Study of UF-retentate concentration degree in low-and lactose-free products’ development

. The article provides a comparative evaluation of some organoleptic characteristics of low-and lactose-free milk samples produced on the basis of ultrafiltration concentrates of skim milk with different mass fraction of solids. Using the ranking method, it was found that the sample produced on the basis of UV concentrate with a mass fraction of solids of 27 % followed by enzymatic hydrolysis of lactose has the best indices of taste and sweetness. The results obtained were necessary for the development of the recipe and technology for the production of low-and lactose-free milk


Introduction
The annual increase in global production of lactose-free milk is 8.7%, which analysts attribute to the strong interest of consumers in healthy lifestyles and the detection of diseases related to lactose intolerance.In Russia this segment accounts for no more than 1% of the sales volume of dairy products for large retail chains, most of which, until recently, were imported.In 2022 several major foreign suppliers of long-life lactose-free milk (UHT, ultra-pasteurised) suspended their operations on the Russian market.According to experts, Russian producers are fully prepared to occupy the niche left by companies such as Valio and Arla Foods [1].In this regard, research aimed at establishing the technological parameters for lactose-free milk production is relevant.
According to TR CU 033/13 "On the safety of milk and milk products", in low-lactose milk products the lactose content should not exceed 1% (10 g/l), while in lactose-free products the lactose content should not exceed 0,01% (0.1 g/l) [2].
Currently, there are several ways to produce lactose-free milk, the main one being enzymatic.Domestic companies produce low-and lactose-free milk in this way.A major disadvantage of this method is that when lactose is broken down by enzymes, the resulting hydrolysis products, the simple sugars (glucose and galactose) -give increased sweetness to the milk.As a result, the digestive qualities of the natural product lose their usual taste and characteristic to the consumer [3,4,5].
The second method uses baromembrane technology to extract individual components from raw milk.Thus, in the process of ultrafiltration of skim milk, the protein phase is concentrated (UV concentration), while most of the lactose and mineral salts pass through the filtrate (UV permeate) [6].With subsequent normalisation of the UV concentrate in terms of protein and fat, low-lactose milk can be produced.When lactose-free milk is produced in the resulting UV concentrate, the remaining lactose is hydrolysed enzymatically with the enzyme β-galactosidase.This method makes it possible to regulate the sweetness of the finished product to the level of natural milk and requires a smaller amount of enzyme to be added, so it was the basis of our research.Preliminary studies have shown that the degree of concentration of the milk by ultrafiltration influenced the sweetness of the product.
The aim of the work was to justify the degree of concentration of skimmed milk by ultrafiltration based on the study of organoleptic characteristics of samples of low-and lactose-free milk, which is necessary for the development of recipes and technology of designed products.
The object of the research was low-and lactose-free products obtained from skim milk ultrafiltration retentate of different degrees of concentration.In order to realise the objective a two-factor experiment was planned, in which enzyme mass fraction (X1) and degree of concentration at ultrafiltration (X2) were selected as factors, and organoleptic quality indicators (Y) were chosen as the response.On the basis of literature review and preliminary studies, upper and lower factor values in natural and coded terms were selected and a complete factor experiment (CFE) was designed (Table 1).Based on the data obtained, a recipe was developed (Table 2).Samples were then drawn and their organoleptic quality indicators evaluated.
The results of the ranking of the organoleptic quality indices of the four samples by the five assessors are shown in Table 3.
The samples were prepared as follows.The skimmed milk was concentrated using an industrial UV unit at a pressure of up to 0.55 MPa.UV concentrates with a mass fraction of solids of 16 and 27 % were taken at different sections of the plant.The samples obtained were normalised for solids and fat by treated water and cream to drinking milk 1.5 % fat content, pasteurised and separated into two samples.Enzymatic hydrolysis of lactose using the enzyme β-galactosidase was carried out in one sample from each UV concentrate sample.All samples were then examined for organoleptic quality indicators: taste and sweetness [7,8].The organoleptic quality indicators of all samples were ranked by five experts for quantitative assessment (Table 3).A comparative analysis of the tasting results shows the following picture.Sample no.2 was prepared according to the low-lactose milk technology, in which lactose was only removed by transferring it to the filtrate during ultrafiltration to a dry matter mass fraction of 16 % (enzymatic hydrolysis was not carried out).Due to the lower lactose content, the sweetness of this sample was lower than that of the original milk.Sample no.3 had the same milk base as no.2, but differed in that it was hydrolyzed with lactose after pasteurization.The resulting simple sugars led to a higher sweetness of the product compared to the original milk.
Samples no.no.4 and 5 were prepared using a UV concentrate with a solids mass fraction of 27% and corresponded to the maximum lactose removal with the filtrate.As no hydrolysis was carried out in sample no.4, it was characterised by an empty unsweet taste compared to the control sample.In sample no.5, after ultrafiltration, normalisation and pasteurisation, the remaining lactose was cleaved with an enzyme, resulting in minimal hydrolysis products (glucose and galactose) and hence a less sweet balanced taste compared to natural milk.

Processing the results of the assessments
We determine the sum of the ranks for each parameter (sample): , here:   is the rank of the i-th parameter by the j-expert; i defines the parameters i=1, …k; jexperts, j=1, …m.
Then we calculate the difference between this sum and the average sum of the ranks: Then the sum of the squares of the deviations is determined: We calculate the correlation coefficient: , here ∑   = ∑(  3 −   );   is a number of identical ranks in the j-ranking.The correlation coefficient varies between W=0÷1.

Calculation results
Average value: Sum of the squares of deviations: The correlation coefficient will be: To assess the significance of the concordance coefficient W, Pearson's test was used χ 2 [9].
With number of degrees of freedom f=5-1=4 the tabular value is: χ 2 tab = 7.815 [5,6].Since χ 2 calc˃ χ 2 tab, the consistency of expert opinion is not accidental.The lower the sum of the ranks of a given sample, the higher its organoleptic quality indices.For example, all the samples tested are arranged in descending order of their organoleptic quality indices as 5, 1, 2, 3 and 4 (Table 1).
Hence, the best organoleptic indicator is sample no.5, in which hydrolysis of UVconcentrate with a mass fraction of solids 27 %.
The results were used in the formulation of the low and lactose-free product (Table 4).

Conclusion
The introduction of cream into the projected product not only allows normalisation by fat but also improves the organoleptic characteristics of the product.
The addition of buttermilk to the proposed product makes it possible, first, to carry out fat normalisation and also to increase its nutritional value due to the presence of biologically active substances and, above all, phospholipids with antisclerotic properties in the composition of buttermilk.A high content of phospholipids contributes to the accumulation of protein in the body and their absence contributes to fat deposition.Buttermilk is also a source of highly valuable protein and a complex of vitamins.

Table 1 .
CFE plan in coded and physical expression of the factors.

Table 2 .
Recipe for low-and lactose-free products in kg per 1000kg of finished product excluding losses.

Table 3 .
Results of ranking of organoleptic indicators of the samples.

Table 4 .
Recipe for low and lactose-free products.