Basics of Dry Curing
Eben van Tonder
19 August 2018
See Bacon & the Art of Living, Chapter 02: Dry Cured Bacon
See Chapter 2 of Bacon & the Art of Living: Dry Cured Bacon
Whether one cures bacon or hams, the basic principles of dry curing remain the same. Kobus and I are staring dry cure products and here I try and understand what those principles are.
Lets put matters of meat quality and breed of pigs aside for a moment, as vitally important as they may be. This, for us, will be lesson 101. Meat quality and pig breed will form part of lesson 201. In this article, we stick to the actual drying process.
Drying is the main process in dry-cured products. The initial “drying” is accomplished by the action of salt which removes moisture through osmotic dehydration during the curing and resting step. This prepares the meat for further drying by “reducing the energy costs by partial water removal” (Petrova, et al., part 2, 2015) and water binding.
The two most important tools used to accomplish drying are air temperature and relative humidity. Air velocity is an important factor in drying food, but in dry cured products it plays a minimal role and should be kept very low (generally around 0.1–0.5 m s−1) to have minimal influence. Salt content and pH play important roles mainly as far as they affect the drying rate. It has been shown that the higher the salt concentration, the slower the drying rate will be (Petrova, et al., part 2, 2015) The role of pH in water binding has to do with the isoelectric point (pI). This is the point where equal positive and negative charges on the protein molecules result in a maximum number of bonds between peptide chains and a net charge equal to zero. The further away the pH range is from this point, the better the water binding will be and the slower the rate of dehydration. The Isoelectric point of Myosin is a pH of 5.4
The oldest form of dry curing was to apply a fixed ratio of salt, sugar, spices and sodium nitrate (saltpeter) to solid meat cuts such as hams. After World War I, nitrate started to be replaced by nitrite or a mixture between the two.
There are two main methods of salting. Either by unlimited salt supply or by exact salt supply, called Equilibrium Curing (EQ) method. “Salting is accompanied by an osmotic dehydration process. While salt is diffusing into muscles, the moisture is going out at the same time.” (Petrova, et al., 2015)
The mixture is rubbed in dry form onto the meat and kept at a temp between 2 – 4 deg C to cure. The curing mix is solubilized in the moisture present in the meat. As a very general guide, the rate of penetration of the salt into the meat is estimated at around 2.5cm/ week. (Pegg and Shahidi.; 2000) Meat should not be frozen during Curing or the Equalizing stage since it will hinder the diffusing of the salts.
Micrococcal reduces nitrate to nitrite which signals the start of the curing process, imparting the characteristic pinkish/ reddish colour and flavour to the meat. More than one application of salt is required and the meat must be turned over and restacked daily. (Pegg and Shahidi.; 2000)
There are another important differentiation between different hams related to salting. In certain hams, nitrate and/ or nitrite are added and in others, salt only is employed. I dealt with the reactions in salt only curing systems in a fair amount of details in an article which deals with the curing reaction sequence. The specific section is, Bacterial/ Enzymatic Creation of Cured Colour.
Equalizing or Post-Salt Resting
After the initial cure is complete, the excess cure is washed off with cold water and equalized for the same amount of time as the initial curing time at the same refrigeration temperatures of between 2 – 4 deg C. This is to ensure that the cure spreads evenly through the meat. (Pegg and Shahidi.; 2000) Temperatures remain low to avoid microbial spoilage.
Generally, this step is done for the same amount of time as the salting step. Some hams call for this step to be a few months. It generally depends on the ham size, the ratio of lean surface to mass, pH, the presence of intramuscular fat. The relative humidity is generally progressively decreased as this step progresses. Weight loss of between 4 to 6% can be expected. French hams are normally heated to between 22 and 24 deg C for a week to dry them and “fix” the colour. (Toldra, 2002).
If smoke is applied, the meat is first dried for 2 – 3 days, with high humidity around 66% to 75% with a very light breeze/airflow. High air velocities will influence the quality of dry-cured ham negatively. The surface layer of ham or bacon will dry out and collapse. “Internal and external diffusions should be the same to achieve an efficient and uniform drying process. The air velocity ought to be kept low, but the air circulation must be uniform to ensure uniform air temperature and relative humidity through the curing chamber. Otherwise, the meat could be spoiled by microorganisms.” (Petrova, et al., part 1, 2015) In the end, the meat needs to be tacky to the touch for the smoke to adhere. (RG)
Whether smoking is done or not, after equalizing, the meat should be rinsed off and dried before aging or maturation. The reason for this is that the meat pores should be closed leading to a hardening of the surface and a considerable reduction in the drying rate.
RG recommends an ambient temperature for dry cured bacon of between 7 – 13 deg C. After drying, the meat will be well prepared for the smoking or the ripening stage.
Traditionally smoking was done in regions where drying was more difficult. It imparts a characteristic flavour to the hams and acts as a preservative. In bacon production, it is interesting that smoking was applied as an additional preservative for bacon destined for long sea journeys. This is one of the reasons why countries far from England, accessed by such long voyages by sea, became accustomed to smoked bacon and green, unsmoked bacon is not generally known in those countries while in England and Europe, it is a well-known version of bacon, enjoyed till today. Here in South Africa, I had the request from many British Expats, to produce unsmoked bacon for them while for South Africans, this is never an option.
When doing cold smoking there is “no” heat at all. Do not run any heat through the chamber at all. All you need is a thin blue smoke. (RG)
Smoke duration is between 8 and 48 hours. Rest overnight at room temperature. In between cold smoking, hang back at room temperature and not in the fridge or you will be making product wet again. (RG)
Typically a total of 48 hours:
Day one 8 hours
Day two 8 hours
Day three 8 hours
Day four Rest
Day five 8 hours
Day six 8 hours
Day seven 8 hours
This will give you 48 hours (6 x 8 = 48) RG
Keep an eye on the colour of the bacon/meat when you take it out of the smokehouse and again the following morning before placing into the smokehouse —this alone will give you an indication of the depth/colour of the smoke you like — keep records. (RG)
The meat is now held in an air-conditioned chamber and ripened. Depending on the objective and the product, this can take anything from 14 days to 3 years. The longer time it takes to mature, the better the quality will be. “Increased time of ripening gives a higher degree of enzymatic degradation, contributing to taste and flavor of the final product and as a consequence yields a higher quality of dry-cured ham.” (Petrova, et al., 2015)
Generally, temperatures vary between 5 deg C and some take this up to as high as 14 or even 20 deg C. Relative humidity is between 70 and 90%. Temperatures for drying-ripening of hams are different since the drying and ripening stage flows one another since there is not normally a smoking step for most hams (there are some with a smoking step and bacon will usually have a smoking step). Even so, drying-aging temperatures for hams vary greatly. Iberian ham, for example, has the “drying–ripening split into three time intervals: the first phase is maintained at 6–16 °C, the second at 16–26 °C and the third at 12–22 °C. This temperature range with the adjusted air relative humidity provides necessary moisture diffusivity and allows the adequate activity of meat enzymes that leads to the formation of distinctive quality of the final product.” (Petrova, et al., part 2, 2015)
The higher the temp, lower the humidity and the higher the airspeed, the dryer the end product and the greater the weight loss. (Pegg and Shahidi.; 2000), RG
Ripening/ Maturing: Its purpose and the role of moisture and temperature
The biochemical reactions are mainly proteolytic and lipolytic which is responsible for the characteristic flavour development. (Pegg and Shahidi.; 2000) “During ripening, endogenous enzymes degrade proteins and lipids to amino and fatty acids correspondingly, which are mainly responsible for the flavor of dry-cured ham. Free amino and fatty acids are further degraded and converted by enzymatic and chemical reactions, including oxidation, to volatile compounds. Free amino acids contribute directly to taste, while further protein degradation products participate in the generation of many odorants. A total of twenty-eight odorants were identified in Iberian ham by Carrapiso et al. including aldehydes, sulfur- and nitrogen-containing compounds, ketones, esters, and alcohol.” (Petrova, et al., 2015)
The purpose of the salting and equalizing phase is for the salts to diffuse through the meat. In order for this to happen, there should be no drying and the meat must be at a temperature to prevent micro growth.
During the maturing stage, conditions must be favourable for the “rates of the enzymatic reactions” to achieve the final organoleptic characteristics of dry-cured ham.” (Petrova, et al., 2015) Dryness and temperature are both important and must be balanced carefully.
The dryer the meat becomes, the higher the salt concentration in the muscle tissue which will affect the rates of the enzymatic reactions. It is important to keep the conditions favourable for these reactions to run to completion till the required results have been achieved.
The two factors that must be balanced are the moisture content in the meat (regulated by the relative humidity) and the temperature.
The break down of a complete peptide to amino acids (protease) increases with the increase in temperature. In fact, most of the activities that will govern the final texture and product taste are increased with an increased temperature such as lipase activity and lipid oxidation rates. Lipid oxidation at moderate temperatures contributes to a better product and “promotes the generation of desirable chemical compounds” which are responsible for the distinctive final flavour of dry cured products. (Petrova, et al., 2015)
In order to balance temperature and moisture loss, hams in the ripening stage are often stored in cellars where it is damp and moist till the required qualities have been achieved.
The internal moisture in the meat is regulated by the relative humidity. “The vapor pressure gradient which occurs between the meat surface and the environmental drying air causes water evaporation from the surface and simultaneous diffusional water transport from the inner meat tissues toward the interfacial layer.” (Petrova, et al., 2015)
It is important for hams not to dry too fast. If the outer layer of the hams dry too fast, this retards the transport of moisture from deeper inside the ham to the outside and thus evaporation. This may cause the penetration and growth of microorganisms inside the hams, resulting in spoilage. It also negatively impacts the overall texture of the finished product. (Toldra, 2002)
The rate of moisture loss is measured by weighing. One other way to control it is to smear the ham with a layer of lard to prevent excessive dehydration once the desired weight has been achieved. Weight loss as high as 20% to 25% is not uncommon during this step and in relation to the initial warm ham, total loss at the end of this step can be as high as 32% to 36% or even slightly higher. (Toldra, 2002)
Ripening/ Maturing: Drying Kinetics
During the Ripening stage, the product loses the weight by dehydration and obtains the necessary textural and flavor characteristics by enzymatic activity. “Dehydration during the drying–ripening contributes to stabilizing the product by decreasing water content and, consequently, the water activity value.” (Petrova, et al., 2015)
We saw the impact of environmental parameters, such as airspeed, ambient temperature and relative humidity on the water activity and drying kinetics. The other factors impacting on these are the salt content of the hams and the pH value of the raw material.
Drying kinetics in dry-cured meat production is complex. When planning and managing the drying step, adequate modeling of water losses must be considered. There are two general approaches used to describe the drying kinetics:
The drying rate is governed by the rate of the two processes — heat and mass transfers. A balance between these two processes is crucial; otherwise, the quality of the product drops; for example, in the case when heat transfer is bigger than mass transfer, the effect called case hardening can appear. It leads to restricted water transfer in the product, and microbial spoilage can occur under the hardening case. (Petrova, et al., 2015)
- empirical (semi-empirical).
In the empirical approach, for hygroscopic products (products tending to absorb moisture from the air, relating to relative humidity or its measurement), the drying process is controlled by internal diffusion mechanism and the drying rate is so small that heat transport mechanisms are not influencing the process. (Petrova, et al., 2015)
Just as we have seen that the temperature at which hams are kept for ripening are stepped up to favour different reactions, relative humidity is handled in the same way. If it is maintained at too high levels, it can promote such undesirable effects as microbial contamination or mold growth. If it is too low, too quickly, case hardening will be promoted. Salting is usually done at RH of 75–95% and drying-ripening, at 65–80%. But it is stepped. For Iberian dry-cured ham, for example, “the first phase is maintained at 60–80 %, the second at 55–85 % and the third at 60–90 %.” (Petrova, et al., 2015)
Low relative humidity reduces the moisture content in the meat. It facilitates drying which helps to avoid microbial spoilage. On the other hand, low RH suppress proteolytic activity (break down of complete peptides to amino acids) since the reduction in water content along with water activity happens together with the increase in salt content.
Drying Room Design
Petrova, et al., 2015 did an excellent overview of drying room design and I give the section from their work verbatim.
Drying rooms allow’s for the production of hams that is not weather dependent. The most important drying agent is air and ensuring proper air flow in the chamber is of paramount importance. In large drying rooms, the control of airflow is notoriously difficult to control which is the reason why large rooms are divided into smaller rooms. Such is the importance of uniform air flow.
“The mass flow of the drying air must be enough to ventilate the whole drying chamber and to remove the evaporated water from the product. Since the drying air is generally the only energy supply into the drying chamber, the heat losses of the process must be also taken into account. In most cases, the local velocity in drying chambers for dry-cured ham is quite small in comparison with drying systems for other products due to the specific drying rate of hams. The size of the ventilation system is depending on the correct determination of the necessary mass flow of the drying air. An over-dimensioned ventilation system can result in a sufficient air flow and uniform air distribution, but causes also higher operation costs since more air than necessary is moved around in the system.” (Petrova, et al., 2015)
Traditionally, ambient air was used for drying products. This is still in use by large processors where local climate conditions allow this.
Below is a diagram representing such a system.
One may want to install a filter before the air inlet for hygienic reasons. If the outside air is too warm, the heat exchanger may be bypassed and a cooling system can be employed. Whichever way it is done, this is the main way that the temperature inside the drying room is regulated.
The other important factor to control is the humidity. “The humidity of the
drying air can be adjusted by mixing a part of the wet air at the outlet of the drying chamber with the fresh drying air; the mixture is supplied back to the drying chamber. By adjusting the mass flow of a heating or cooling medium through the exchanger and the degree of mixing between fresh and moist drying air, it is possible to obtain the necessary parameters of the drying air.” (Petrova, et al., 2015)
Closed-looped Drying System
When the drying air is circulated through the process in a closed loop, there will be no influence or disturbance with the ambient air and the process is more stable and needs less regulation after initial adjustments (see figure below). Still, the evaporated water which was taken up from the product needs to be removed from the drying air. Commonly, this is done by cooling the air down below its dew point, so that the moisture is condensed out. The dehumidified air needs then to be heated up again in a second heat exchanger to its desired drying temperature. The heat exchangers for cooling use a cooling medium which is tempered by a separate cooling system. The same principle can be also used for the heat exchanger for re-heating of the drying air; however, in some cases also direct heating is used here in order to avoid another heating system and to minimize heat transfer losses. The regulation of a closed loop system is done by controlling the mass flow of the heating or cooling media by their respective pumps. It is also possible to use excess heat from other processes in the production plant for reheating of the drying air; however, for cooling normally a refrigeration cycle is necessary. The investment costs for a closed loop drying system are moderate but can be high due to the need of two different thermal operations (cooling followed by heating) or when more sub-systems are needed. The drying efficiency of such system is therefore quite often lower than 30%. However, the operation is easy to control and the process is stable, which is why this system is very common in industrial applications.” (Petrova, et al., 2015)
Heat pump drying
“In a closed loop cycle, it is necessary to cool the air in order to dehumidify it so that the drying air can be used again. Heat pumps are characterized by the possibility to produce cooling energy at the evaporator and heating energy at the condenser. For the case of closed loop drying, this combined heat and cool load can be used to recover the drying energy (basically the latent heat of evaporation) and deliver this energy back into the drying process in the form of dehumidified and re-heated drying air. Heat pump drying consists of two loops: one closed loop for the drying air and one closed loop for the refrigerant of the heat pump (see figure below). At the evaporator of the heat pump, the drying air is cooled down and the moisture from the air is condensed. Energy is hereby transferred to the refrigerant, which is evaporated. The evaporated refrigerant is then compressed and can now be condensed back to the condenser at higher temperature. Hereby, the formally transferred energy is given back to the drying air which is then reheated to its initial desired condition. Since both loops are closed, it is necessary to install a second, external condenser in order to transfer the excess heat out of the system. The external condenser is normally installed parallel to the main condenser, and the mass flow is controlled by a three-way valve. The main source for the excess energy is the compressor, which also should be equipped with a rotation speed control in order to ensure optimum working conditions at varying heat and cooling loads. It is also recommended to install a bypass valve for the drying air, so that only the necessary amount of drying air is cooled and reheated; this makes the operation more efficient.
The drying efficiency of heat pump drying is normally between 80 and 90 % and therefore significantly higher compared to conventional HAAD or closed loop drying. Correctly designed and operated heat pump driers can save up to 80 % of the drying energy from conventional driers. However, installation costs are higher and the energy price for the primary energy can reduce the economic benefit. In Nordic countries, heat pump driers are widely used in dry-cured fish production since the 1980s, which kept the industry competitive on international markets. Some commercial household cloth driers are also equipped with heat pumps in order to fulfill energy requirements. It is expected that energy prices as well as political regulations will force the industry to implement this technology in larger numbers.” (Petrova, et al., 2015)
“Certain materials (e.g., silica gel) have very good sorption characteristics toward air humidity, which can be used for dehumidification of drying air. Still, the sorption material can only take up a limited amount of water from moist air and the regeneration of the sorption material is therefore of a key requirement for closed loop drying cycles with sorptionbased dehumidification. Most often, this technology is implemented as a slowly rotating sorption wheel, where one part of the wheel is dehumidifying the drying air, while the other part is regenerated by a separate regeneration cycle (see figure below). The regeneration cycle uses also the air, but at significantly higher temperatures, so that the water is desorbed from the sorption material and leaves the system. The only energy supply to the system is a heat exchanger in order to heat the regeneration air. Different available heat sources can be used as an energy source including the excess energy from other operations of the plants. Operational conditions of sorption wheels require low temperatures, high humidity of the drying air and moderate air flow rates, which confirms well with process parameter in dry-cured ham production. The drying efficiency of sorption drying is similar to closed loop drying or HAAD. The operational costs can be low, if excess heat from other process in the production facility is available as a heat source. The operation of a sorption wheel is not complicated, and the system can be controlled through the amount of ventilated drying and regeneration air as well as the capacity of the heat exchanger.” (Petrova, et al., 2015)
Comparison of different drying systems according to energy efficiency is presented below.
Evaluation of the Final Product
The “cala” process is involves using a bone needle, usually from beef or a horse to probe for spoilage. The Ham Master will perform the “cala” of the ham. After removing the needle, he will smell it to determine the hams quality. One of the things he will try and detect is any off-flavours. Once he approves the ham, it can be sold.
Pegg, R. B. and Shahidi, F.. 2000. Nitrite Curing of Meat. Food & Nutrition Press, Inc.
Petrova, I., Aasen, I. M., Rustad, T., Eikevik, T. M.. 2015. Manufacture of dry‑cured ham – a review Part 1 Biochemical Received: 10 April 2015 / Revised: 18 June 2015 / Accepted: 20 June 2015; Springer-Verlag Berlin Heidelberg 2015, Eur Food Res Technol DOI 10.1007/s00217-015-2490-2
Petrova, I., Aasen, I. M., Rustad, T., Eikevik, T. M.. Manufacture of dry-cured ham – A review Part 2 Drying kinetics modeling and equipment Received: 14 April 2015 / Accepted: 14 June 2015 / Published online: 25 June 2015, Springer-Verlag Berlin Heidelberg 2015; Eur Food Res Technol (2015) 241:447–458, DOI 10.1007/s00217-015-2485-z
RG Robert Goodrick. Private Communication