**1. Introduction**

Meat plays a key role in human nutrition and evolution thanks to its components, including proteins and essential micronutrients such as Zn, Se, Fe, vitamin A, vitamin B12, and folate [1,2]. Most often, raw meat is subjected to various cooking methods such as boiling in water, grilling, steaming, microwave radiation, and sous vide to enhance its digestibility, sensory characteristic, and to improve its hygienic quality [3–5]. In each type of cooking method, several changes occur as a consequence of heating, such as denaturation, aggregation, and degradation of proteins, fiber shrinkage, and collagen solubilization [5–7]. The bio-accessibility of nutrients also can be affected during the cooking process mainly due to the degradation of vitamins, amino acids, and minerals [8]. Therefore, selecting an appropriate cooking method is a critical step before consumption which directly affects physicochemical, textural, and microbiological properties. Among different cooking methods, sous vide cooking has received considerable attention from catering, restaurants, ready-to-eat industries, and homes [9,10]. This technique provides more efficient heat transfer from water to food compared to other cooking methods [11], resulting improvement in eating quality characteristics such as texture, tenderness, juiciness, color, flavor, and

A.M.; Masino, F.; Minelli, G.; Lo Fiego, D.P.; Pulvirenti, A. Effect of Time and Temperature on Physicochemical and Microbiological Properties of Sous Vide Chicken Breast Fillets. *Appl. Sci.* **2021**, *11*, 3189. https://doi.org/ 10.3390/app11073189

**Citation:** Haghighi, H.; Belmonte,

Academic Editor: Isidoro Garcia-Garcia

Received: 25 February 2021 Accepted: 30 March 2021 Published: 2 April 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

also provides high nutritional value [6,12,13]. Besides, this technique is simple to apply for cooking different kinds of food (e.g., meat, cereals, legumes, etc.) [14]. The term "sous vide" is a French word that refers to the uniform cooking of food inside the food grade and heat-stable vacuumed pouches incubated in a circulating water bath with monitored conditions of temperature and time followed by chilled storage [15,16]. Sous vide cooking has been reported to enhance the quality attributes, inhibiting off-flavors from lipid oxidation, reducing aerobic bacteria and the risk of post-cooking contamination during storage [9,17–19]. Besides, it is beneficial for preserving vitamins, antioxidant compounds, essential amino acids, and unsaturated fatty acids during solubilization, volatilization, and high-temperature application [11,15].

Selecting the right temperature and time combinations plays an important role in sous vide cooking to reduce the risk of overcooking, loss of volatile compounds, and heat-sensitive nutrients [8]. In this context, the effect of cooking temperature and time in sous vide has been reported on the physicochemical properties and eating quality of pork [20–23], lamb [7], beef [24,25], turkey [26,27], and chicken [28–30]. Sánchez del Pulgar et al. [23] found that sous vide pork cheeks cooked at 60 ◦C had lower water losses, more moisture content, more lightness (L\*), and redness (a\*) compared to those cooked at 80 ◦C. Roldán et al. [7] reported that sous vide lamb loins cooked at 60 ◦C had the highest lightness and redness compared to those cooked at 70 and 80 ◦C. Besides, increasing cooking temperature caused an increase in cooking loss and a decrease in moisture content. However, the interaction between time and temperature was only effective on microstructural properties. Bıyıklı et al. [26] found that sous vide turkey cutlet cooked at 65 ◦C had a lower cooking loss, thiobarbituric acid reactive substances (TBARS), and pH compared to those cooked at 70 ◦C and 75 ◦C. Besides, the cooking loss, fat content, and pH were increased by increasing cooking time from 20 min to 60 min.

According to the United Nations Food and Agriculture Organization (FAO), poultry meat is the second most widely eaten meat in the world after pork. It is estimated that global poultry consumption will reach 133 million tons by 2024. This is mainly due to the high consumer demands for a healthier diet with high protein content, good amino acid composition, low levels of fat and cholesterol, as well as lower selling price [31]. Because of these features, poultry meat, including chicken breast fillets, has received much attention recently. To the best of our knowledge, literature concerning the combinations of temperature and time on physicochemical and microbiological properties of sous vide chicken breast fillets is still limited. Therefore, the focus of this research was to evaluate the effect of these parameters on eating quality characteristics such as moisture content, cooking loss, lipid oxidation, pH, shear force, color, and microbial safety of sous vide chicken breast fillets.

#### **2. Materials and Methods**

#### *2.1. Experimental Design*

Fresh skinless and boneless raw chicken breasts were purchased from the local market (Reggio Emilia, Italy) supplied by the same producer within 24 h postmortem and transported to the Department of Life Sciences, University of Modena and Reggio Emilia, Italy using a thermocol box filled with ice and used immediately. Surface fat was trimmed off and samples were cut into pieces with 125 ± 5 g weight and 2.5 ± 0.2 cm thickness. Samples were randomly assigned into the 13 groups. Twelve groups were vacuum-sealed in the food-grade nylon-polyethylene plastic pouches (150 × 200 mm2) using a vacuum sealer (La Grandispensa, Elegen, Reggio Emilia, Italy) with a pump flow rate of 30 L per minute to create 98% vacuum degree inside the pouches. Plastic pouches had wide thermal stability (−<sup>40</sup> ◦C–+120 ◦C) with O2 permeability of 9 cm3/day m2 (4 ◦C/80% relative humidity), and water vapor permeability of 1.2 g/day m2 (Joelplas SL, Barcelona, Spain). As a control group, chicken breast fillets sealed in plastic pouches without a vacuum (0% vacuum degree) were boiled at 100 ◦C for 60 min. The samples were cooked in a sous vide cooker (Elegen, Reggio Emilia, Italy). Three independent replicate trials with two repeats

based on different combinations of temperature (60, 70, and 80 ◦C) and time (60, 90, 120, and 150 min) were analyzed (Table 1). Overall, a total of 78 chicken breast fillets were analyzed (13 groups of samples × 3 independent replicate × 2 repeats). The sous vide chicken breast fillets were cooled in an ice bath for one hour and overnight in the fridge at 2–4 ◦C. Moisture content, cooking loss, pH, color, TBARS, and shear force were measured the day after the cooking process [7].


**Table 1.** Temperature, Time, and Vacuum Conditions Applied in This Study for Cooking Chicken Breast Fillets.

### *2.2. Moisture Content and Cooking Loss*

The moisture content and cooking loss were determined according to the AOAC International 950.46 method [32]. The moisture content of the chicken fillets (5 g) was calculated as the percentage of weight loss to a constant weight (Md) after drying in an oven at 105 ± 2 ◦C and the initial weight (Mi) according to Equation (1):

$$\text{Moisture content (\%): (M\_i, M\_d)/(M\_i) \times 100 \tag{1}$$

The cooking loss was measured by the weight difference of meat samples (5 g) before (W1) and after cooking (W2) according to Equation (2):

$$\text{Cooling loss (\%): } (\text{W}\_1.\text{W}\_2)/(\text{W}\_1) \times 100\tag{2}$$

Moisture content and cooking loss measurements were performed in triplicate.

*2.3. pH*

The pH value was measured before and after cooking according to the AOAC 981.12 method [32] using a pH meter equipped with a Xerolite electrode (Crison Instrument, Allela, Spain). The pH was determined by blending a 10 g sample with 50 mL distilled water for 60 s in a homogenizer (IKA, Labortechnik, Staufen, Germany). The analysis was performed in triplicate.

#### *2.4. Color*

The color of meat samples before and after cooking was measured on the external surface of each fillet with a colorimeter (CR-400, Konica Minolta, Osaka, Japan) equipped with a standard illuminant D65 and 10◦ observer angle [33]. The results are reported as L\* (lightness), a\* (redness/greenness), and b\* (yellowness/blueness). The instrument was calibrated with a white standard (L\* = 99.36, a\* = −0.12, b\* = −0.06) before each measurement [34]. The average of six measurements at different positions was calculated.

#### *2.5. Warner-Bratzler Shear Force (WBSF)*

The WBSF was performed according to Honikel [35] with slight modification. Texture analyzer (Z1.0, Zwick/Roell, Ulm, Germany) with loading cell of 1000 N and crosshead speed 250 mm/min was used to perform shear force analysis on cooked chicken breast fillets (3 × 1.5 × 1 cm3) using a Warner-Bratzler blade [22]. The data was obtained from TestXpert® II 161 (V3.31) software (Zwick/Roell, Ulm, Germany). The maximum peak force (kg) to shear the sample was reported as a shear force. The average of five measurements was recorded.

#### *2.6. Thiobarbituric Acid Reactive Substances (TBARS)*

TBARS measurement was carried out based on Siu and Draper [36]. A total of 2.5 g of minced meat sample and 12.5 mL distilled water were homogenized at 9500 rpm for 120 s using an ultra-turrax homogenizer (IKA, Labortechnik, Staufen, Germany). The homogenized sample mixed with 12.5 mL of 10% trichloroacetic acid (TCA) (CAS Number: 76-03-09, Sigma-Aldrich, Milan, Italy) and centrifuged for 20 min at 2000 rpm at 4 ◦C. The supernatant was filtered by a filter paper (Whatman No. 1). A total of 4 mL of the filtrate aliquots was mixed with 1 mL of 0.06 M 2-thiobarbituric acid (TBA) (CAS Number: 504-17-6, Sigma-Aldrich, Milan, Italy) and the solution was heated in a water bath at 80 ◦C for 90 min. A distilled water-TCA-TBA reagent was also prepared and presented as a blank. The absorbance at 532 nm was measured in duplicate by a spectrophotometer (Jasco Corporation, Tokyo, Japan). Results were expressed as mg of malondialdehyde (MDA) equivalents/kg sample. The average of three measurements was recorded.
