China has a long, glorious history and a splendid dietary culture. Many traditional meat products have been developed in China, among which Jinhua ham is the most famous. Jinhua ham has an attractive color, unique flavor and bamboo leaf-like shape. Its rose-like muscle, golden yellow skin and pure white fat make Jinhua ham one of the most preferred items in Chinese cuisine. The processing technology of Jinhua ham was introduced to European countries by Marco Polo during the 13th to 14th century and had an important impact on the development of drycured ham processing technology outside China.
Jinhua ham is formed and produced in Jinhua District, Zhejiang Province in China. The earliest legend regarding the method of processing Jinhua ham may be traced back to the Tang Dynasty (A.D. 618–907); however, the name “Jinhua ham’’ was formally bestowed by the first emperor of the South Song Dynasty about 800 years ago (Wu, Sun, & Sun, 1959). Typical Jinhua ham processing generally takes 8–10 months, starting in winter and finishing in autumn of the following year. During a long ripening process, muscle protein and fat are hydrolyzed to some extent by internal enzymes, and many small peptides, free amino acids, free fatty acids and volatiles are produced, which eventually contribute to the unique flavor of Jinhua ham.
Jinhua ham is traditionally processed under natural conditions where air temperature and relative humidity depend on climate and weather condition. Its unique quality is not only due to the elaborate processing technology, but also related to the unique local geographic terrain and climate. About 70% of the Jinhua District is mountainous with four distinct seasons. Air temperature fluctuates regularly without extremes, which is desirable for processing dry-cured meat products. Usually, air temperature in winter is between 0 and 10 C, favorable for ham salting. In spring, air temperature goes up to 20 C or higher, suitable for sun-drying. Summer in the Jinhua region is relatively hot. The temperature can reach 40 C, ideal for ham ripening. Post-ripening begins in autumn when the temperature falls. The entire processing of Jinhua ham, beginning in winter and finishing in the following autumn, takes 8–10 months.
Processing of Jinhua ham consists of six stages: green ham preparation, salting, washing, sun-drying and shaping, ripening, and post-ripening. In all, there are more than 90 steps within the six stages.
1. Green ham preparation
Traditionally, only hind legs from the Jinhua ‘Liangtouwu’ pig or its cross offspring could be used for producing Jinhua ham. Desirable legs should be fresh, with a thin skin and slim shank bone, well-developed muscle, and a thin layer of white fat. Broken bone should be particularly avoided. The exposed part of the bones as well as the fat, tendon and muscle membrane on the meat surface of selected legs are removed and the hams then trimmed into a shape like a bamboo leaf. The remaining blood should be squeezed out. A leg weight of between 5.5 and 7.5 kg after trimming is preferred .
2. Salting
Salting is a critical stage in Jinhua ham processing and inappropriate salting may cause spoilage. Ambient temperature and humidity have great effects on the salting process. With regard to temperature, it’s difficult for salt to penetrate meat when it is below 0℃, while fast growth of microbes will occur when temperature is above 15℃. With regard to humidity, when ambient humidity is below 70% RH, undesirable water loss will occur, which also causes insufficient salt penetration. When ambient humidity is above 90%, salt will flow away in the form of brine, causing pastiness on the surface of the ham. Therefore, the desirable ambient temperature and humidity for salting is 5–10℃ and 75–85% RH, respectively. Average duration of salting is about 30 d, varying from 25 d for small hams (<5 kg) to 35 d for large hams (more than 8 kg), during which time each ham is salted 5–7 times. Usually only dry salt is used during salting, however, nitrate may also be used during the first two times of salting when irregular weather condition is encountered.
3. Soaking and washing
The purpose of this procedure is to remove excess salt and wash off any dirty substances on the surface of the meat. Usually, hams are initially soaked in water for 4– 6 h and then washed with bamboo brushes. Water is changed after initial washing, and hams are again soaked in water for another 16–18 h.
4. Sun-drying and shaping
Achieving appropriate dehydration is the objective of sun-drying as insufficient dehydration may cause spoilage.For balance, a pair of hams of similar weight are tied with a rope and hung on a rack. Sufficient ventilation and exposure to sunshine are considered when positioning the hams. When hams are hung, hoofs are removed, water and dirt on skin is razed off with blade and the brand is sealed on the skin. Then hams can be removed from the racks and shaped into a bamboo leaf-like shape. Sun-drying can be terminated when hams start to drop liquidified fat, which generally requires about 7 sunny days.
5. Ripening
Ripening is the key process for generating ham flavor substances. During this period, muscle proteins and fat are hydrolyzed mainly by endogenous enzymes, which results increased amounts of peptides, free amino acids and free fatty acids. These products constitute the main part of ham flavor substances and may continue to react with one other or be hydrolyzed to produce volatiles that contribute to the unique aroma of Jinhua ham. Ham pairs are fastened to a centipede rack (named because of its centipede-like shape) with the meat surface toward the windows in the ripening room. Ham quality is susceptible to the microclimate in the ripening room. High temperature with low humidity stimulates weight loss and fat oxidation, while high humidity may result in ham spoilage. On the other hand, low temperature, especially in the later phase of ripening, slows aroma formation in hams. The ripening room should be well ventilated. Room temperature should increase from 15 Cto37 C gradually and humidity is controlled to within 55–75% during the 6–8 months ripening. During ripening, skin and muscle shrink to some extent because of moisture loss and bones around joints protrude out of the surface of the meat. Therefore, hams are usually removed from the centipede rack and retrimmed in midApril. This is normally the last shaping and it requires cutting off protruding bones, superfluous skin and fat. After reshaping, hams are retied to the centipede rack for completion of ripening. Usually ripening process terminates in mid-August when temperatures begin to drop.
6. Post-ripening
After ripening, the surface of the meat becomes very dry and is often covered with a thin layer of mould spores and dust. For this reason, hams are first brushed clean and a thin layer of vegetable oil applied to soften muscle and prevent excessive fat oxidation, then stacked with skin side up for post-ripening. The post-ripening stage is a process designed to stabilize and intensify ham flavor; it is carried out in a warehouse and usually takes two months. During post-ripening, the ham piles are turned over from time to prevent unexpected fermentation, which affects ham quality.
7. Grading and storage
It’s common practice to grade Jinhua ham into different categories according to quality, depending primarily on aroma intensity. A grader appointed by the government assesses ham’s aroma by inserting a bamboo probe into a ham and smelling the probe when removing it. There are three fixed locations: ‘up’, ‘middle’ and ‘lower’ position on a ham for this special purpose. Jinhua ham can be stored for years and peak flavor is reached when stored for around 12 months.
Traditional Jinhua ham processing consists of six stages: green ham fabricating, salting, washing, sun-drying and shaping, ripening, and post-ripening. Intense proteolysis, lipolysis and oxidation reactions occur over the course of processing and, as a result, many characteristic volatile flavor compounds are produced. The content of free amino acids in final Jinhua ham products is 14–16 times that of green ham, and 191 volatile compounds have been identified during processing. Endogenous enzymes may play an important role in the development of the characteristic flavor of Jinhua ham.