MATERIALS AND METHODS

Materials: Fresh buffaloes milk (contain 6% fat and 17% TS) was obtained from the herd of the Faculty of Agriculture, Cairo University, Giza Egypt.

Pure cultures (S. salivaricus subsp. thermophilus and L. delbruechii subsp. bulgaricus) were obtained from Chr. Hansen’s Laboratories, Corenhangen, Denmark. Cultures were propagated in sterile skim milk at 37°C. Olive oil was extracted by pressure system from Koroneiki olive fruit variety by using 2% NaCl during malaxation step as mentioned by Hassan et al. (2015). Its properties were as follow in Table 1.

Yoghurt cheese preparation: Samples were prepared by a procedure normally used in the Middle East (El-Sayed et al., 1993). Milk was heated at 85°C for 15 min, then cooled to 40°C, inoculated with 2% starter culture (S. thermophilus+L. bulgaricus, 1:1). Milk was divided into three portions; the first was served as control while the two other portions were fortified by 1 and 2% extracted olive oil (w/v), respectively. The fermented milks were incubated for 12 h at 40°C for complete coagulation. Resultant coagula were held in refrigerator (4-5°C) for 3 h, then hanging in the refrigerator overnight in cloth bags to drain the whey. The resulting curds were mixed with 1.0% salt and stored for 21 days in refrigerator. Samples were analyzed fresh and during cold storage (7, 14 and 21 days) for chemical, microbiological and organoleptic properties.

Table 1:	Physical and chemical properties of extracted olive oil

Analytical methods
Sensory evaluation: Sensory evaluation was carried out for all treatments, using a score scheme (10, 40 and 50 points) for appearance, body and texture and flavor, respectively by a panel of 20 trained Jude’s of stuff members at Dairy Laboratory NRC, according to score card suggested by Keating and White (1990).

Chemical analysis: Total solids, total protein and soluble nitrogen were determined according to AOAC (2007). The pH values were measured using a digital laboratory PH meter (HI 94 1400, Hanna instruments). Total Volatile Fatty Acids (TVFAs) values were determined according to Kosikowski (1986) and expressed as milliliter of 0.1 N NaOH/100 g. Acetaldehyde content (mmol/100 g) was estimated according to Lee and Jago (1969) and diacetyl content (mmol/100 g) was determined as reported by Pack et al. (1964).

Microbiological examination: Total viable count was determined by using plate count agar medium as described by Klose (1968), coliform detected by Violet Red Bile Agar medium (VRBA) using the method described by the APHA (1978). Moulds and yeasts were also detected by acidified potato dextrose agar according to Harrigan and McCance (1966). Lactic acid bacteria count was carried out on MRS medium for Lactobacillus bulgaricus and M 17 for Streptococcus thermophilus as described by Gruev (1982) and Krusch et al. (1987).

Statistical analysis: Statistical analysis was performed using the GLM procedure with SAS (2004) software. Analysis of variance (ANOVA) and Duncan’s multiple comparison procedure were used to compare the means. A probability of p<0.05 was used to establish statistical significance.

RESULTS AND DISCUSSION

Sensory properties: From Table 2, it could be noticed that cheese yoghurt samples contained olive oil (T₁ and T₂) were more acceptable than control and they gained a highest total score. It is also clear that (labneh contained 1% olive oil, T₁) had gained the highest total scores followed by T₂ and control. Gradually decreases in scores were observed with the progress of storage to reach the minimum scores at the end of storage. Slight differences were, also observed in the appearance of control and treatments.

Table 2:	Organoleptic properties of yoghurt cheese samples when fresh and during storage at 5±1°C for 21 days
C: Control, T 1: Sample with 1% olive oil, T 2: Sample with 2% olive oil, Means follower by different letters are significantly different

T ₁ had gained the highest score for flavor and their values were gradually decreased until the three weeks. These results are in agreement to El-Sayed et al. (1993), Salem et al. (2007), Zaky et al. (2013) and Thabet et al. (2014). On the other side, T ₂ sample which prepared with (2% olive oil) had a highest score for body and texture than control and other treatments. Control and other treatments behaved the same trend.

Chemical parameters: Figure 1-2 showed the chemical composition of fresh samples and that stored at 5±1°C for 3 weeks. It could be notice that control had the highest pH values followed by T ₁, T ₂. Generally, the pH values (Fig. 1a) of control as well as T ₁ and T ₂ samples were decreased during cold storage. These results were in accordance with Ersoz et al. (2011) and Zaky et al. (2013). For total solids contents (TS%), it is clear that control had the highest TS followed by T ₁, T ₂, (Fig. 1b). The TS contents increased in all treatments as the cold storage increased until 3 weeks. These results were in agreement also to Mahdi et al. (1990) and El-Sayed et al. (1993). On the other side, control had the highest content of TP% followed by T ₁, T ₂ and gradually increased (either control or treatments) during storage until 3 weeks and T ₂ had lowest TP%. These results were in agreement with Al-Otaibi and El-Demerdash (2008) and Thabet et al. (2014).

Soluble nitrogen content (%) was the highest at (T ₂) followed by (T ₁) and control (Fig. 1d). They contents were gradually increased during cold storage at 5±1°C until 3 weeks. These results were in agreement to Ismail et al. (2006).

From Fig. 2a, the Total Volatile Fatty Acids (TVFAs) contents were affected by addition of olive oil. T2 had the highest content of TVFAs where control had the lowest content of TVFAs.

Fig. 1(a-d):	(a) pH values, (b) Total solids, (c) Total protein and (d) Soluble nitrogen of cheese yoghurt samples as affected by adding olive oil during 21 days of storage at 5±1°C, C: Control, T 1: Sample with 1% olive oil, T 2: Sample with 2% olive oil

Fig. 2(a-c):	(a) Total volatile fatty acids, (b) Acetaldehyde content and (c) Diacetyl content of cheese yoghurt samples as affected by adding olive oil during 21 days of storage at 5±1°C, C: Control, T 1: Sample with 1% olive oil, T 2: Sample with 2% olive oil

Increasing the concentration of oil led to increase of TVFAs. The TVFAs had gradually increased during cold storage until (3 weeks). These results were in agreement to Zaky et al. (2013) and Al-Otaibi and El-Demerdash (2008). The increases in TVFAs could be explain on the basis that the increase in the cheese proteolysis as amino acid can serve as precursors for the development of certain volatile fatty acid (Salem et al., 2007) and the TVFAs of olive oil itself. Similar results were also reported for Edam cheese by Kebary et al. (2002) and Abd El-Salam et al. (2011) for probiotic labneh.

Acetaldehyde and diacetyl contents: Figure 2b-c indicated the acetaldehyde and diacetyl contents (mmol/100 g) of labneh fresh and during cold storage. It could be notice that level of acetyldehyde was higher than diacetyl either fresh or during cold storage. T ₂ samples had a higher content of both acetaldehyde and diacetyl than control and T ₁ samples. It was obvious that both acetaldehyde and diacetyl contents were increased to reach maximum values till 14 days of cold storage then deceased until the end of experiment (21 days) in all treatments. These results are in agreement to Salem et al. (2007) and Zaky et al. (2013).

Microbiological examination: Figure 3-5 reflected the microbiological changes of yoghurt cheese (labneh) through 3 weeks of storage. Total viable count were gradually increased during storage up to 7th day, then they decreased though the remaining period of storage in control and other treatments (Fig. 3). These results were in agreement to Al-Otaibi and El-Demerdash (2008), who found that total viable counts were gradually increased up to 7th day of storage then decreased thereafter.

Fig. 3:	Total viable count of cheese yoghurt sample as affected by addition of olive oil during 21 days of storage at 5±1°C, C: Control, T 1: Sample with 1% olive oil, T 2: Sample with 2% olive oil

Fig. 4:	Starter culture ( S. thermophilus) count in cheese yoghurt sample as affected by of addition olive oil during cold storage, C: Control, T 1: Sample with 1% olive oil, T 2: Sample with 2% olive oil

Fig. 5:	Starter culture ( L. bulgaricus) count in cheese yoghurt sample as affected by addition olive oil during cold storage, C: Control, T 1: Sample with 1% olive oil, T 2 sample with 2% olive oil

Figure 4 and 5 indicated that the counts for S. thermophilus and Lactobacillus delbrueckii spp. bulgaricus (in control and treatments) were increased gradually up to 7th days of storage and then decreased thereafter. The control had the highest content of streptococcus and lactobacillus at 7th day of cold storage. From the same figures, it could be notice that increase in the oil concentration (T ₂) lead to decrease in lactic acid bacterial count. Abou Ayana and Gamal El Deen (2011), Khaleel (2000) and Sharaf et al. (1996) reported that addition of some aromatic and essential oils to yoghurt and labneh had stimulatory effect on lactic acid bacteria by enhancing their growth and acid production. Coliform group was not detected in control or treatments. Yeast and moulds were also not detected in labneh containing olive oil throughout storage; whereas control had yeast and moulds at the end of storage period (21st day). These results were in accordance to El-Sayed et al. (1993) and Thabet et al. (2014).

CONCLUSION

Olive oil which extracted in the laboratory by pressure system from Koroneiki olive fruit variety by using 2% NaCl during malaxation step was successes in manufacture of cheese yoghurt by using 1.0 and 2.0% olive oil and improved body and texture and gave good appearance and flavor than control. On the other hand it improved the quality. So, it is concluded that 1.0% olive oil is the best treatment for preparation of yoghurt cheese.