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Dry-aged beef
Dry aging beef at a steakhouse
Dry-aged beef is beef that has been hung or placed on a rack to dry for several weeks. After the animal is slaughtered and cleaned, it is hung as a full or half carcass. Primal (large distinct sections) or sub primal cuts, such as strip loins, rib eyes, and sirloin, are placed in a refrigerator unit, also known as a "hot box". This process involves considerable expense, as the beef must be stored near freezing temperatures. Subprimal cuts can be dry aged on racks either in specially climate-controlled coolers or within a moisture-permeable drybag. Moreover, only the higher grades of meat can be dry aged, as the process requires meat with a large, evenly distributed fat content. Because of this, dry-aged beef is seldom available outside of steak restaurants and upscale butcher shops or groceries. The key effect of dry aging is the concentration and saturation of the natural flavour, as well as the tenderization of the meat texture.
The process changes beef by two means. Firstly, moisture is evaporated from the muscle. The resulting process of desiccation creates a greater concentration of beef flavour and taste. Secondly, the beef's natural enzymes break down the connective tissue in the muscle, which leads to more tender beef.
The process of dry-aging usually also promotes growth of certain fungal (mold) species on the external surface of the meat. This does not cause spoilage, but rather forms an external "crust" on the meat's surface, which is trimmed off when the meat is prepared for cooking. These fungal species complement the natural enzymes in the beef by helping to tenderize and increase the flavor of the meat. The genus Thamnidium, in particular, is known to produce collagenolytic enzymes which greatly contribute to the tenderness and flavor of dry-aged meat.
Dry-aged beef is typically not sold by most supermarkets in the U.S. today, because it takes time, the meat loses weight, and there is a risk of spoilage. Dry-aging can take from 15 to 28 days, and typically up to a third or more of the weight is lost as moisture. This type of beef is served in higher-priced steakhouses and by select restaurants.
Dry-aging can be done at home under refrigeration by three means: open air, with the presence of salt blocks, and with the use of a moisture permeable drybag to protect the meat while it is aging. Since the mid-2010s, some chefs have experimented with a "quick" or "cheat" dry-age by coating a cut of beef with ground koji (rice inoculated with Aspergillus oryzae, traditionally used to ferment savory sauces) to simulate the effect of traditional dry-aging; the results are not quite the same, but can be achieved within 48 to 72 hours. The koji technique can also be applied to chicken and shrimp.
When dry aging using a moisture-permeable material, surface mold growth is not present, flavor and scent exchange within the refrigerated environment is not a concern, and trim loss of the outer hardened surface is measurably reduced. The flavor and texture profile of the beef is similar on all dimensions to the traditional open air dry-aged results.
Historically, it was common to store mutton or beef joints at room temperature for extended periods; even after the invention of refrigeration hanging sides of beef in large coolers for a few weeks as part of the processing was standard.
Wet-aged beef
Wet-aged beef is beef that has typically been aged in a vacuum-sealed bag to retain its moisture. Since the 1970s, with the development of vacuum packing machines and related technology, this has become the dominant mode of aging beef in the US and UK. It is popular with producers, wholesalers and retailers because it takes less time: typically only a few days and there is no moisture loss, so any given piece of meat sold by weight will have a higher value than a dry aged piece where moisture loss is desired for taste at the expense of final weight. The beef is usually kept for a period of 4 to 10 days in wet aging. Modified-atmosphere packaging (MAP) is usually employed for the vacuum packaging of meat; typically between 60 and 80 percent oxygen to retain its appetizing color, with red meat such as beef needing a higher oxygen level than less vividly colored meat such as pork. The vacuum packed beef is stored at a temperature of 32 to 45 °F (0 to 7 °C).
熟成の過程
代表的な牛肉の乾燥熟成プロセスとしては、ブロック又は枝肉（半身）などを乾燥熟成庫内に一定期間貯蔵する。庫内の温度を0 - 4℃、湿度は80%前後に保つ。常に肉の廻りの空気が動く状態を作り、その中で14 - 35日間熟成させる[4]。帯広畜産大学の島田謙一郎准教授の研究によれば、35日以上の熟成が望ましいという[5]。
温度が高ければ肉は熟成ではなく腐ってしまい、低過ぎれば凍ってしまい、熟成にならない。除湿や通風も、肉の水分活性を下げて腐敗を防ぐためである。そのため温度調整にも繊細な手間が掛かり、気候の変化にも影響を受けやすい。熟成期間中に、肉の中にある酵素等の働きで肉の繊維（蛋白質）がゆっくりと壊れてペプチドやアミノ酸に変化し、旨味が増すとともに肉が柔らかくなっていく。
保管中にカビが自然に生えるだけでなく、保管庫内に置いた数年物の肉についたカビを意識的に肉に移して、熟成を促すこともある。どちらの場合でも、カビが広がった肉の表面近くを調理前に削り取る「トリミング」が行われる[6]。熟成に適したカビの胞子を付けて、有毒なカビや腐敗・食中毒菌の侵入を防ぎつつ熟成を進められる「エイジングシート」も開発されている。熟成肉専門店を運営するフードイズム社と明治大学農学部教授の村上周一郎が共同開発したエイジングシートは、ヘリコスティラム属の菌を、村上と協力したミートエポック社はケカビを[9]利用する。
肉に元々含まれる酵素以外に、カビが持つ酵素（リパーゼ）が脂質を分解することで熟成香が生じる効果もある[10]。このように、微生物を用いる熟成肉は発酵食品と位置付けられることもある。
乾燥させることで21日後には重量が20%程度減少し、減少した分、肉の味や香りも濃厚なものに変わっていく。乾燥熟成が相当進んだ状態では、肉の外観は赤黒く変色し、薄く白カビなどが発生する場合もあるが、それが乾燥熟成で最高の状態とも言われ、食しても問題はない。乾燥による重量減少の上に外側の乾燥した部分を切り抜いてステーキとするため、最終的に残るのはプロセス前の60%以下と言われる。従って、歩留りロス、保管冷蔵庫（熟成庫）などの設備費・電気代などの経費がかかり、また保管するだけの空間が必要となる上、熟成期間のキャッシュフローも悪くなるため、諸経費が大きく増えてしまう。上記のエイジングシートを使い、さらにミートラッパーで包むことで削り捨てる部分はほとんどなくすこともできる[8]。
上記の理由で乾燥熟成はごく一部の高品質な牛肉に対してのみ行なわれ、鮮度落ちが早い鶏肉や熟成期間の短い豚肉などではほとんど行わない。その他の肉では羊肉やジビエ（鹿肉など）で行われる場合がある。羊肉の場合は、臭みが抜けて味が上品になるという[12]。一方で、乳牛としての役割を終えた廃用牛などを美味しくする手法として使われている例もある。
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業者によっては肉の熟成期間が100日間にも及んだり、「熟成肉は生食できる」と誤解していたりする例がある。このため食中毒など衛生面でのリスクを指摘する意見もある。
