Effect of volume reduction ratio on the concentration and retention factors during ultrafiltration of goat ’ s milk

This experimental work aimed to study the effect of volume reduction ratio on the concentration and retention (selectivity) factors during ultrafiltration of goat’s milk with UF25-PAN polyacrylnitrile membrane. They were calculated on the basis of dry matter, total protein, fat and ash content in retentates and permeate at volume reduction ratios of 2 and 3. The results showed that the increase in the volume reduction ratio from 2 to 3 led to an increase in the concentration factors of dry matter from 1.34 ± 0.05 to 1.87 ± 0.03, total proteins – from 1.70 ± 0.02 to 2.72 ± 0.06, fat – from 1.71 ± 0.03 to 2.71 ± 0.03, ash – from 1.13 ± 0.02 to 1.52 ± 0.02 (p  0.05). At these conditions, the retention factor (selectivity) of ash increased from 37.40 ± 2.19% to 53.50 ± 2.21% (p  0.05). There was no significant difference (p  0.05) of the selectivity of the membrane according to the proteins when volume reduction ratio increased. The results showed that these two volume reduction ratios and this membrane could be successfully used for the production of yoghurts with improved quality making them an excellent functional food.


Introduction
In recent years, more attention has been paid on the development and consumption of functional foods. These products are fortified, enriched or enhanced with components providing health benefits when they are consumed at efficacious levels (Brasil et al. 2011). After cow's milk, goat's and sheep's milks are the two basic milks produced, consumed and used in the production of dairy products (Silva and Costa 2019). Goat's milk has many essential components, especially proteins with high biological quality, nutritional and functional properties (emulsifying, foaming and gelling). It is a good source of vitamins, minerals like calcium, phosphorus, riboflavin and medium-chain fatty acids (Bergilos-Meca et al. 2015;Clark and García 2017;Verruck et al. 2019). Goat's milk can be used in the production of many dairy products, such as beverages, yoghurt, condensed milk, cheese, ice cream, buttermilk, butter, flavored milk (Aryana and Olson 2017;Fazilah et al. 2018;Miller and Lu 2019;Pal et al. 2017;Sepe and Argüello 2019). Concentration, purification and fractionation of milk components can be carried out by means of membrane processes where ultrafiltration takes the first place (Chen et al. 2019;Zhang et al. 2020). Membrane filtration processes which use a hydrostatic pressure to force low-molecular components through a semi-permeable membrane are well known as ultrafiltration, nanofiltration and reverse osmosis (Reis et al. 2007). Ultrafiltration is preferred concentration method in comparison with the conventional evaporation due to the following advantageslow energy costs, novel non thermal environmentally friendly technology that minimizes the adverse effect of temperature rise such as change in phase and denaturation of proteins (Ali et al. 2017;He and Hägg 2012;Macedonio and Drioli 2017;Kumar et al., 2013). Retention factor (selectivity) is the qualitative characteristic of ultrafiltration process which generally determines the effectiveness of membrane concentration (Fang et al. 2015;Le et al. 2014;Polyakov and Zydney 2013). The successful application of ultrafiltration depends on the knowledge about factors which affect the process such as the type and pore size of the membrane, operating conditions as well as the interactions between the membrane and the feed solution (Tamime 2013). The concentration of goat's milk by ultrafiltration is one of the most promising research areas for the modern goat's dairy industry (Verruck et al. 2019). The aim of the experimental work was to study the effect of volume reduction ratio on the concentration and retention (selectivity) factors during ultrafiltration of goat's milk with UF25-PAN polyacrylnitrile membrane.

Materials and Methods
Milk. The experimental work was carried out with commercially purchased whole goat's milk Olympus (Bulgaria).

Membrane.
UF25-PAN polyacrylonitrile membrane with molecular weight cut-off of 25 kDa was used for the experimental work. The membrane was prepared by dry-wet phase-inversion method of polymer solution with dimethyl formamide as solvent (Sigma Aldrich, Germany). Then it was heat-treated in aqueous medium for 10 min at 80°C.

Ultrafiltration equipment.
Ultrafiltration experiments were carried out on laboratory equipment with a replaceable plate and frame membrane module fitted with UF25-PAN polyacrylonitrile membrane shown on Fig. 1. This equipment contained a plate and frame membrane module with membrane surface area of 0.125 m 2 , a three-plunger high-pressure pump (max 15 MPa) with a capacity of 330 dm 3 .h -1 ; a pipeline system with two manometers (0 MPa to 15 MPa) for measuring the inlet and outlet pressure; and a special working pressure regulating valve. Ultrafiltration was carried out under the following operating conditions: working pressure of 0.2 MPa, temperature of 20°С and volume reduction ratios of 2 and 3. All experiments were carried out at a feed flow rate of 330 dm 3 .h -1 .

Calculation of volume reduction ratio (VRR).
The volume reduction ratio (VRR) was calculated by the following formula: where: V0, dm 3 was the volume of the initial milk and VR, dm 3 was the volume of the retentate.

Determination of concentration and retention factors.
Concentration factor was calculated as follows: where: CR, % and CO, % were the dry matter, fat, total protein or ash content in ultrafiltration retentates and initial goat's milk.
The retention factor (selectivity) was calculated as follows: where: Cp, % and CR, % were the protein or ash content in ultrafiltration permeate and retentates, respectively.

Statistical analysis.
Fisher's least significant difference test at a significance level of 0.05 was used for the comparison of the experimental values using Excel 2010 by a one-way analysis of variance (one-way ANOVA).

Results and Discussion
The chemical composition (dry matter, fat, total protein and ash content) of initial goat's milk and ultrafiltration retentates at volume reduction ratios of 2 and 3 during ultrafiltration of goat's milk with UF25-PAN membrane, as like as permeate, are presented in Table 1. It could be seen that the increase in volume reduction ratio led to an increase (p < 0.05) in the dry matter content from 12.30 ± 0.26% to 23.00 ± 0.15%, the total proteinsfrom 3.53 ± 0.10% to 9.60 ± 0.05%, the fat contentfrom 3.5 ± 0.10% to 9.50 ± 0.20 %, the ash contentfrom 0.87 ± 0.02% to 1.32 ± 0.03%. This was probably due to the molecular weight cut-off of the membrane used in this experimental research and its ability to maintain high molecular weight components, such as proteins and milk fat, and to allow the passage of low molecular weight components, such as minerals and lactose. The proteins have molecular weight in the range from 14 kDa to 900 kDa, except proteose-peptones; the milk fat from 0.1 μm to 15 μm; the lactose -350 Da; the minerals -up to 100 Da (Norazman et al. 2013;Iritani et al. 2014). According to Moreno-Montoro et al. (2015), the dry matter, total proteins and ash increased at volume reduction ratio of 1.7 with 50 kDa ultrafiltration membrane. Domagala and Kupiec (2003) studied the composition of retentates from goat's milk at 2-fold ultrafiltration concentration with three various membranes (10 kDa, 30 kDa and 100 kDa) and found an increase in the total solids, proteins, fat, ash, and a decrease in the lactose content with all membranes investigated. Meena et al. (2015) determined the highest increase in the total solids, fat, proteins, ash and the highest decrease in the lactose content in the 3.87 fold retentate in comparison with 1.55 and 1.28 fold retentates from buffalo's milk with 50 kDa ultrafiltration membrane.
The concentration and retention factors of the main components at volume reduction ratios of 2 and 3 are presented in Table 2. The results showed that the concentration factor of the dry matter increased from 1.34 ± 0.05 at VRR = 2 to 1.87 ± 0.03 at VRR = 3 (p < 0.05). At the same time, the protein concentration factor increased from 1.70 ± 0.02 to 2.72 ± 0.06, respectively (p < 0.05), fat concentration factorfrom 1.71 ± 0.03 to 2.71 ± 0.03 (p < 0.05), ashfrom 1.13 ± 0.02 to 1.52 ± 0.02 (p < 0.05). Our results are in agreement with the experimental work of Baldasso et al. (2011), Luo et al. (2015 and Macedo et al. (2012) which found that the concentration and retention factors increased with the increase in the volume reduction ratio for other types of milk. The results showed the possibility of the membrane and volume reduction ratios studied to concentrate the main components of goat's milk in order to be used for obtaining of yoghurts and to improve their quality. According to Moineau-Jean et al. (2019), the use of ultrafiltration of milk prior to fermentation led to a higher protein content which favoured the growth of Lb. helveticus R0052 and S. thermophilus, and increased the functionality of the final product. The experimental work of other researchers also proved the positive effect of ultrafiltration of milk to obtain high quality plain yoghurt, without addition of stabilizers, which is otherwise widely used commercially for the manufacture of yoghurt to control its wheying off and body (Narayana and Gupta 2016). In our experimental work, the retention factor of the ash increased from 37.40 ± 2.19% (VRR = 2) to 53.50 ± 2.21% (VRR = 3) (p < 0.05), while according to the total protein contents there was no signifficant difference between VRR = 2 and VRR = 3 (p > 0.05).  Note: Different lowercase letters (a and b) showed a significant difference between the concentration or retention factors at volume reduction ratios of 2 and 3 (p < 0.05).

Conclusions
The increase in the volume reduction ratio from 2 to 3 during ultrafiltration of goat's milk with UF25-PAN membrane led to an increase (p < 0.05) in the dry matter content from 12.30 ± 0.026% to 23.00 ± 0.15%, the total proteinsfrom 3.53 ± 0.10% to 9.60 ± 0.05%, the milk fatfrom 3.5 ± 0.10% to 9.50 ± 0.20%, the ashfrom 0.87 ± 0.02% to 1.32 ± 0.03%. At these conditions, the concentation factors of dry matter, fat, total proteins and ash increased. Similar results were obtained for the retention factor of ash, while according to the total proteins there was no statistically signifficant difference between volume reduction ratio 2 and 3. These volume reduction ratios and UF25-PAN membrane can be used for the production of yoghurts.