The Effect of Sodium Bicarbonate on the Water-holding of Pork Loin

Xiao-Liu Chu, Peter Török, Carl Paterson and Tom Aikens

Imperial College London


We investigated the effect of a 3% concentration solution of sodium bicarbonate (treatment A) and pure sodium bicarbonate (treatment B) on pork loin samples. These were compared to a 5% salt brine (treatment C), controls consisting of tap water (treatment D) and untreated samples (treatment E).

Pork loin samples under treatment A experienced a (50 ± 14)% higher increase in weight after 390 minutes of marinating as compared to treatment D. This suggests an increase in water holding capacity. The result of heating pork samples subjected to treatments A and B lost, in comparison to control, (39 ± 7)% and (33 ± 10)% less in weight respectively. This implies that sodium bicarbonate caused the meat to better retain its water content. The mechanical testing also showed that meat fibers from treatment A required a smaller force to cut compared to treatment D. These results indicate that sodium bicarbonate significantly improved the quality of the pork loin.

Keywords: Pork; Bicarbonate; Water-holding capacity

1 Introduction

It is common in the food industry to enhance the quality of meat, making it more desirable for consumers. A widespread technique includes injecting salt and phosphate solutions into meat post-mortem to enhance the water content and the general appearance of meat products (Davis et al., 2004; Jensen et al., 2003; Robbins, 2002). However, additives such as salt risk exposing consumers to possible health hazards related to an increasing intake of sodium chloride. In the UK, the average daily consumption of salt already exceeds the recommended daily intake, of which a major source of sodium chloride comes from processed foods (Chobanian, 2003; Scientific Advisory Committee on Nutrition, 2003). Studies have shown a clear link between an excess salt intake and long-term illnesses such as hypertension (Appel, 2001).

A few studies have been carried out on the potential of sodium bicarbon- ate to possibly combine or replace the salt and phosphate compounds used to improve meat quality (Sheard & Tali, 2004; Wynveen et al., 2001). Wyn- veen et al. showed that sodium bicarbonate injected post-mortem appeared to have improved the quality of pale, soft, exudative pork. With this study, we want to investigate water-holding capacity and qualitative improvements of normal pork. However, most of the research in this area has been focused on the effect of salt and phosphate alone (Collignan et al., 2001; Hamm, 1960; Offer and Trinick; 1983; Schmidt et al., 2008), despite research indi- cating that sodium bicarbonate has additional desirable qualities, such as inhibiting bacterial growth (Corral et al., 1988).

Water-holding capacity is often described as the ability of the meat tissue to hold on to its water content when subjected to an external force, which varies with the method used to test the meat (Hamm, 1960). The swelling of myofibrils within muscle cells is described to be the main cause of increase in water content (Hamm, 1960; Offer and Trinick, 1983), in particular by the expansion of the inter-filament space of actin and myosin. These changes are in turn argued to be mainly influenced by the structure and charges of the protein (Hamm, 1960). Offer and Trinick (1983) suggest that the mechanism of the swelling is due to the presence of chloride ions in between the actin and myosin filaments causing electrostatic repulsion, adding that the cross-bridges form a constraint to this swelling. They conclude that the presence of sodium chloride also helps to weaken the binding of myosin to actin. These results support the use of sodium chloride in the food industry as a tenderising agent. However, it still remains unclear if sodium bicarbonate has a similar effect on the change of protein structure, specifically myofibrils. It has nevertheless been shown that meat treated with sodium bicarbonate post-mortem has experienced an increase in water content and decrease in shear force value (Sheard & Tali, 2004; Wynveen et al., 2001). The aim of this study is to gain a further understanding of the changes in meat tissue caused by sodium bicarbonate, and compare with the changes produced by sodium chloride. A proper understanding of the process would allow applications on a bigger scale in the food and restaurant industry.

2 Materials and methods

2.1 Preparation of meat samples

Raw skinless and boneless pork loin, from the same animal at any one time, was purchased from a local butcher. The samples were cut into rectangular cuboids (≈ 0.8 cm × 3.5 cm × 7.5 cm). The preparation took place in room temperature (22◦C).

2.2 Preparation of treatments

Marinade solutions were prepared with sodium bicarbonate (NaHCO3)) and tap water, and sodium chloride (NaCl) and tap water. A 3% and 5% w/w concentration marinade solution for sodium bicarbonate (treatment A) and sodium chloride (treatment C) respectively, were used. A tap water solution of the same weight was prepared as control. Each meat sample was placed in 100 g of the solutions (which corresponded to approximately 4× the mass of the meat sample) cooled to 3◦C.

A second treatment method was prepared, in which a ’dry marinade’ con- sisting of sodium bicarbonate powder, was used to test its effect on cooking loss of pork loin (treatment B). 0.5 gram of sodium bicarbonate was sprinkled inside a rectangular frame of 6.5 × 3.0 cm. The dimensions of the frame were made to match the surface area of the pork samples. The layer of sodium bicarbonate was evened out to create a rectangular surface on which the samples were placed, covering their surfaces completely.

2.3 Method of measuring weight gain

The individual pork samples were placed in containers with treatment A, treatment C and the control consisting of tapwater (tretament D). The weight was measured using a high precision scale (PS-60, Fisherbrand). Each sample was removed from its container using tweezers, let drip for 5 seconds, and then placed on the scale. The measurements took place at regular intervals every 30 minutes. The weight gain measurements were performed both at 3◦C and for comparison, at 22◦C. The scale was cleaned in between measurements to avoid contamination between the marinade and control samples.

The normalised weight gain over time was determined for both tempera- tures, using the following formula:

Normalised weight gain = (Wt − W0)/W0A                                                                   (1)

where the symbols have the following meaning:
Wt – the weight of the sample at time t
W0 – the initial weight of the sample (t = 0)
A – the initial surface area of the sample, determined by mechanical mea- surement

Note: For the rest of the paper, the normalised weight gain will be stated in percentage (%) only, but all the values are per square centimetre (cm2).

2.4 Method of heating

A water bath containing 18 litres of tap water was preheated up to 60◦C using an immersion circulator (SV100, Grant Instruments). Pork loin samples were immersed in treatment A at 3◦C and after 18 hours, they were placed into the water bath. Weight measurements were taken every 30 minutes, using the method described earlier. Treatment C and D were put through the same procedure to serve as comparison and control.

The effect of heating with treatment B was investigated as follows. Two rectangular sides of each pork sample were selected to be covered in sodium bicarbonate. Each surface was placed on top of a sodium bicarbonate layer once, for the same length of time, ensuring that the samples were treated identically. Immediately afterwards, the samples were individually vacuum packed (model V410 TI, Sammic) for heating. Treatment E, non-treated samples, served as control. Each pork sample was heated at 60◦C for different periods of time; 30 min, 60 min, 90 min, 120 min, 150 min and 180 min. Weight measurements before and after heating were taken.

2.5 Method of Mechanical testing

A separate set of pork loin samples, with surface area (73 ± 4.5) cm2, was prepared for mechanical testing. Pork loin samples were left in treatment A at 3◦C. The same procedure was performed with treatment C and D for comparison. The samples were treated for 12 different periods of time (30 min, 60 min, 90 min, 120 min, 180 min, 240 min, 360 min, 420 min, 540 min, 720 min, 960 min, and 1440 min). Treated samples were removed from their individual package, separately vacuum-packed and kept at 3◦C before being placed directly into the water bath. Weight measurements before and after marinating were taken.

All samples were heated for 158 minutes at 60◦C. The heating time was determined by adding the time required to heat the centre of a 20 mm thick slab (thickness of the samples) to within 0.5◦C of its surrounding temperature and to reach a 6D reduction for Listeria Monocytogenes (Salmonella and Escherichia Coli both have higher reduction rates at this temperature). This is to imitate the expected cooking time to prepare pork loin for consumption. The required time to heat the centre of a slab of 20 mm thickness, initially at 3◦C, to within 0.5◦C of 60◦C, takes 30 minutes and 14 seconds (Bleecker, 1995). To reach a 6D reduction in L. Monocytogenes at 60◦C, adds an additional 128 minutes of heating time (Juneja et al., 1997; Murphy et al., 2000).

Immediately after heating, the samples were removed from the container and placed in an ice water bath to prevent any furhter cooking. Mechanical testing was done using a biomechanical single column testing system (Model 5543, 5500 series, Instron). A 2 mm thick blade was fitted to the compression machine and allowed to cut through the centre of each sample at a speed of 30 mm/min. The relative mechanical load was measured as a function of relative distance moved by the blade every 0.1 seconds.

3 Results and Discussion

3.1 Weight Gain

The normalised weight gain of pork loin samples in various treatments was found using eq. 1 and plotted as a function of time and shown in Fig. 1 and 2. A summary of the weight changes due to various treatments can be found in Tab. 1. Pork loin samples exposed to treatment A experienced a significant increase in weight over time. Their respective controls showed no or significantly smaller weight increases. From Fig. 1, at 3◦C, the weight increase was (0.34 ± 0.06)% after 390 minutes, which is (50 ± 14)% higher than the controls. For comparison, the samples were also treated at room temperature. Treatment A samples showed a (0.62 ± 0.14)% weight gain after 390 minutes, which is (73 ± 20) % higher than for the controls. The water uptake was faster at the higher temperature, indicating a temperature dependence. Comparing with the meat samples prepared in treatment C (Fig. 3), it was observed that treatment A and C were very similar, with the samples that underwent treatment C showing a slightly faster increase in this particular case. These results suggest that sodium bicarbonate causes a similar increase in the flow of water into the meat tissue as sodium chloride and that the magnitude of the water uptake is comparable in the two cases, also mentioned by Sheard & Tali (2004).

The normalised weight gains plotted in Figs. 1, 2 and 3, all show approx- imately linear behaviour over the range plotted, with the fastest weight gain

Normalised weight gain of pork loin samples in 3% w/w sodium bicarbonate marinade over time, at 3◦C. 4 samples are shown in this graph (2 for treatment A and 2 for treatment D) with the dotted line showing the fitted curve. (Error: ±0.06% (treatment A);  ±0.02% (treatment D);
Normalised weight gain of pork loin samples in 3% w/w sodium bicarbonate marinade over time, at 3◦C. 4 samples are shown in this graph (2 for treatment A and 2 for treatment D) with the dotted line showing the fitted curve. (Error: ±0.06% (treatment A); ±0.02% (treatment D);

measured within the first 60 minutes of treatment. Samples left for an ex- tensive period of time were seen to reach saturation, with the rate of weight gain slowly decreasing.

Normalised weight gain of pork loin samples in 3% w/w sodium bicarbonate marinade over time, at 22◦C. 6 samples are shown in this graph (3 for treatment A and 3 for treatment D) with the dotted line showing the fitted curve. (Error: ±0.14% (treatment A);  ±0.11% (treatment D);
Normalised weight gain of pork loin samples in 3% w/w sodium
bicarbonate marinade over time, at 22◦C. 6 samples are shown in this graph (3 for treatment A and 3 for treatment D) with the dotted line showing the fitted curve. (Error: ±0.14% (treatment A); ±0.11% (treatment D);
Normalised weight gain of pork loin samples for different treatments; 3% sodium bicarbonate and 5% sodium chloride (w/w) over time, at 3◦C. Three samples are shown in this graph (one for each treatment) with the dotted line showing the fitted curve. (Error: ± 0.06 % (treatment A); ± 0.03 % (treatment C); ± 0.02 % (treatment D)
Normalised weight gain of pork loin samples for different treatments; 3% sodium bicarbonate and 5% sodium chloride (w/w) over time, at 3◦C. Three samples are shown in this graph (one for each treatment) with the dotted line showing the fitted curve. (Error: ± 0.06 % (treatment A); ± 0.03 % (treatment C); ± 0.02 % (treatment D)

3.2 Heating Results

Pork loin samples treated in marinade and subsequently heated for 18 hours were observed to lose weight over time. The normalised cooking loss was significantly different for the various treatment types (Fig.4). Treatment A samples lost weight at a slower rate than the samples treated with treatment C or treatment D. The weight loss was 0.22 ± 0.01 % after 30 minutes for the treatment A sample, which is found to be 49 ± 5 % less than that shown for treatment C and 39 ± 7 % less than treatment D. Surprisingly, treatment C samples lost weight at a faster rate than any other treatment, surpassing even the normalised weight loss of control samples after 200 minutes. A similar result was reported by Robbins et al. (2002), who observed a higher cooking loss for beef samples injected with sodium chloride and phosphate solutions.

Normalised weight loss of pork loin samples treated in different marinades (treatment A-E) and heated at 60◦C. 5 samples are shown in this graph (1 for each treatment) with the dotted line showing the fitted curve. (Error: ± 0.01 % (treatment A); ± 0.02 % (treatment B, C, D and E))
Normalised weight loss of pork loin samples treated in different marinades (treatment A-E) and heated at 60◦C. 5 samples are shown in this graph (1 for each treatment) with the dotted line showing the fitted curve. (Error: ± 0.01 % (treatment A); ± 0.02 % (treatment B, C, D and E))

In the same figure, it is seen that the pork loin samples treated with sodium bicarbonate powder (treatment B) showed a similar normalised weight loss during heating as treatment A, which after 390 minutes had lost 33 ± 10 % less weight than the control (treatment E). Both treatment A and treat- ment B improved the cooking loss. This suggests that leaving pork samples in a sodium bicarbonate solution for an extensive period of time does not improve significantly on the cooking loss.

These results imply that the change of the protein fibres caused by sodium bicarbonate causes an enhanced ability of the tissue to retain its water content, and that this change occurs rapidly. This is an important factor, be- cause the time period associated with marinating is one of the biggest concerns with the method in question (Barham et al., 2010). Whether or not this is related to a decrease in denaturation of protein fibres is yet to be confirmed.

From these results, it also appears likely that the structural changes in protein caused by sodium chloride are different from that produced by sodium bicarbonate. Consequently, the water uptake during marinating with respect to the two additives may also be caused by two different mechanisms.

Similarly to the weight increase during marinating, the weight loss curve resembles a linear behaviour after approximately 60 minutes and slowly ap- proaches saturation.

3.3 Mechanical Force Test results

The results of the mechanical force testing showed that the meat fibres of samples from treatment A and C reduced the cutting force compared to the control samples (treatment D). This test yielded slightly varied results depending on the marinating time of the test samples. Generally, the longer the pork loin sample had been immersed in its specific treatment, the smaller the force required to cut it.

Comparison of the average normalised weight gain after 390 minutes and cooking loss after 180 minutes caused by the various treatments on pork loin samples.
Comparison of the average normalised weight gain after 390 minutes and cooking loss after 180 minutes caused by the various treatments on pork loin samples.
The force to cut pork loin samples, marinated for 24 hours and heated at 60 ◦C, over distance.
The force to cut pork loin samples, marinated for 24 hours and heated at 60 ◦C, over distance.

Fig. 5 shows results for samples left in the different treatments for 24 hours prior to cooking. It is apparent that the samples in treatment A and C were cut through by the blade, indicated by the coarse portion of the curve. The sodium bicarbonate treated sample was cut at <20 N, which was the lowest measured value. The smooth parts on the curves indicate the compression of the meat sample due to the blade, clearly seen for the control sample in Fig. 5.

The obtained results were affected by factors such as alignment of tissue fibres with respect to the blade. It is however prominent that sodium bicar- bonate and sodium chloride treated samples indicate that there is a texture difference compared to the control. This could be related to the weight gain and decrease in cooking loss caused by the treatments, also mentioned by Sheard & Tali (2004), and Wynveen et al. (2001).

3.4 Appearance and eating quality of samples

The pork samples immersed in treatment A and C exhibited an increase in size, a few mm after more than 1200 min. This supports the significant increase in water content that was measured, suggesting higher water-holding capacity compared to their respective controls.

Also mentioned in similar studies (Bruce et al., 1996; Sheard & Tali, 2004), sodium bicarbonate treated samples exhibit a porous appearance after cooking. We observed the presence of gas in the sealed vacuum bag of sodium bicarbonate samples as the bag expanded during heating, from ≈ 1 ml to slightly less than 100 ml. We conclude that the thermal expansion of the gas present after vacuum packing could not account for the volume found inside after heating, which would theoretically only be ≈ 1.2 ml. This expansion occurred after less than 30 minutes, both for treatment A and B, suggesting that carbon dioxide gas was formed during heating, also mentioned by Bruce et al. (1996).

Treatment A samples displayed marks on the surface, along with pores between the bundles of muscle fibres. Treatment B samples lacked the surface marks, but exhibited similar pores. These were observed to increase in size with increasing cooking times. Gill & Penney (1990) suggested in their study that this effect is caused by the carbon dioxide gas dissolving in the meat tissue due to ionic effects, creating a change to the muscle tissue structure. Such a structure change results in a porous appearance of the meat sample. Bruce et al. (1996) suggested that the pores between the muscle fibre bundles are related to the development of carbon dioxide gas during the cooking process. Our results show that it is likely that sodium bicarbonate reacts with the proteins in the meat and form carbon dioxide as a by-product during the heating process. It is however unknown if the pores on the inside of the sample contained carbon dioxide gas and it can therefore not be determined whether or not the gas pressed apart the muscle bundles. This reaction can however change the protein structure such as to cause an effect on the water retention and hence the cooking loss. The details of the temperature dependence are still to be investigated. The existence of pores inside the meat for the sample that was covered in sodium bicarbonate powder indicates that the change of the myofibrils must have occurred on the surface, which consequently separated the muscle bundles within. The exact mechanism for this is should be subject of further study.

During heating, all samples shrank in size by different amounts. The most apparent difference was observed for control, whereas treatment A and B both gave much smaller changes. Treatment A exhibited a very small change even after cooking for over 7 hours. This is thought to be related to its higher water content and pores.

A test panel, consisting of people involved in this experiment, found that both sodium chloride and sodium bicarbonate caused the meat to become more succulent than that of control. The sample described as most succulent was that treated with treatment B. This was rather surprising since the samples in treatment A and treatment C had been allowed to gain weight prior to cooking. It was also noted that very little or no trace of sodium bicarbonate could be tasted.

4 Conclusion

Water holding capacity related to meat quality has previously mostly been studied for sodium chloride and phosphate solutions. Our investigation has shown that sodium bicarbonate can cause similar effects to meat tissue when applied under identical conditions, supporting the results of other studies in the area (Sheard & Tali, 2004; Wynveen, 2001). Our results show that sodium bicarbonate both increases the ability of the meat to take up and keep its water content. A significant decrease in cutting force further sup- port the fact that sodium bicarbonate causes a physical change to the meat tissue, which could possibly partly be caused by the higher water content and the formation of pores in the meat samples. The changes to the appear- ance and texture are comparable, but not entirely similar to that caused by sodium chloride. We can therefore not conclude that the explanation pro- vided by Offer and Trinick (1983), regarding the swelling of the sarcomere due to electrostatic repulsion provided by ions, may be applied to the case of sodium bicarbonate. It is clear however that our results show that sodium bicarbonate causes a change to the protein structure, which has an effect on the water holding capability of the meat. We also suggest that part of this alteration takes place between the bundles of muscle fibres and the space between them.

More detailed research into the mechanism of water flow and water reten- tion is required to fully understand the effects of sodium bicarbonate on meat tissue. The results are however seen to be very promising with regard to the food industry and enhancement of meat quality and safety for commercial use.

5 Acknowledgments

The authors are grateful to Dr Maria Charalambides for provision of mechanical testing laboratory equipment and Idris Mohammed for guidance using the equipment. XLC’s work was partly supported by the Nuffield Foundation.


This study was carried out in July, 2011


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