Fatty Acid Composition and Fat Stability of Raw Milk and Pasteurized Milk from Laoshan Goats

In this study, the fatty acid profile and fat stability for seven consecutive days of raw milk and pasteurized milk from Laoshan goats have been evaluated by gas chromatography-mass spectrometry (GC-MS) after fatty acid methyl ester. The results showed that the concentrations of short chain fatty acids (SCFA) and saturated fatty acids (SFA) significantly increased by 47.36% and 11.68% after pasteurization respectively, while the concentrations of unsaturated fatty acids (UFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) decreased by 26.08%, 26.45% and 22.15% respectively. The C10:0 (5.39%-8.57%), C12:0 (3.13%-5.28%), C14:0 (8.12%-11.87%), C16:0 (25.59%-28.53%), C18:0 (14.60-13.69%) and C18:1 (33.9124.92%) are the most predominant fatty acids of Laoshan goat milk with significant differences. Moreover, the fat stabillity for seven consecutive days of raw milk and pasteurized milk was detected by sedimentation rate (R). The fat stability in pasteurized milk was more stable than that in raw milk, the sedimentation rate of raw milk and pasteurized milk consisted in a progressive decrease in the seven days by 82.99% and 79.77% respectively. What’s more, significant difference was observed from 1st day to 4th day between raw milk and pasteurized milk, however, there was no significance from 5th to 7th. This is the first report to fully characterize the fatty acid contents and fat stability of Laoshan goat raw milk and its pasteurized milk and it provided a certain theoretical basis for the research and development of goat milk functional product.


Introduction
Caprine agriculture is an important part of the national economy in many countries especially in the Mediterranean and the Middle East (García et al., 2014), but few people drink goat milk in China because it has a unique flavor that people find intolerable.However, many studies have shown that goat milk has many nutritional benefits that surpass those of bovine milk; moreover, goat milk is a great source of easily digested nutrition.For example, there are more than 200 nutrients and biologically active substances in goat's milk with lower allergenicity (Durand et al., 2003).Furthermore, the epithelial growth factor (EGF), superoxide dismutase (SOD) and Vitamin E in goat's milk prevent skin aging and make skin smoother.Goat milk is suitable for lactose intolerant people because there is less lactose in goat milk than bovine milk (Frances, 2001).
Fat is one of the most important components in milk.The fat in goat milk includes more unsaturated fatty acids than cow's milk, which can reduce the risk of obesity and is closely related to human health.For instance, omega-3 and omega-6 fatty acids are known to benefit for cardiovascular disease (Simopoulos, 2008), and conjugated linoleic acid (CLA) has anti-carcinogenesis and anti-atherosclerosis activity while strengthening immunity (Benjamin & Spener, 2009).Other studies have shown that the flavor of goat milk is primarily influenced by short-chain fatty acids especially C8:0 and C10:0.Sumarmono et al. investigated the fatty acids profiles of fresh milk, yogurt and concentrated yogurt, the results showed that oleic acid (C18:1) was the most common fatty acid (Sumarmono et al., 2015).Not only pasteurization can kill all the pathogenic bacteria, but also maintain the main nutrition and natural flavor of milk.The research revealed that pasteurization increased the content of trans fatty acids significantly (Herzallah et al., 2005).The study also found that with the treatment of high temperature short time (HTST) pasteurization (75 °C, 16 s) and ultra-high temperature (UHT) sterilization (140 °C, 4 s), the content of conjugated Linoleic Acid (CLA) decreased compared with raw cow milk, the reason may be that the hydrogen peroxide causes the decomposition of CLA and the temperature promotes the oxidation process (Costa et al., 2011).What's more, many factors can modulate the fatty acid composition including animal feed (Morales-Almaráz et al., 2011;Zervas & Tsiplakou, 2011).
Dairy products often occur the phenomenons of fat floatin and aggregation during the process of production and storage.To restrain the bad phenomenons and improve the stability of milk fat, we often do by adding emulsifiers in the actual process.It is considered that the adsorption rate of the emulsifier must be large enough to stabilize the emulsion as soon as possible in the emulsification process (Jafari et al., 2008).Emulsion stability of milk from Ettawah Crossedbred goats changed after 30d of storage during frozen storage，the fat globule in goat milk only began to clustering, whereas the fat globule in cow milk already occurred coalescence (Nurliyani et al., 2015).The following order was derived for the stability of whey protein in milk: raw whole > HTST, homogenized, homogenized and pasteurized > skimmed and pasteurized, and skimmed UHT > homogenized UHT (Qi et al., 2015).However, little work has been done to quantitate the fatty acids and fat stability of Chinese goat milk.The aim of this study was to evaluate the fatty acids compositions of both raw and pasteurized goat milk by gas chromatograph-mass spectrometry (GC-MS) and to analyze the stability changes via a 7-day sedimentation assay from Laoshan goat milk.To the best of our knowledge, this is the first report to quantitate the fatty acids and sedimentation rate of milk from Laoshan goat.Our findings have important implication for the dairy industry and provides a theoretical basis for application of pasteurization in goat milk industry.

Materials
In the current study, 50 Laoshan goat milk samples were collected at 10:00 AM from the Tai'an Three Hi Goat Farm in Shandong province.These 50 Laoshan goats, in the age of 38-41 months, were healthy, no acute mastitis visible clinical disease, and were at the peak lactation.All goats were well organized by utilizing the free stall bar raising technique, feeding the TMR silage diet, milking in central milking hall, clearing the fecal by labor.Whole milk samples were transported to the lab on ice and stored at -20 °C.
The GC-MS (QP2010) was purchased from Shimadzu.The digital thermostatic water bath (DK-98-IIA) was purchased from Jintan Jin Nan Instrument Factory.An electronic balance (AL104) was purchased from Mettler Toledo Instruments (Shanghai) Co., Ltd.A refrigerated centrifuge (TGL-18m) was from Jinan Longrui Trade Co., Ltd.An ultrasonic cleaner was purchased from Shanghai Kedao Ultrasonic Instrument Co., Ltd.A room temperature centrifuge (TG16-ws) was purchased from Changsha Xiangyi Centrifuge Instrument Co., Ltd.a visible spectrophotometer (V-1100D) was purchased from Shanghai Mapada Instruments Co., Ltd.
Hexane was purchased from Tianjin Guangfu Fine Chemical Research Institute.Potassium hydroxide was purchased from Tianjin kemi'ou Chemical Reagent Co., Ltd.Methanol was purchased from Shandong Yuwang Industrial Co., Ltd.

Fatty Acid Compositions of Laoshan Goat Milk
No spoiled goat's milk was selected, followed by melting at room temperature.Then Milk fat was centrifuged in a refrigerated centrifuge at 4 °C and 8000 r/min for 10 min-the fat accumulated in the upper layer (Palmquist & Jenkins, 2003).We used an alkali-catalyzed method to perform methyl esterification of the fatty acids in goat's milk (Talpur et al., 2009).Firstly, 0.2 g of milk fat was added into a 100 mL beaker, followed by the addition of 5 mL of hexane with 3 min of ultrasonic treatment to dissolve fat.Then 5 mL of methanol and 5 mL of methanol-potassium hydroxide solution (0.5 mol/L) were added, the mixture was placed in a blender shock with water bath at 55 °C for 25 min.Next, saturated brine was added to remove more impurities after heating.After the reaction, 300 µL of the supernatant was obtained, diluted by hexane (4 mL) and filtered with a 0.22 μm organic membrane.
The C18:1 is the main indicator of fatty acids in milk.It is a neutral fatty acid that has no effect on the concentration of serum cholesterol.The content of C18:1 in Laoshan goat raw milk accounted for 33.91%, which is much higher than that in pasteurized milk (24.92%)-the concentrations were very significant (P < 0.01).Furthermore, the concentration of C18:1 was higher than palmitic acid (33.91% versus 25.59%) in raw milk but lower in pasteurized milk (24.92% versus 28.53%).This implies that unpasteurized milk has a lower risk of causing cardiovascular disease than pasteurized milk.
The C18:2, C18:3 and C20:4 are essential fatty acid that decrease the concentration of serum cholesterol-they cannot be synthetized in the human body.They also have a major influence on childhood development.The content of C18:2 and C18:3 were 1.87% and 1.15% in raw milk, but 1.42% and 1.11% in pasteurized milk.No significant differences were found in C18:2 and C18:3 between the two milk types.The concentration of C20:4 was 0.23%, however, it was not detected in pasteurized milk.

Stability of Milk Fat in Raw and Pasteurized Milk
Distribution of fat globule in goat's milk appeared homogen, the size of fat globule in goat's milk showed uniform and smaller than the bovine milk fat globule (Nurliyani et al., 2015).Smaller fat globules are usually better dispersed and provide a more homogeneous mixture of fat in milk (Attaie & Richter, 2000).Temperature also determines the physical state of oil droplets in O/W emulsions, which influences the emulsion properties such as viscosity and stability (Nurliyani et al., 2015).

Table 1 .
The chemical components and their relative fatty acid contents in raw goat's milk and pasteurized goat's milk Table 1 and t's milk, but 10 ds in pasteuriz ience acids in pasteur red at different t's milk and p lk and 19 fatty nd 22.18:8.02:olleagues (Ma SFA:MUFA:PU k and its produ

Table 2 .
Changes in the sedimentation rate of milk fat in raw goat's milk and pasteurized goat's milk for 7 days