Traditionally, pigs were reared at home and slaughtered by the end of November or early December so that hams could be salted and then left for salt diffusion during the coldest months. During the spring and summer, hams were ripened and dried, becoming ready for consumption by autumn (almost one year of total process). The production sites were usually located in the mountains, with cool and dry weather conditions favoring this process. The windows of the rooms were opened or closed depending on visual and tactile assessment by an experienced operator. Of course, this method was transmitted from fathers to sons, but the subjective assessment resulted in a great variability in the final quality. Today, most modern factories use computer-controlled drying chambers that allow a full control of air speed, temperature, and relative humidity. The final quality depends on the length of the process because time is needed for the enzymatic and chemical development of flavor, as will be discussed later. In general, the process is as schematized in Fig. 1 and consists of the following stages.
Pork legs are classified when they arrive at the factory in order to facilitate their correct processing. This classification depends on the particular area but is usually based on ham weight, pH, and fat thickness (3). The composition of fatty acids in the fat mainly depends on the animals's feed (6) and, to a minor degree on the crossbreed used (7) but is of extreme importance for correct flavor development. Depending on the composition of certain polyunsaturated fatty acids, hams may develop an adequate flavor or may experience undesirable oxidation and develop rancid off-flavors. Fat may be controlled through the iodine index (as an indicator of unsaturation) and the acid index (as an indicator of freshness). The exudative hams, with a condition known as pale, soft, and exudative (PSE), have a low water-binding capacity and may sustain important weight losses, substantially higher than normal hams (8). In addition, PSE hams have a pale color and a wetted surface that facilitates the dissolution and penetration of the added salt but, on the other hand, results in an excessively salty taste. Other groups of hams, those having high ultimate pH and known as dark firm, and dry (DFD), must be rejected in order to avoid microbial contamination.
Although the modern meat industry uses standard pigs, hams produced from older pigs usually are of better quality due to the higher amount of myoglobin (improved color) and different enzyme profile (better flavor profile) (9,10).
The skin is partially removed, leaving an area where salt will penetrate and water will evaporate. Hams are then registered to facilitate traceability, subjected to pressing rollers for bleeding, and left for 1 or 2 days under refrigerated storage (2-4 °C) to reach a uniform temperature. Frozen hams are allowed to thaw till a temperature of about —4°C is reached inside the ham.
This is a short stage during which nitrate is added to the hams in the form of a curing salt (sodium chloride with 4% potassium nitrate) for a few minutes within a rotary drum (i.e., Spanish Serrano hams). The curing salt may be directly applied in the salting stage (i.e., for French and country-style hams). Nitrate and/or nitrite are used as protective agents against botulism (11). Nitrate is reduced to nitrate by the action of nitrate reductase, a bacterial enzyme present in the natural flora (i.e., Micrococcaceae) of ham. This reduction is slow due to the low bacterial counts. Further formation of nitric oxide is achieved at slightly acid pH, as found into the ham and favored by curing adjuncts, such as ascorbic or erythorbic acids, that act as reducing substances. The maximum amount allowed in the European Union is 150 ppm potassium nitrate or 300 ppm for combination of potassium nitrate + sodium nitrite, and in the United States 156 ppm sodium nitrite (1/4 ounce per 100 pounds of meat). In some cases, the use of nitrate and/or nitrite is banned (i.e., Italian Parma ham).
Salt inhibits the growth of spoilage microorganisms by reducing the aw; it also imparts a characteristic salty taste and increases the solubility of myofibrillar proteins. The main objective of the salting stage is to supply the necessary amount of salt to the outer surface of the hams. Absorbed salt is then slowly diffused through the whole piece during the post-salting stage. The amount of salt may be tightly controlled, on a weight basis, allowing time enough for its penetration into the piece (exact salt supply). So, hams are weighed one by one and the exact amount of salt per kilogram of ham is added on the lean surface. For instance, Parma hams receive 20-30 g medium-grain salt per kilogram on the lean surface and 10-20 g of wet salt per kilogram on the skin (12). Then, salt is hand-rubbed and left to be absorbed into the ham (14-21 days, depending on size).
In other cases, the amount of salt is undetermined but time of salting is strictly controlled. Hams are entirely surrounded by rough sea salt or refined mineral salt and then placed by layers into stainless steel bins with holes for the elimination of drippings. Salt may be rubbed onto the lean surface, and the hams are placed on shelves. This stage may last up to 13 days under refrigeration with 3-4% weight losses. In some cases, hams are salted again. Once the salting stage is finished, the excess salt is removed by brushing and water rinsing.
The main objective of this stage is to achieve salt equalization through the entire piece. The required time may vary between 40 and 60 days, depending on many variables such as the size of the ham, pH, amount of fat, and conditions in the chamber. The relative humidity in the chamber is progressively reduced with time, and the typical weight losses are around 4-6%.
The use of smoke is one of the oldest preservation technologies, and it is used for short-term processed hams like American country-style or German Westphalia ham. The use of smoking is typical in areas where drying was originally more difficult (i.e., Northern countries) and gives a particular flavor to the hams. The smoke compounds also protect hams against molds or yeasts growth due to their bactericidal effects.
Hams are placed into modern computer-controlled drying chambers; some may contain up to 30,000 hams per chamber. Temperature, relative humidity, and air speed must be as homogeneous as possible and are carefully controlled and registered. Each type of ham has a specific set of variables along time of processing. For instance, Spanish hams are subjected to a progressive slow increase in temperature whereas French hams are heated to 22-26°C just after the post-salting stage. In all cases, these conditions allow the action of the endogenous enzymes, as will be described later. The length of the process depends on the type of ham (pH, size, amount of intramuscular fat, etc.) and drying conditions. The final expected weight loss (around 32-36%) is usually achieved within 6 to 9 months. Then, hams are covered with a layer of lard to avoid further dehydration and prevent any growth of molds and/or yeasts on the outer surface. Hams quality is monitored through a sniff test consisting of the insertion of a small probe in a specific area of the ham prone to spoilage and immediately smelled by an expert for detection of any off-flavor (12). The rapid development of commercial electronic noses and probes to get an objective assessment of flavor quality has led to their increased use for quality classification of hams (13).
Hams of high quality are further ripened in cellars for several months under mild conditions in order to get a full, rich flavor development. This is the case with Iberian hams, which may undergo 24 to 30 months of total processing time.
Hams may be sold either as an entire piece (usually those of higher quality) or boned. Commercial distribution of sliced ham in vacuum-packages or under controlled atmosphere is increasing very fast. Boned hams are usually vacuum-packaged and distributed through retailers for final cutting into pieces or slices (see a slice in Fig. 2). Hams are sliced by retailers or directly by consumers at home.
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