Peng YU

Shanghai Engineering Research Center of Dairy Biotechnology/State Key Laboratory of Dairy Biotechnology/Dairy Research Institute/Bright Dairy & Food Co., Ltd., Shanghai 200436, China

Abstract The objective of this study was to study the stability of oat kernel-supplemented milk from two perspectives of formulation and processing process, with centrifugal precipitation rate, viscosity and sensory score as evaluation indices.After optimization, the results show that the optimal compound stabilizer formula was obtained as follows: 0.04% gellan gum, 0.020% carrageenan, 0.020% sodium carboxymethyl cellulose and 0.20% microcrystalline cellulose.With single-factor experiments and orthogonal array design, the optimal processing process that provided the best stability as well as good taste was determined as follows: batching temperature 50 ℃, hydration time 40 min, homogenization pressure 30 MPa and filling temperature 15 ℃.

Key words Milk, Oat kernel, Stability, Formulation, Processing process

1 Introduction

Oats are rich in nutrients such as β-glucan, protein, unsaturated fatty acids, peptides,etc.The nutritional effects brought by these ingredients, such as prevention and treatment of cardiovascular and cerebrovascular diseases caused by hyperlipidemia, regulation of lipid and blood sugar metabolism, and anti-oxidation have been widely certified[1-3].At present, the common oat products are mainly instant oats, however, the comprehensive utilization of oats is limited.Meanwhile, the oat milk drinks on the market are relatively simple, and mostly with oat flavor, or adding a little oat flour as selling points[4-6].In 2020, the health trends reports milk and milk beverages of Mintel showed that more and more consumers pay more attention to the application and nutritional ratio of raw materials with the increasing of younger health-care groups[7].Therefore, designing a drink with oats as the main raw material to combine whole kernels with milk and develop oatmeal drinks with balanced nutrition and convenient consumption will have a good market prospect.The problem with the design is that the milk density is low and the oats are easy to sink, so that consumers’ taste experience and perception are not good.If only the suspension of oat kernels is considered, it is easy to cause the problem of thicker texture and poor taste.Therefore, in order to solve the problem of oats sinking, this research screened the stabilizers from the formulation, and from the actual production, the process parameters were optimized to avoid affecting the stability of the system during the production process and obtain an oat milk product with stable properties and good taste.

2 Materials and methods

2.1 Raw milk and reagentsThe raw milk was produced by Bright dairy farm.The reagents used included cream(Bright Dairy Co., Ltd.), oat flour and oat kernels(Heilongjiang Green Manor Co., Ltd.), white sugar(Guangxi Shangshang Sugar Co., Ltd.), gellan gum[CP Kelco(China)Co., Ltd.], sodium carboxymethyl cellulose and microcrystalline cellulose[Fumanshi(Shanghai)Trading Co., Ltd.], sucrose fatty acid ester(Hangzhou Ruilin Chemical Co., Ltd.), and monostearate and distearin(Guangzhou Meichen Technology Industrial Co., Ltd.).

2.2 Instruments and equipmentThe used instruments and equipment mainly included rotational viscometer(Brookfield, USA), EBA 270 centrifuge(Hettich, Germany), UHT sterilization equipment(GEA, Germany), Panda Plus 2000 high-pressure homogenizer(APV, Germany)and filling equipment[Evergreen Packaging(Shanghai)Co., Ltd.].

2.3 Methods

2.3.1Preparation of oat kernel-supplemented milk.(i)Basic formulation.Fresh milk 70%(mass fractions), white sugar 5%, oat flour 1%, oat kernel 3%, sucrose fatty acid ester 0.05%, monostearate and distearin 0.05%, stabilizers and water(supplement to 100%).(ii)Process flow.The process flow of oat kernel-supplemented milk is shown in Fig.1.

Fig.1 Flow chart for the production of oat kernel-supplemented milk[8]

2.3.2Index determination.(i)Viscosity.Oat kernels were first filtered out, and then the viscosity(mPa·s)of the sample was determined using the No.2 rotor of Brookfield rotary viscometer(mPa·s).(ii)Stability.The stability is judged by centrifugal precipitation rate.The specific method was as follows.A certain amount(10 mL)of sample with oat kernels filtered out was centrifuged at 4 000 r/min for 20 min, and the precipitate was collected and weighted.The centrifugal precipitation rate was calculated according to the following formula[9]:

Centrifugal precipitation rate(%)= Weight of precipitate(g)/Weight of milk with oat kernels filtered out(g)×100

(iii)Sensory evaluation.The sensory characteristics of oat kernel-supplemented milk were evaluated by 20 professional sensory evaluation members, mainly including appearance state, taste and organization state.The sensory evaluation criteria are shown in Table 1.

Table 1 Criteria for sensory evaluation of milk supplemented with whole oat kernels[10]

2.3.3Experimental design.(i)Determination of compound stabilizer formulation.For single-factor experiments, gellan gum, carrageenan, sodium carboxymethyl cellulose, microcrystalline cellulose and modified starch were supplemented according to different proportions(mass fraction in the system), and the optimal supplementation amount was explored by centrifugal precipitation rate, viscosity and sensory score.The addition amounts of gellan gum were 0.02%, 0.04%, 0.06%, 0.08% and 0.10%; the addition amounts of carrageenan were 0.010%, 0.015%, 0.020%, 0.025% and 0.030%; the addition amounts of sodium carboxymethyl cellulose were 0.010%, 0.015%, 0.020%, 0.025% and 0.030%; the addition amounts of microcrystalline cellulose were 0.05%, 0.10%, 0.15%, 0.20% and 0.25%; and the addition amounts of modified starch were 0.3%, 0.6%, 0.9%, 1.2% and 1.5%.

On the basis of the results concluded from single-factor experiments, an orthogonal experiment was conducted, with centrifugal precipitation rate was the judgement index, so as to optimize the formulation of the compound stabilizer.

(ii)Optimization of production process of oat kernel-supplemented milk.The batching temperature, hydration time, homogenization pressure and filling temperature were taken as factors for single-factor experiment.The parameters of other process points are the same as the process flow.The centrifugal precipitation rate was used to determine the effect of parameter changes on the stability of oat kernel-supplemented milk.The effect on viscosity was investigated.In single-factor experiment of batching temperature, taking into account the other raw materials in the formula and the melting temperature of gellan gum, the temperature of the mixing tank was increased to 30, 40, 50, 60, 70 ℃, respectively.The materials were mixed under different temperatures and stirred for 15 min.The centrifugal precipitation rate and viscosity of the final product were compared to examine the stability.

In the single-factor experiment of hydration time, after the raw materials were mixed at 40 ℃, the hydration time was set as 10, 20, 30, 40, 50 min, respectively.The stirring time was 15 min.The centrifugal precipitation rate and viscosity of the final product were compared to examine the stability.

In the single-factor experiment of hominization pressure, the raw materials were homogenized under different pressures(20, 25, 30, 35, 40 MPa), and the centrifugal precipitation rate and viscosity of the final product were compared to examine the stability.

In the single-factor experiment of filling temperature, according to the actual operation of the factory, the filling temperature was set as 5, 10, 15, 20 and 25 ℃ respectively, and the stability of the samples was compared.

Based on the results of single-factor experiments, an orthogonal experiment was designed to obtain the best combination of the technological parameters for preparing oat kernel-supplemented milk.

2.4 Data processingThe experiment was repeated 3 times.Data processing and analysis was performed using Excel and SPSS software.Desigh Expert 7.16 and SAS 9.0.2 software were used for experiment design.

3 Results and analysis

3.1 Single-factor experiments of stabilizersTaking into account the particularity of the oat kernel-supplemented milk system, it should have a good taste while maintaining the suspension of oat kernels.Different addition amounts of stabilizers have different effects on the viscosity and texture of the samples.Gellan gum, carrageenan and modified starch have better gel effects and strong stability[11-13].Microcrystalline cellulose and sodium carboxymethyl cellulose have a good effect on improving fat floating and system stratification during the shelf life[14-16].Therefore, the influence of different stabilizers on the stability of oat kernel-supplemented milk system was investigated through single-factor experiments.

As shown in Table 2, when the centrifugal precipitation rate was used as the criterion for judging the amount of stabilizer added, gellan gum, modified starch and carrageenan were all better choices.From the perspective of viscosity and sensory score, modified starch and carrageenan were likely to cause the product to have high viscosity and poor taste, leading to lower sensory score.Too high viscosity can also cause practical problems such as large material loss during the production process, impaired system stability and pipe coking.Therefore, gellan gum was selected as the best stabilizer after measuring the three indices of centrifugal precipitation rate, viscosity and sensory score.As the amount of gellan gum added increased, the centrifugal precipitation rate of the oat kernel-supplemented milk system showed a downward trend.When the amount of gellan gum added exceeded 0.06%, the centrifugal precipitation rate almost did not change any more.From the perspective of centrifugal precipitation rate and cost, 0.06% is more suitable, but from the perspective of sensory score, the score was higher when the gellan gum was added at 0.04%.Thus, further optimization was needed to determine the optimal addition amount of gellan gum.

Table 2 Effect of stabilizer type and concentration on stability of milk supplemented with whole oat kernels

3.2 Orthogonal experiment of complex stabilizerConsidering the centrifugal precipitation rate, viscosity and sensory score, modified starch had a greater impact on viscosity, resulting in poor taste and low sensory score.Therefore, it was considered to compound gellan gum, sodium carboxymethyl cellulose, microcrystalline cellulose and carrageenan according to a certain proportion.These four stabilizers have good compatibility, and their combination can improve the taste of the product while reducing the centrifugal precipitation rate.

On the basis of the single-factor experiments, carrageenan, sodium carboxymethyl cellulose and microcrystalline cellulose that are commonly used in beverages and have stabilizing and emulsifying effect were selected to compound with gellan gum.The centrifugal precipitation rate was used for analysis.An orthogonal experiment was designed.

Table 3 and Table 4 show that the intensity of influence of various factors on the centrifugal precipitation rate of oat kernel-supplemented milk was in the order asA(addition amount of microcrystalline cellulose)>B(addition amount of carrageenan)>C(addition amount of sodium carboxymethyl cellulose)>D(addition amount of gellan gum).The optimal combination wasA1B3C3D3,i.e., 0.04% of gellan gum, 0.020% if carrageenan, 0.020% of sodium carboxymethyl cellulose and 0.20% of microcrystalline cellulose.Under the optimal conditions, the centrifugal precipitation rate of the product was 3.02%, the viscosity was 28 mPa·s, and the sensory score was 7.6 points.

Table 4 Results of orthogonal experiment for optimizing stabilizer formulation

Table 3 Factors and levels of orthogonal experiment for optimizing stabilizer formulation

3.3 Single-factor experiments of processing process of oat kernel-supplemented milkIn addition to the formulation, another important factor affecting the stability of milk containing oat kernels is production process.The gelling process of gellan gum which plays a major role is a process in which its molecules change from random coils to an ordered double helix structure, and then to a continuous three-dimensional network structure.When the temperature reaches 72 ℃, gellan gum solution will form a soft and elastic gel.As the temperature rises, the viscosity of the gellan gum solution will drop rapidly.After the temperature is reduced, it returns to its original viscosity[17].During the heating-cooling process of the production line, if the added amount of gellan gum is higher, the viscosity loss will be very obvious as the shear rate increases.The high-speed continuous shearing force on the production line destroys the network structure to a certain extent, leading to a drop in efficient suspension.In this study, how to ensure the taste and stability of the product through process optimization was further discussed.

Taking into account the above characteristics of gellan gum and the characteristics of oat flour and other stabilizers in the formulation, the batching temperature and hydration time will have an important impact on the stability of the product[18].On the other hand, most factories produce several products on the same line, so it is difficult to change the pumping pressure of the existing equipment, and instead, only parameter debugging can be performed on the basis of the existing equipment to achieve process optimization.Therefore, the four aspects of batching temperature, hydration time, homogenization pressure and filling temperature were discussed.

3.3.1Effect of batching temperature on the stability of oat kernel-supplemented milk.When the batching temperature increased from 30 to 70 ℃, the viscosity of the product was in the range of 20-23 mPa·s, and the batching temperature had little effect on the viscosity.When the batching temperature increased, the centrifugal precipitation rate of the product also increased.Especially when the batching temperature exceeded 50 ℃, the centrifugal precipitation rate increased significantly(Fig.2).It means that if the batching temperature was too high, the stability of the product decreased, and within the operable range, 30-50 ℃ was the best batching temperature.

Fig.2 Effect of batching temperature on stability of oat kernel-supplemented milk

3.3.2Effect of hydration time on the stability of oat kernel-supplemented milk.Fig.3 shows that as the hydration time was extended, the viscosity of the product showed a rising trend.When the hydration time was extended from 30 to 50 min, the viscosity was almost unchanged, and the centrifugal precipitation rate also changed slightly, indicating that a good stable state can be reached after 30 min of hydration.

Fig.3 Effect of hydration time on stability of oat kernel-supplemented milk

3.3.3Effect of homogenization pressure on the stability of oat kernel-supplemented milk.Due to the addition of oat flour and compound stabilizers in oat products, the particle size distribution of the mixed liquid was considered to be improved by increasing the homogenization pressure, thereby improving the stability of the final product[19-20].

As shown in Fig.4, as the homogenization pressure increased, the centrifugal sedimentation rate of the product dropped significantly, indicating that the homogenization pressure had a great influence on the stability, and the greater the homogenization pressure, the better the product stability.When the homogenization pressure exceeded 35 MPa, the product centrifugal precipitation rate almost did not change as the pressure continued to rise.Homogenization pressure had little effect on viscosity, and the viscosity of the product changed little during the increase process of pressure.

Fig.4 Effect of homogenization pressure on stability of oat kernel-supplemented milk

3.3.4Effect of filling temperature on the stability of oat kernel-supplemented milk.As the filling temperature increased, the centrifugal sedimentation rate of the product also increased.It shows that the lower the filling temperature, the more conducive to the stability of the product.The filling temperature had almost no effect on the viscosity of the product(Fig.5).It can be seen that the suitable filling temperature was 5 ℃.

Fig.5 Effect of filling temperature on stability of oat kernel-supplemented milk

3.4 Optimization experiment of processing process of oat kernel-supplemented milkThe intensity of influence of various factors on the stability of oat kernel-supplemented milk ranked asA(batching temperature)>D(filling temperature)>B(hydration time)>C(homogenization pressure)(Table 5, Table 6).That is, batching temperature was a sensitive factor affecting the centrifugal precipitation rate.The optimal combination isA3B3C1D3,i.e., batching temperature of 50 ℃, hydration time of 40 min, homogenization pressure of 30 MPa, and filling temperature of 15 ℃.The oat kernel-supplemented milk obtained under these process parameters had the lowest centrifugal precipitation rate, only 1.43%, and viscosity of 23 mPa·s and sensory score of 7.92 points.

Table 5 Factors and levels of orthogonal experiment for optimizing production process of oat kernel-supplemented milk

Table 6 Results of orthogonal experiment for optimizing production process of oat kernel-supplemented milk

4 Conclusions

The stability of oat kernel-supplemented milk was studied from the aspects of formulation and production process.Taking centrifugal precipitation rate and sensory score as evaluation indices, the influence of different stabilizers on the stability of oat kernel-supplemented milk was analyzed.On the basis of single-factor experiments, gellan gum, carrageenan, microcrystalline cellulose and sodium carboxymethyl cellulose were compounded.By orthog-onal experiment, it was concluded that the optimal combination of the stabilizers was 0.04% gellan gum, 0.020% carrageenan, 0.020% sodium carboxymethyl cellulose and 0.20% microcrystalline cellulose.In order to reduce the impact of the shear force of the production line on the viscosity and avoid the destruction of the stable system, within the adjustable ranges of equipment parameters, the four process points that affect the stability of the system, batching temperature, hydration time, homogenization pressure, and filling temperature, were adjusted to achieve the optimal process.The best combination of parameters obtained through orthogonal experiment was as follows: batching temperature 50 ℃, hydration time 40 min, homogenization pressure 30 MPa and filling temperature 15 ℃.Under the optimal process, the milk supplemented with whole oat kernels has uniform suspension, fine taste, uniform texture and good quality.