairy products in places where raw milk is not available.
Because UHT milk has bad rennetability and acid coagulation properties, it is impossible to use it for dairy products manufacture which it''s manufacture based on either enzymatic or acid coagulation unless improving these properties. That was carried out in part (II) section A and section (B), respectively. Also, the properties of resultant cheese and yoghurt were followed.
Part II (Section A):
The pilot experiment which carried out to determined the best addition for improving the rennetability properties of UHT milk revealed that acidified milk to pH 5.9 together with the addition of 0.02% CaCl2 was the best. On the other hand, pilot experiment showed that UHT milk did not coagulate by rennet for these we use pasteurized milk or raw milk in the manufacture of two controls cheese in the original experiment.
Results in the original experiment showed that:
I-Modified UHT milk had low rennet coagulation time which approach to that of raw milk.
2-Curd tension of modified UHT milk cheese is greatly improved comparing with that of pasteurized milk cheese (control). It had curd tension close to the raw milk cheese (control).
3- Modified UHT milk had greater curd synersis than pasteurized milk and raw milk cheese (controls).
4- There were no significant differences in ripening properties between modified UHT milk cheese and pasteurized milk one except in acidity and pH.
5- Modified UHT milk had higher yield of cheese than pasteurized milk (control).
6- Cheese made from modified UHT milk had higher total organoleptic scores than cheese made from pasteurized milk
(control). The superability of former than the latter was due to the higher body & texture and appearance scores.
Part II (Section B):
The pilot experiment in part II section 8 revealed that the use of 0.5% gelatin or 0.3% carrageenan, was the best stabilizer for improving yoghurt making properties from UHT milk.
Origin experiment:
Origin experiment appeared that:
1- The acidity rate development during incubation time was slightly higher in both stabilizer treatments compared with the control of UHT milk.
2- The total solids and fat / dry matter decrease in both treatment and control as storage period increased.
3- Total nitrogen / dry matter increased in both treatment and control as storage period increased.
4- Stabilizer had no effect on the chemical composition of yoghurt (T.S, fat /DM, TN /DM) when it fresh and along all storage period.
5- Stabilizer had no effect on the quality properties during storage (pH, acidity, total volatile fatty acids (TVFA), non-protein nitrogen (NPN), diacetyl and acetyl methyl carbinol (DA +AMC).
6- The curd tension greatly increased in yoghurt made with the addition of stabilizer, it followed by five times as compared with the curd tension of the control.
7 - The rate of syneresis decrease in yoghurt made with either gelatin or carrageenan comparing with the control.
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