1. Introduction
Sous vide cooking was adopted as a heat treatment technique in catering industries in the 1970s, and since that time, with increasing access to less expensive cooking equipment, it is gaining popularity in industrial food processing, gastronomy and home cooking [
1,
2,
3]. This technique involves vacuum packaging a food item, cooking it in a water bath or a steam chamber at a relatively low temperature for a relatively long time, and finally rapid cooling, for example in an ice-water bath [
4] or a blast chiller. The main feature of this technique is precise cooking temperature control which leads to reproducible culinary procedures and together with vacuum packaging facilitates the handling of ready-to-eat foods [
3,
5,
6]. Sous vide cooking preserves nutritive compounds in food products and ensures their adequate texture, juiciness and flavour, the former due to the mild conditions of cooking and therefore limited changes in food components and the latter due to mild changes in protein structures and keeping moisture and volatile compounds within bags [
7]. In addition, sous vide processing inhibits oxidative changes in products as a result of the lack of oxygen in bags [
1,
3,
8] and allows food preparation that is microbiologically safe and prevented from cross contamination after cooking [
8,
9].
The advantage of sous vide cooked meat is its appealing texture and colour [
10], while its flavour intensity is described as low [
11]. The relatively low temperatures used in sous vide cooking were reported to positively affect the tenderness and juiciness of meat. Nevertheless, it was observed that particularly long cooking times resulted in more tender but less juicy meat [
11]. The myofibrillar proteins begin to denature at 35–40 °C, causing shrinkage of muscle fibres, and this process continues to 80 °C [
12]. The extent of myofibrillar protein coagulation influences the final texture of cooked meat as the shrinking of these proteins leads to an increase in meat toughness [
3]. Collagen, a main protein of the connective tissue, requires long cooking and a temperature of at least 55 °C to hydrolyze, which leads to reduction in interfibre adhesion and to meat tenderness [
3,
12]. Christensen et al. [
13] observed that meat toughness increased between 40 and 50 °C and then between 60 and 80 °C, while toughness decline occurred between 50 and 60 °C. The authors explained the reduction in toughness as a result of decreased breaking strength of the perimysial connective tissue caused by partial denaturation and shrinkage of the collagen fibres.
Colour determines the doneness of meat and therefore can affect the consumer’s acceptance of a meat product [
14,
15]. The well-done state of meat requires reaching internal meat temperature of around 70 °C [
12]. In raw meat, meat pigment myoglobin exists in three forms, i.e., oxymyoglobin, deoxymyoglobin and metmyoglobin, which show bright red, purple-red and brown colour, respectively. As a result of heat treatment, globin denaturates and precipitates with other meat proteins, and red ferrohemochrome and brown ferrihemochrome are formed. Denaturation of myoglobin begins between 55 °C and 65 °C and is almost completed by 80 °C [
16,
17]. Lien et al. [
15] observed a 51.9% myoglobin denaturation in pork loin chops cooked to 62.8 °C end point temperature and its gradual increase to 85.3% when end point temperature reached 82.2 °C. Christensen et al. [
11] suggested that when prolonged cooking time is applied, denaturation of myoglobin occurs below 60 °C.
The crucial factor in the sous vide technique is a balanced combination of temperature and cooking time [
18,
19]. Sánchez del Pulgar et al. [
3] reported that chefs cook pork primals at 60–63 °C, while catering temperatures used for pork are around 75–80 °C. In the literature, sous vide cooking temperatures of pork for eating quality studies start at 48 °C [
20] and reach 71 °C [
21], while cooking times are between 45 min [
21] and 32 h [
11]. For a study of physical aspects, Zielbauer et al. [
22] cooked pork at 45–74 °C for 10–2880 min. Most publications on sous vide cooking of pork investigate the quality of meat when temperatures between 50 and 60 °C are applied. Vaudagna et al. [
23] suggested that beef sous vide cooking at 60–65 °C assures its safety and desirable yield and tenderness. In relation to pork safety, recommended end point temperatures range between 65 and 75 °C [
24].
As the literature shows, the ranges of temperature and time regimes applied even to the same meat cuts are very wide, which makes the practical use of this technique quite difficult. Moreover, the emphasis on long cooking times to ensure microbiological safety of meat makes this technique time and energy consuming. In addition, combined scientific data on physicochemical and microbiological characteristics as well as sensory properties of sous vide cooked pork are scarce. Having in mind the practical application of sous vide cooking of meat in gastronomy and other catering services, the objective of the present study was to investigate the effect of different temperature-time combinations, featuring relatively shorter cooking times than usually applied in sous vide pork cooking, on cooking loss, instrumentally measured colour and texture, microbiological quality as well as sensory properties of pork loin. Correlation coefficients between selected physiochemically, instrumentally and sensorially assessed features of meat samples were also investigated.
2. Materials and Methods
2.1. Preparation of Samples
Pork loins (M. longissimus thoracis et lumborum) of commercial crossbred pigs PIC (5–6 month-old female pigs of around 110 kg weight) were purchased 24 h after slaughter from a local meat supplier and transported to the laboratory in chilled conditions, vacuum packed and stored at 4 °C for 4 days. After storage each muscle was trimmed and cut into 2.5 cm thick slices. The slices were individually weighed and vacuum-packed in PA/PE pouches (15 μm polyamide/60 μm polyethylene; heat resistance of −20 °C/+110 °C; Hendi, Austria) using chamber vacuum sealer (Edesa VAC-20 DT, Barcelona, Spain). Seven slices were randomly assigned to each treatment.
Sous vide cooking was performed using a water bath with an immersion circulator equipped with a temperature sensor (Diamond Z, Julabo GmbH, Seelbach, Germany). The samples were heat-treated at 60 °C and 65 °C for 2 h, 3 h and 4 h, and at 70 °C and 75 °C for 1 h, 1.5 h and 2 h, after the sample core reached the temperature set for the water bath. Cooking temperatures and times were selected on the basis of preliminary study and the literature data [
5,
11,
12,
21]. After cooking, the samples destined for physicochemical, instrumental and microbiological analyses were cooled in an ice-water bath and stored overnight at 4 °C before analysis. The samples destined for sensory analysis were served after cooking. Three independent replicate trials of the whole experiment, with the use of meat purchased on three separate occasions, were conducted.
2.2. Cooking Loss
Cooking loss was calculated on the basis of the difference in meat weight before and after heat treatment.
2.3. Moisture Content
Moisture content in raw and cooked comminuted meat samples was determined by drying to constant weight at 105 °C according to AOAC procedure 950.46 [
25] using a forced draught laboratory oven (UF55; Memmert, Schwabach, Germany).
2.4. pH Measurement
Five grams of comminuted raw and cooked meat samples was homogenized with 45 mL of distilled water using the HO 4 A homogenizer (Edmund Bühler GmbH, Hechingen, Germany) for 2 min at 5000 rpm, and the measurements were registered after the equilibrium was reached using a pH meter (pH 210, Hanna Instruments, Woonsocket, RI, USA) calibrated with pH 4 and pH 7 buffers.
2.5. Water Activity
Water activity was determined at 20 °C on comminuted raw and cooked meat samples placed in measuring containers in the analyzer chamber (AWC-200, Novasina, Pfäffikon, Switzerland) calibrated with a set of Novasina humidity sources.
2.6. Colour Determination
Coordinates
L* (lightness/darkness),
a* (redness/greenness) and
b* (yellowness/blueness) of the CIE
L*
a*
b* colour space were measured on the surface of raw and heat-treated samples using a CR-400 Chroma meter (Konica Minolta Sensing Inc., Osaka, Japan) equipped with standard observer 2° and illuminant D65 and calibrated with a white ceramic tile supplied by the manufacturer. Six measurements were taken for each treatment. Chroma (
C*), hue angle (
h°) and total colour difference (Δ
E*) were calculated according to the following equations:
2.7. Instrumental Texture Analysis
Texture analysis of cooked meat samples was based on shear test and texture profile analysis (TPA) using TA.TXplus Texture Analyzer (Stable Micro Systems Ltd., Godalming, UK) equipped with a 50 kg load cell. Shear test was performed with a Warner-Bratzler shear blade with a v-shaped notch. The crosshead speed during the test was 250 mm/min. The TPA test was a two-cycle compression using the P/100 compression platen of 50 mm diameter, with sample deformation to 50% of its original height. The crosshead speed was 50 mm/min. For each treatment, 20 specimens cut parallel to the muscle fibres (10 × 10 × 25 mm for the shear test and 16 mm diameter, 20 mm height cores for the TPA) were analyzed.
2.8. Microbiological Analysis
The total number of mesophilic microorganisms [
26], the number of coagulase-positive staphylococci [
27] and the number of
Enterobacteriaceae [
28] were determined in the raw and cooked meat samples. For this purpose, 10 g of meat was homogenized with 90 mL sterile 0.1% peptone in a Stomacher (Lab Blender, Model 400, Seward Medical, London, UK) for 120 s. Appropriate dilutions were made with 0.1% peptone broth, and 1 mL was plated onto the culture media and incubated under optimal conditions: total mesophilic counts on a Plate Count Agar (PCA, Oxoid) for 72 h at 30 °C; coagulase-positive staphylococci on a Baird Parker Agar RPF (Baird Parker Rabbit Plasma Fibrinogen Agar; BPA, RPF RPF Agar; Merck) for 72 h at 37 °C and
Enterobacteriaceae on a VRBD Agar (Violet Red Bile Dextrose; Merck) for 24 h at 37 °C. Typical colonies for each media were counted in plates from the dilution with 10–150 colonies.
The presence of pathogens
Salmonella spp. [
29] and
Listeria monocytogenes [
30] in the meat was also determined. The presence of
Salmonella spp. was determined in 25 g of meat. The sample was homogenized in 225 mL of buffered peptone water and incubated at 37 °C for 24 h. Then, 1 mL of the culture was transferred to a GranuCult™ RVS Broth (RAPPAPORT-VASSILIADIS-Soya; Merck) and incubated at 37 °C. After 24 h of incubation, the XLD Agar (Xylose Lysine Deoxycholate Agar; Merck) and BPLS Agar (Brilliant-green Phenol-red Lactose Sucrose; Merck) media were inoculated. The presence of
Listeria monocytogenes in 25 g of meat was determined after precultivation in Half-Fraser broth (Merck) and cultivation in Full Fraser broth (Merck) on Chromocult
® Listeria Agar; ALOA (Merck) and Palcam Listeria Selective Agar (Merck).
2.9. Sensory Analysis
The evaluation was conducted in the sensory analysis laboratory of the Department of Human Nutrition. The sensory panel consisted of 15 employees of the Faculty of Food Science, trained according to [
31] and experienced in sensory evaluation of food. Before evaluation two training sessions familiarized the panelists with the samples represented in the experiment. The assessment of experimental samples was repeated twice during each experiment replication. The panelists were provided water and bread to clean the palate between samples. Cooked meat samples were diagonally cut into 1 cm thick slices and served one slice per assessor. Each panelist received all treatments in random order. Samples were evaluated using a 10 cm structured graphic scale, according to [
32], for overall appearance, flavour acceptability and overall acceptability (0–not acceptable, 10–very acceptable), colour uniformity (0–not uniform, 10–highly uniform), aroma intensity and meat flavour intensity (0–low, 10–very intense), tenderness (0–tough, 10–tender) and juiciness (0–low and 10–very high).
2.10. Statistical Analysis
The results of the study were presented as mean values and standard error of the mean (SEM). The experiment was conducted using randomized factorial design with temperature and treatment time as fixed effects and experiment replicates as random effects. In each experiment replicate, when not otherwise stated, the measurements were conducted in three replications. Data were analyzed using the General Linear Model procedure of Statistica 13 (TIBCO Software Inc., Tulsa, OK, USA). The results of the measurements were subjected to two-way ANOVA to identify the effects of cooking temperature, cooking time and their interaction on the investigated features of meat samples. The means were separated with the Tukey’s test, and differences were considered significant if p < 0.05. Additionally, to examine potential relationships between selected attributes of the samples, Pearson’s correlation coefficients were calculated.
4. Conclusions
The quality of cooked meat is a multidimensional feature, which is affected by meat cut characteristics and physico-chemical and sensorial properties as well as microbiological safety of final products. The research presented in the literature most often deal with selected aspects of the sous vide method: different meat cut quality, e.g., storage stability of pork loin cooked at only one temperature/time combination and assessed on the basis of physico-chemical and sensory analyses [
43]; physico-chemical properties of pork cheeks cooked using different combinations of two temperatures and two times of heat treatment [
3] or physico-chemical and basic microbiological quality aspects of pork ham cooked using combinations of two temperatures, two times and two vacuum degrees [
21].
The present study, where we applied a wide range of cooking parameter combinations and a wide range of analyses (physico-chemical, sensorial and microbiological), revealed that cooking at 60 or 65 °C for 4 h ensured the most attractive and acceptable sensory traits of pork loin, which was also partially confirmed by the results of TPA analysis. Instrumentally measured hardness, cohesiveness and chewiness showed the lowest values for the 60 °C/4 h sample, while the lowest springiness was observed for the 65 °C/4 h sample. The sensory features that influenced the overall acceptability of sous vide cooked pork loin in the highest degree were tenderness and juiciness, as can be seen based on coefficients of correlation. That means the texture attributes were the most important for sous vide cooked pork perception. Regarding cooking loss, meat preparation at 60 °C/4 h was more beneficial than at 65 °C/4 h, however it was not reflected in moisture content and sensorially assessed juiciness. The results obtained in the research indicate that the applied parameters of the sous vide processing were sufficient to reduce the microflora in the pork loin to the level safe for consumption.