Influence of various chilling methods on the sustainable beef production based on high voltage electrical stimulation

Among the challenges of sustainable management of meat production, the key issue is to improve the energy efficiency of production processes, which will consequently affect the reduction of greenhouse gas emissions. Such effects are achieved by combining various chilling systems with electrical stimulation that determines the quality of meat at the slaughter stage. The novelties of the research undertaken included determining the impact of various variants of meat production (chilling method: slow, fast, accelerated + HVES/NES) on changes in the basic (industrial) quality indicators (pH and temperature) of beef produced from Polish Holstein-Friesian breed cattle, and then indicating the optimal variant for energy-efficient (sustainable) beef production. The HVES and the fast chilling method yielded positive economic (meat weight loss), technological (high quality, hot-boning), energetic (lower electricity consumption), and organizational effects (reduced chilling and storage surfaces and expenditures for staff wages) compared to the slow and accelerated methods. Reaching the desired final temperature with an increased amount of chilled meat enables obtaining a few-fold decrease in the specific energy consumption and a higher energy efficiency of the process. This allows recommending the above actions to be undertaken by entrepreneurs in the pursuit of sustainable meat production.


Introduction
One of the strategies of sustainable production in the agri-food sector, apart from striving to ensure high quality and safety of raw materials, envisages also effective management of energy and the environment throughout the food chain. Pursuant to this strategy, agricultural production is realized through changes in farm management, where energy is used with maximum efficiency (the so-called carbon footprint). Practical tools for the implementation of sustainable beef production at the rearing stage include measures aimed to improve: animal productivity (health and welfare), feed quality, soil fertility, and the efficiency of use of electricity and fertilizers [1]. Proposals for good cattle breeding practices in the Western European the most advantageous production variant (fast chilling + ES), taking into account the varied amount of raw material chilled in the chamber.

Material and methods
The research material consisted of carcasses/half-carcasses of meat cattle (heifers, bulls-age around 18 months, cows-age around 10 years) of the Polish Holstein-Friesian Black and White breed. The hot carcass standard weight was approx. 240 kg (min.178.9 ± 27.7kg; max. 298.5 ± 41.5 kg). Stunning and post-stunning procedures were executed under industrial conditions following provisions of the Council Regulation (EC) No 1099/2009 of 24 September 2009 on the protection of animals at the time of killing. The study was carried out in a mediumsized meat processing plant with a capacity of the slaughter line reaching 100 carcasses per shift. The slaughter was controlled by the State Veterinary Inspector to keep required conditions concerning animal welfare and especially to minimize suffering. The slaughter was done by qualified persons using a pneumatic cattle gun.

High-voltage electrical stimulation (HVES)
Approx. 40 min, after stunning, carcasses were treated with a high-voltage electrical stimulation (HVES) device designed by Żywica, & Banach [29]. The left half-carcasses were subjected to ES using alternating electrical current with the following parameters: U = 330V, f = 17Hz, rectangular shape of impulses, and pulse-duty factor 0.9 for 120 sec. The right half-carcasses served as the control non-electrostimulated sample (NES). Afterwards, the half-carcasses were washed with water having a temperature of ca. 10˚C, weighed, and transferred to chilling chambers. This comparative system (ES/NES) was used to determine the changes in the pH values of meat of heifers, bulls, and cows since stunning. In the last stage of the study addressing the energy balance of the fast chilling method, ES was applied to whole carcasses. In all chilling chambers, the half-carcasses were moved in a continuous pendular motion with strong forced convection.

Temperature measurements
Meat temperature measurements were performed over the chilling time using a TES 1310 TYPE-K spike thermometer (accuracy of ± 0.1˚C), at three different localizations of non-stimulated (NES) half-carcasses: semimembranosus muscle, longissimus dorsi muscle, and around a shoulder blade, at a depth of ca. 7cm. The measurements were performed 1, 3.5, 8, and 24h after stunning, and their results are presented in Table 1 as mean values ± standard deviation. The number of examined carcasses reached n = 60 in each chilling method. Due to the harsh industrial conditions (low temperatures and very high air humidity) and the need to obtain approval from plant management to enter the chilling chambers, the number of measuring points was reduced to the necessary minimum.

pH measurements
Measurements of pH of stimulated (ES, n = 15) and non-electrostimulated (NES, n = 15) beef chilled with the slow, accelerated, and fast methods were performed 2/3, 2, 6, and 24h after stunning with a pH-meter (CP-411, electrode OSH 12-01, Elmetron, Poland) in three different localizations in m. longissimus thoracis et lumborum between 7 and 8 th rib. The decision to  [30][31][32]. Before the measurements made for meat from a given experimental group (ES, NES), the electrode was washed with distiller water and the pH-meter was calibrated.

Weight losses of meat (WLM)
The effect of the fast chilling method on weight losses of stimulated meat (WLM) was determined based on the difference between the weight of carcasses (n = 390) before (M i ) and after (Mf) the chilling process (initial-W i and final-W f weight, respectively) and expressed in per cents (Eq 1).
Energy balance, specific energy consumption, and energy efficiency The heat balance of the chilling chambers used in the fast chilling method was based on the amount of heat necessary to be removed from the fast chilling chamber (area of 84 m 2 ) and the chilling tunnel (area of 57 m 2 ) during one chilling cycle. The measurements were carried out in the spring season for 8 working days (D1-D8), with varying amounts of carcasses (n = 10 to n = 144) in the chamber. The total heat (Q) collected from the chilling chamber (Eq 2) and its components were calculated using the following formulas (Eqs 3-6): where; Q 1 -heat removed form products (half-carcasses); Q 2 -heat transferring to the environment; the so-called transfer heat; Q 3 -ventilation heat; Q 4 -engine work heat; Q d -additional losses, in small chambers.  The size of a building partition was assumed to be the size of the outer walls and ceiling of the chamber.
where: n-number of air exchanges in a chamber, for chilling chambers: n = 2 per day Engine work heat (Q 4 ) was assumed to be the electric energy consumed by engines installed in the chamber. It was calculated based on engine power and work time.
The heat of lighting and the heat of men's work did not occur in the applied chilling technology. Data regarding: casing losses (Q 2 -heat penetrating from the environment), total lighting, engine work, losses due to door opening, and fan work were derived from the 'Operation and Maintenance Documentation' available at the technical department of the audited plant.
Energy consumption of the production process, meaning the demand for heat energy necessary to accomplish a specified production process, was determined based on measurements of electric energy consumption by chilling devices (compressors, FChCh+ChT). Indicators of the total specific energy consumption (SEC 1 and SEC 2 ) and energy efficiency, i.e., reduction of energy consumption (EE 1 , EE 2 ), per number or weight of slaughtered animals, were used to determine the effect of employing innovative solutions (fast chilling method + HVES). They were computed using the following formulas (Eqs 7-10): where: Q d -total heat removed from chilling chambers-FChCh + ChT,

Statistical analysis
The statistical analysis of the effects of chilling duration and method and of electrical stimulation on changes in meat temperature and pH was carried out using Statistica 13.1 software based on one-way analysis of variance (ANOVA, p<0.01, p<0.05).

Effect of the chilling method on changes in temperature of nonelectrostimulated (NES) meat
Analyses carried out in the first part of the study were expected to allow understanding the mechanisms of the impact of various chilling methods on the basic meat quality attributes, expressed by changes in the temperature inside the meat and in meat pH, determining the rate of post-stunning changes, as well as quality and safety of beef during further storage or processing.
Results of meat temperature measurements demonstrated that the type of the chilling method (slow, accelerated, fast) had a significant effect on the rate of changes in muscle tissue temperature. Beef chilling with the slow method caused very slow but significant (p<0.01) changes in its temperature. The storage of beef half-carcasses in a chilling chamber for 2.5h at the elevated temperature (ca. +10˚C/6h) contributed to meat temperature decrease by barely ca. 3˚C, i.e. to ca. 36˚C. In turn, after 8 and 24h of slow chilling, meat temperature dropped to ca. 25 and 5˚C, respectively. Compared to the slow chilling method, slightly higher rate and significant (p<0.01) changes in meat temperature were demonstrated during accelerated chilling. As soon as after 2.5h of chilling, meat temperature decreased by ca. 13˚C and reached ca. 26˚C. After 8h and 25h after stunning, it reached ca. 12˚C and 4˚C, respectively ( Table 1). The above results corroborate literature data indicating that achieving this rate of changes in meat temperature in the first hours of post-stunning slow and accelerated chilling can pose the risk of microbiological contamination caused by the rapid proliferation of microorganisms on the surface of half-carcasses [21]. In contrast, the rate of changes in meat temperature was consistent with the '10/10 rule of thumb', indicating that meat chilling to a temp. not lower than 10˚C within 10 h post-slaughter, at pH below 6.2, allows avoiding the 'cold shortening' phenomenon. The effect of counteracting this adverse phenomenon can be intensified by delayed/ slow chilling, which involves keeping the half-carcasses outside the chilling room for some time [33] or under either elevated temperatures, i.e., 10-16˚C / 3-12h [34][35][36] or lower temperatures, i.e., 5-6˚C / 24h [37].
The chilling of beef half-carcasses with the fast two-phase method had a strong effect on meat temperature decrease compared to the slow and accelerated methods. Keeping the halfcarcasses in the chilling chamber for 2.5h caused its temperature to drop from ca. 38˚C to ca. 18˚C. During further meat keeping in a chilling tunnel, its temperature dropped to ca. 10˚C after 8h and to 1.4˚C after 24h. The final temperature of meat chilled with the fast two-phase method was lower than of the meat chilled with the slow (ca. 5˚C) and accelerated (ca. 4˚C) methods. The statistical analysis of the effects of chilling methods on changes in meat temperature also demonstrated significant (p<0.01) differences between mean meat temperatures recorded after 3.5 and 24h of chilling with SM, AM, and FM as well as between mean meat temperatures measured after 8h of chilling with SM/AM and FM/SM ( Table 1).

Effect of chilling method and electrostimulated on changes in meat pH
The subsequent part of the study aimed to determine the effect of different meat production variants (chilling method slow, fast, accelerated + ES/NES) on changes in the industrial indicators (pH and temperature) of beef quality [32,38]. This complex approach was expected to allow identifying the optimal variant of meat production, ensure its high quality, and reduce its harmful effect on environment.
Results of pH measurements of the longissimus dorsi muscle of ES and NES heifers, cows, and bulls chilled with the slow method showed that ES significantly (p<0.001) accelerated the post-stunning transformations during 24-h chilling. The highest and significant differences in pH values were obtained after 2 and 6 h of chilling when meat temperature was very high (ca. 37-30˚C). The pH 2h value of ES heifer, cow, and bull meat ranged from 5.98 to 6.10 and was lower by ca. 0.9 units compared to the pH 2h value of control samples. After 6h post-stunning, the pH value of ES meat was still significantly lower (by 0.7-0.8 units, p< 0.001) compared to the pH values of meat of non-stimulated heifers, bulls, and cows that reached 6.42, 6.54, and 6.63, respectively. The final pH values (pH 24h ) of both ES and NES meat were similar and reached before 5.7; however, the pH 24h values of ES and NES meat of heifers and cows differed significantly at p<0.05 ( Table 2). The analysis of meat temperature recorded after ca. 3.5h of chilling (Table 1) and pH value of ES meat measured 2h post-stunning allows concluding that the use of HVES enables the hot-boning of meat as soon as 2h after stunning without the fear of its cold shortening during consecutive ripening. Moreover, electrostimulation allows eliminating the phase of meat retention in the chilling chamber (2.5h/ ca.10˚C) and increasing production surface [39,40].
The analysis of pH values of meat chilled with the accelerated method (AM) demonstrated the rate of changes in pH of NES meat over the chilling period had similar values (did not differ significantly) as in the slow method. Therefore, these changes were not presented in Table 2. In turn, the pH values of ES meat of heifers, cows, and bulls were insignificantly higher than in the slow chilling method (SM). The pH values measured in ES meat 2, 6, and 24h post-stunning fitted within the following ranges: pH 2h = 6.01-6.06, pH 6h = 5.73-5.87, and pH 24h = 5.6-5.7. The gender of animals had no significant effect on the final pH values (pH 24h ) of ES meat. However, the pH 24h value of bulls was higher and more differentiated (5.73 ± 0.17) compared to the pH 24h value of heifers (5.60 ± 0.05) and cows (5.65 ± 0.04)- Table 3. Higher and more differentiated pH values of meat of the stimulated bulls proved that bulls are more susceptible to the pre-slaughter stress than heifers and cows [21]. Negligible differences in the rate of pH changes between ES and control meat indicate that the AM chilling method is more rational and efficient than the SM, from the practical point of view.
The use of lower temperatures during the fast chilling of ES meat (-8˚C/2.5h, 1-2˚C/ca. 16-18h) than in the accelerated and slow methods contributed to the slower post-stunning Table 2. Changes in the pH value (� x ± SD) of the longissimus dorsi muscle of non-electrostimulated (NES) and electrostimulated (ES) heifers, cows, and bulls chilled with the slow method.  Table 3. biochemical changes of meat and, consequently, to pH values higher by ca. 0.3 units at particular measuring points. While the temperature of stimulated meat chilled with the fast method reached 18 and 10˚C after 2 and 8h since stunning, its pH 2h was at ca. 6.2 and 6.3, and its pH 6h was at 5.95 (Table 4). This means that the coupling of HVES treatment performed with the own-construction device and the fast chilling method allows achieving a very rapid temperature drop before the onset of rigor mortis and producing high-quality meat without the fear of its cold shortening [41][42][43][44]. In the case of NES meat, at pH 2h = 6.7-6.8 and pH 6h = 6.5-6.6, the biochemical processes are still in progress, and meat is still before the rigor mortis state; therefore, its rapid chilling without ES is not recommendable [45]. Given the results presented above, it has been concluded that the coupled use of HEVS performed using an own-construction device and a fast chilling method is justified considering both the economic and meat quality class established based on pH [32].

Energy consumption of the meat chilling with the fast method
The rational management of electrical energy in the production process entails not only accomplishing economic effectiveness (increasing savings) but also reducing the energy consumption of technological processes and facilities, which determine the magnitude of effects of technical and man activities on the natural environment [23,46]. The chilling systems are the largest electricity consumers in the meat industry-they account for 50-93% of the total electric energy consumed in a typical slaughterhouse [5,47]. Therefore, as part of internal cost control / production savings and preparation for the energy audit [48], entrepreneurs (with individual production specifics) should keep records of the total consumption of energy carriers. In addition, determination of the electric energy consumption of production processes allows entrepreneurs to compare their production expenditures with those of the competition. The extent of implementing the sustainable energy policy in the meat (food) industry is most of expressed by the specific energy consumption-SEC [49][50][51].
Results of measurements and calculations of the heat balance of the fast chilling chamber (FChCh) and the chilling tunnel (ChT) demonstrated that the heat of penetration, lighting heat, and engine work heat in these rooms had constant values (FChCh = 33.94 kJ/h, ChT = 23.26 kJ/h). The total amount of heat necessary to be removed from the chambers depended on the duration of their work and the amount of raw material being chilled (halfcarcasses, quarter-carcasses). In turn, the weight and type of chilled raw material, as well as the final temperature of chilled meat (0-2˚C) determined the amount of heat that had to be Table 4 removed from it [52,53]. Apart from the mentioned factors, the total amount of heat depended on additional heat losses associated with door opening (10-15%) and heat emitted by ventilation fans (20%)- Table 5.

Experimental group
Results of measurements and calculations demonstrated that the total heat needed to be removed from chilling chambers increased along with the increasing number of carcasses intended for chilling (10-114 animals). However, ca. 11-fold increase in the number of chilled carcasses caused over 4-fold increase (391.54-1717.23 kJ) in the fast chilling chamber (FChCh), and a 2-fold increase (660.52-1331.37 J) in the chilling tunnel (ChT) in the amount of heat that had to be removed from these rooms (Table 5). This indicates that the heat load of the chilling chamber (production of the amount of chill and electrical energy consumption) increases along with the increasing load of products placed in it within a day, with the increasing specific heat of the chilled product, and with the increasing difference of temperatures [47]. Keeping the temperature lower than the ambient temperature in cold rooms requires also discharging heat energy from them, which entails higher losses (longer work of compressors) that will generate greater costs of energy consumption. The amount of heat that needs to be Table 5  removed from chilling chambers is also strongly affected by the ambient temperature related to the season of the year. According to Neryng et al.

No
[54], greater by ca. 32% electric energy consumption per chilled product unit is recorded in the spring than in the winter. The higher temperature around the chilling chamber contributes to, e.g., slower cooling of half-carcasses after slaughter, the higher temperature in the chamber before the chilling process, and a higher value of the transfer heat.
The results of analyses of the energy consumption of production per carcasses are in contrast to those of the total amount of heat calculated (Table 5, item 9). With an almost 9-fold increase in the number of carcasses, it was necessary to remove approx. 3 times less heat units from the fast chilling chamber (39.15-15.06 kJ/carcass) and more than 5 times less heat units from the chilling tunnel (60.05-11.68 kJ/carcass). This was due to the large proportion of heat associated with the housing losses (light transmission, motor operation, door opening) in the total amount of heat, that needed to be removed from the chambers. This, in turn, was related  Table 5 indicate that with the comparable weight of chilled meat, i.e. 10,553 kg and 6,616 kg, the SEC values obtained in the present study were substantially lower and reached 0.103 and 0.112 kJ/kg. This indicates that the optimal variant of beef production established in the present research is far more energy efficient than other production variants presented in literature.
Considering that one chilling cycle in a fast chilling chamber lasts 2.5h, its prolongation by 1h with carcass number of 10, by 2-3.5h with carcass numbers of 22 and 26, and by 6.5h with carcass number of 114 was due to the stable time of cooling the chambers before they were filled with half-carcasses (regardless of the amount of chilled material) and the stable chilling time of one half-carcass in the chamber. Besides, the temperature decrease by ca. 20˚C in the fast chilling chamber and by ca. 18˚C in the tunnel (14 to 18h, Table 1) influenced the total unitary amount of heat that had to be removed from both types of chambers (FChCh and ChT). During chilling, the total heat capacity per carcass was 99.20 kJ with 10 carcasses, and ca. 4-fold lower (26.74 kJ) with 114 carcasses (Table 5).
An indispensable side effect of the meat chilling process are its weight losses. They generate economic losses, the magnitude of which depends on the chilling method type. According to Brown et al. [56], the cost incurred by small UK plants due to meat weight losses was higher than the cost of electricity. Therefore, quick chilling methods are recommended [13] to reduce meat weight losses and achieve the best efficiency of the chilling process (shortening the time, ensuring meat safety and quality).
Results of determinations of weight losses of meat (WLM) caused by water evaporation from meat surface during its chilling with the fast two-phase method demonstrate large differences between particular days of measurements assuming various numbers of animal carcasses to be chilled ( Table 1). The highest WLM (4.8%) was obtained during chilling 10 beef carcasses, presumably due to the highest volume of water evaporated from half-carcasses in the fast chilling chamber. On days 2, 3, and 4 of measurements, when the number of chilled carcasses was 21-42, the WLM ranged from 3.1 to 3.5%. The results of measurements allow concluding that smaller WLM were recorded (2.3-2.6%) with carcasses having a higher meat content, more densely packed, and touching one another in the chamber than with the lower number of chilled carcasses (Table 5). According to Klettner [57], the weight loss of meat caused by its chilling with the fast one-phase method was at 1.6% and was higher by ca. 0.3% from the WLM recorded during the shock chilling process (from -3 to -5˚C/2h, 0˚C/16-22h). However, the carcasses intended for chilling should not be moist because of the possibility of meat surface freezing during chilling with this method. As reported by Żywica, Banach, & Gornowicz [55], WLM obtained using the shock and fast chilling methods reached 2.2 and 3.6%, respectively.
Summing up, the analysis of energy consumption of the examined meat production variant-coupled use of HVES and fast chilling method, showed that it enabled achieving a lower value of the specific energy consumption and WLM of 2.8% at highly diversified amounts of meat chilled in the chambers, compared to the values reported by other authors. It may thus be concluded that this production variant contributes to the efficient energy consumption in the production process and sustainable company management through, among others, reducing the carbon footprint of beef and energy costs.

Conclusions
1. The use of HVES and the fast chilling method at the slaughter line enables producing highquality meat, reducing expenditures related to electric energy consumption, and increasing work effectiveness of cooling appliances compared to the slow and accelerated chilling methods. The coupling of these techniques is recommended for both investment and modernization undertakings in pursuit of sustainable meat production.
2. The intensive chilling of beef to ca. 20˚C within 2.5h and the break in the chilling process until temperature equalization between its outer and inner layers allow the producers for getting the optimal rate of pH changes, rational quality management and changes in production organization, mainly through increasing chilling speed and reducing storage surface and expenditures for staff wages.
3. The heat balance of fast chilling chambers (FChCh+ChT) demonstrated that achieving the desired final temperature resulted in several times lower value of the specific energy consumption at the lowest amount of chilled meat, and in higher energy efficiency of the process at the highest amount of chilled meat.
4. Evaluation of the industrial quality indicators of beef and energy consumption of its production process in the variant with the coupled use of own-construction device for HVES at the slaughter stage and meat chilling with the fast method demonstrated that it allowed accomplishing the principles of sustainable beef production. In addition, it was found to indirectly contribute to mitigating the adverse effects of the meat production plant on the natural environment and to improving its image.