1. Introduction
Pasta filata cheese production technology is based on the processing of milk from various species of mammals. Traditionally, cow’s milk is used to produce Provolone del Monaco cheese [
1,
2], buffalo’s milk produces Mozzarella di Bufala Campana PDO [
3,
4,
5], and sheep’s milk is used to produce Oscypek PDO cheese [
6], semi-hard Kasseri PDO cheese [
7,
8] and Vastedda Della Valle del Belice PDO soft cheese [
9,
10]. Among these cheeses, Kashar cheese, which traditionally originates from Turkey, can be made from cow’s milk, sheep’s milk, or a mixture of both [
11,
12]. Currently, the adaptation of the production technology of many pasta filata cheeses allows their more widespread production from cow’s milk and, therefore, more accessible raw material. This is the case for Mozzarella cheese or Caciocavallo cheese. However, as with any modification of the raw material and technical/technological parameters, it is associated with obtaining cheeses with properties different from the original [
4,
13].
Mozzarella is one of the most popular cheeses around the world. Its high or low moisture depends on fat content in the dry matter. For example, when content of fat in dry matter (
m/m) is ≥30% but <40%, then the corresponding minimum dry matter content (
m/m) for low- and high-moisture Mozzarella is 39% and 26%, respectively [
14]. It is characterized by a fresh, milky taste and an exceptionally soft texture, and is stored in covering liquid. This type of cheese shows specific properties, where serum leachate after portioning of the cheese is probably the most particular [
15,
16]. The composition of the covering liquid may be different, including, among others, water, lactic acid or citric acid, NaCl, and CaCl
2. It is a type of brine that maintains high moisture in the cheese, usually above 60%, as well as very soft texture, and prevents the formation of rind on the surface [
16,
17]. Nevertheless, the use of a covering liquid contributes to the reduction of the shelf-life due to high moisture content and water activity, and mass transfer between the cheese matrix and the serum phase [
16,
17,
18]. The presence of water redistribution and enhanced water-holding capacity in cheese during longer storage has been described by Gonçalves et al. [
19].
The physical state of the water in cheese critically influences both structural and functional properties. Cheese has a bi-continuous gel structure consisting of a porous protein matrix (casein) interrupted by fat [
20]. Some of the water in cheese is located near or inside fat clusters [
21], while most of the water is found in the porous casein matrix [
20]. The water fractions of cheese are generally divided into a matrix associated with casein and unbound free serum water [
20].
Mozzarella cheese is widespread among consumers, not only for its sensory properties, but also due to its nutritional and health benefits; however, one undesirable feature in the consumption or production and industrial use of Mozzarella is leachate that appears after its unpacking. This phenomenon is not only negatively perceived by consumers, but also (due to the reduction in cheese mass) is a source of losses for producers. Considering the impact of leachate on the profitability of production and the consumer impression of pasta filata cheese, it is necessary to identify the conditions that favor its formation. To the best of our knowledge, reports have not comprehensively examined the effect of raw material used to manufacture cheese and its post-production processing on the amount of leachate. Therefore, the aim of the present study was to analyze the impact of cheese fragmentation and packaging on the dynamics of water–fat serum release from pasta filata cheese made from cow’s milk and its mixture with sheep’s milk. To enable easy use of the obtained results in industrial practice, the artificial neural network model of water–fat serum released as a function of the above-mentioned parameters was elaborated. In addition to the scientific cognitive value, the developed model will be useful for technologists designing specific properties of cheese and the food industry, especially HoReCa (Hotel, Restaurant, Catering).
2. Materials and Methods
2.1. Cheese-Making Protocols
A detailed description of cow’s and sheep’s milk used and the procedure for making pasta filata cheese was described by Biegalski et al. [
22]. Half of the produced cheeses were vacuum packed using a vacuum sealer (A300/16 type, Multivac GmbH & Co.KG, Wolfertschwenden, Germany). The other half were packed in brine and stored at 3 °C. PA/PE bags with a thickness of 0.08 mm were used. The produced cheeses were shaped into spheres (220 g, Ø = 7 cm). The research material was a whole sphere of cheese, quartered cheese, and sliced cheese.
2.2. Experimental Design
The cheese was quartered by slicing along the geometric center horizontally and vertically to obtain 4 equal quarters of cheese. Slicing was carried out with a food slicer (R506E, Gorenje d.d., Valenje, Slovenia) to obtain 1 cm thick slices.
Table 1 shows the parametric data of fresh pasta filata cheese before and after portioning. The differences in dimensions were closely related to the way the ball of cheese was cut. Hence, the area of the cheese slice was greater than that of the quarter.
The various parameters of cheese quality, sensory test, and the amount of water–fat serum released from cheese were rated after production (after 2 days of storage in packaging at 3 ± 0.5 °C). Storage time for all samples was 2 days after production, which imitates the period of time from the end of production to the moment the product goes on sale. Test specimens were taken from different production batches (n = 6). The cheese was prepared in a pilot plant scale and each batch was analyzed twice.
2.3. Composition and Acidity of Cheese
The composition of the cheese was determined according to moisture [
23], protein [
24], and fat [
25] content. Total protein was calculated as: (TN − NPN) × 6.38. pH was measured using a CP–402 pH-meter (Elmetron, Zabrze, Poland) equipped with a IONODE IJ44A electrode (Ionode Pty. Ltd., Tennyson, Australia). The titratable acidity values were expressed as Soxhlet–Henkel degree (SH, 1 SH = 0.0225 lactic acid %).
2.4. Profile Texture Analyses and Oiling-Off
The hardness and stretching of the cheeses were measured using a texturometer (Stable Micro Systems Ltd., Surrey, UK) with attachments: A/WEG—hardness (Pre-Test Speed 1.0 mm/s, Test Speed 2.0 mm/s, Post-Test Speed 10.0 mm/s, distance 10.0 mm); A/CE (stretch quality) attachment with a PT 100 temperature sensor (test speed 20.0 mm/s, post-test speed 20.0 mm/s, distance 270 mm, temp. 55 °C, samples of 60 g). Results were recorded using Texture Exponent E32 version 4.0.9.0 software (Godalming, Surrey, UK).
Oiling-off (fat-ring test) was determined according to the method of Schenkel et al. [
26] and Hartmann et al. [
27]. The free oil formation was expressed as the percentage of the area soaked by free oil relative to the area of the total filter paper.
2.5. Gloss Measurement and Microstructure
The gloss was measured using the DT 268 gloss meter (TestAn, Gdańsk, Poland), measurement geometry 60.
All samples were evaluated using optical microscopy. Observations were conducted on pieces taken from the central layers of the cheese mass. The fragment dimensions were as follows: width 2 mm, height 2 mm, and thickness 0.3 mm. Samples of cheeses were deposited onto a glass slide surface and covered with a cover slip for observation under an optical microscope ProteOne (Delta Optical, Mińsk Mazowiecki, Poland). Observations were made at 1000× magnification using a ProteOne semiplanachromatic objective (Delta Optical, Mińsk Mazowiecki, Poland) with oil immersion. Images were taken using DLT-Cam PRO microscope camera (Delta Optical, Mińsk Mazowiecki, Poland).
2.6. Acceptability of Appearance and Consumer Penalty Analysis
In the sensory test, consumers (
n = 84; 50 female, 34 male; ages 29 to 69; M
age = 35.5, SD = 8.71) were asked to indicate how much they liked or disliked each product on a 9-point hedonic scale (9 = like extremely; 1 = dislike extremely) based on appearance. Each consumer was given 12 cheese samples for evaluation: whole sphere of cheeses, quartered cheeses and sliced cheeses, packed in brine and vacuum packed, made from cow’s milk and a mixture of cow’s and sheep’s milk. The samples were assessed between 5–6 h after removing from the packaging and portioning (according to preliminary observations, this is the average storage time of the cheese after unpacking by the consumer). Samples were held and served at 6 °C in a refrigerated display case (YG-05025, YATO, Wrocław, Poland). Any assessor who rated the sample at a level of 1 to 4 (dislike) had to rate shininess, leachate and compactness using a 5-point just-about-right (JAR) scale. For this purpose, the methodology described by Costa et al. [
28] was used. Ratings consisted of 1 = not enough, 3 = ideal, 5 = too much.
2.7. Modeling Process
2.7.1. Datasets
The model of water–fat serum release from pasta filata cheese was developed based on experimental data, which described the volume of liquid phase (mL) collected within 24 h after cheese unpacking (according to preliminary research, it is the maximum storage time of the cheese after unpacking by the consumer). The data included observations recorded from cheeses that differed in terms of packaging method, degree of sample fragmentation and type of raw material used in production. During modeling, data were randomly divided into three groups, training, testing and validation. The training dataset used for the network learning process consisted of 604 points, which accounted for 70% of all cases. The other two datasets, the set of testing data used to evaluate the network during its training and the set of validation data not involved in the construction of the model, was used for the final model verification, each contained 130 points, i.e., 15% of the full data set.
2.7.2. Model Development
Artificial neural networks (ANNs) are a universal approximating system capable of mapping dependences existing in multidimensional datasets. ANNs do not require a priori knowledge of the relationship between process variables and offer a simple and straightforward approach to problem identification; hence, they constitute a highly promising modeling technique particularly in the case of non-linear phenomenon [
29]. The most commonly used neural networks consist of several layers of neurons (input, one or more hidden and output layers). Determination of the number of hidden layers, the number of neurons in each of them, and the type of activation functions in neurons of the hidden and output layers is part of the neural network design process. The values of network parameters (weights, biases) are estimated in the optimization process to allow the network to best map the set of independent variables constituting the input signals into the set of dependent variables constituting the output signals.
In our study, feed-forward networks based on Multi-Layer Perceptron (MLP) with a single hidden layer were applied to develop the model of water–fat serum released from pasta filata cheese within 24 h of unpacking. The model inputs were comprised of four independent variables. One was quantitative, i.e., time, while the others were qualitative and took the following states, i.e., type of raw material (cow’s milk, cow-sheep’s milk), the method of sample portioning (whole, quarters, slices), and the packing method (vacuum packed and packed in brine). When designing the network, each of the variables were assigned to several neurons equal to the number of states. For a specific state of a given qualitative input variable, only one of the assigned neurons could take one of two values, i.e., 0 (inactive) or 1 (active). In turn, the dependent variable, which was the leachate volume, was taken as the output of the network. During model development, network topologies based on MLP containing a different number of neurons in the hidden layer (from 2 to 20), various types of activation function in neurons of hidden layer (i.e., hyperbolic tangent (Tanh), logistic (Log) and exponential (Exp)) and linear activation function (Lin) in the neurons of output were examined. The network parameters (weights, biases) were determined using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) training algorithm. The total number of tested ANN topologies was 57,000 (3 types of activation functions in neurons of hidden layer × 19 sizes of hidden layers × 1000 repetitions for each structure). The performance quality of each tested network was assessed during the model-designing process based on the values of training and test errors. The generalization capability of examined networks was evaluated based on a validation error. All errors were calculated with the use of the sum-of-the-squares error function.
2.8. Statistical Analyses
Statistical analysis was carried out using TIBCO Statistica data analysis software, version 13.3.0 (TIBCO Software Inc., Palo Alto, CA, USA). Results are presented as mean ± standard deviation (SD) of triplicate of each analysis carried out in experiments performed in duplicate. A critical level of significance of α = 0.05 was used throughout the study. The influence of milk composition on chemical and physicochemical characteristics of pasta filata cheeses was evaluated by one-way analysis of variance (ANOVA). The effect of milk composition, the packaging method and the sample fragmentation on the leachate amount within 24 h of cheese unpacking was investigated using multivariate analysis of variance (ANOVA). The significance of the effect of individual factors and their interactions was assessed using the F test, while the significance of differences between the mean values of leachate volumes was determined using the post-hoc Tukey’s HSD test. Each analysis of variance began with the verification of its assumptions including examination of the probability distribution for measured variable and homoscedasticity in the standard deviation of its value using the W Shapiro–Wilk and Levene tests, respectively. The evaluation of the ANN model of water–fat serum released from fresh pasta filata cheese performance was carried out based on the determination coefficient (R2) and root mean square error (RMSE).
4. Conclusions
In the present work, we investigated the effect of the portioning of cheese into quarters and slices of previously vacuum-packed or packed-in-brine cheese on the leachate of the water–fat serum. The results showed that the amount of leachate affected by the portioning of cheese was negatively perceived by consumers. Dissatisfaction of consumers was observed in quartered and sliced cheese (in the case of cow’s milk cheeses) and sliced cheese from mixture of cow’s and sheep’s milk (also vacuum-packed). Overall, consumers showed less acceptance of cow’s milk pasta filata cheeses than CS cheeses. The addition of sheep’s milk reduced the amount of leachate from the vacuum-packed cheeses and did not cause considerable loss of gloss, as in the case of C cheeses. This is demonstrated, for example, by plasma/serum retained in the channels of the cheese structure. The leachate has proven to be an important criterion for food quality.
Additionally, this study showed the potential usefulness of neural networks in dairy food processing. The developed predictive artificial neural network model allowed the estimation of the amount of water–fat serum released depending on the milk composition, cheese fragmentation, packing method, and storage time (within 24 h) after its unpacking, therefore, it can be useful in pasta filata cheese production process optimization.