There are clear benefits from the cool temperature storage of foods. As this. thesis evolved there was another development which paralleled it. While slow. in maturing, it also had significant potential. The area dealt with the gas, storage of flesh foods. The matter had appeared in quite an unexpected system─refrigerated dough. In this lesson we will explore the nature and direction. of the technology of gas storage as it is evolving.

    Gas Packed Refrigerated Dough
      
    Cooking is the art of preparing foods by heating until they have changed in flavor, appearance, tenderness and chemical composition. Baking is a form of  cooking that is carried out in an oven. Baked bread is not only one of the most ancient foods manufactured by man, it is also the food most widely eaten in the world. The actual baking process is really the last and most important step in the production of bakery products. Through the agency of heat, an unpalatable dough mass is transformed into a light, porous, easily digestible add appetizing
product.

    Freshly baked products can be produced and distributed effectively However, the storage life of soft baked goods is generally less than a week. During this time, products stale and lost! desirable tastes and textures. The appealing aroma of baked goods is slowly lost after the products are removed from the ovens. For best eating, baked products are generally soon after baking.
 
    An ingenious means of providing consumers with ready-td-bake dough products evolved in the past few decades. This product line is called refrigerated dough. The essential technology involves the preparation of a dough, packaging it in a sealed container capab1e of holding about 5 atm of gas, and causing an initial controlled release of carbon dioxide. Under refrigerated conditions, slightly above freezing, such products can be effectively produced and distributed, and when baked yield excellent products.
 
    Matz(1968) reported that in such commercial refrigerated dough practices. there is ordinarily used, as the leavening agent or gas generating component a slowly rating sodium acid pyrophosphate ot the approximate formula Na2H2P2O7 in combination with sodium bicarbonate. All ingredients are mixed under very rigidly controlled temperature conditions. (Note: dough temperature out of mixer, 10℃ to 15℃) The resulting dough is rolled out and the dough is then sheeted and cut into banks, such as discs about 5 cm in diameter by l.2 cm thick. The cut-outs are dusted with rice flour or oiled to prevent sticking together. They are then stacked and packed in a suitable can. These cans are doughtight but not gas-tight. As a result, air and carbon dioxide may and do escape so that the dough reaches and blocks the gas outlets. Within about l.5 to 3 hr after the package is sealed, for instance, the biscuits will have so expanded as to fill the container and close the original vents for gas and the internal pressure of carbon dioxide gene ated by the leavening materials will have risen to around 1 atm. Pressure within the can will be maintained over a period of 8 weeks or so if the biscuit dough and cans are normal and the storage temperature is between 7℃ to 12℃.

    One of the difficulties in the manufacture of dough using phosphate leavening agents has been the formation of visible phosphate crystals (disodium phosphate dodecahydrate). This crystal formation occurs at storage temperatures below 10℃, and is frequently most abundant at about 7℃. This crystallization is prevalent in the canned refrigerated doughs because of their extended storage, including and normal period of transportation and storage in the stores and homes. These visible crystals cause consumer rejection of the product because of their glass-like appearance and an uneven coloration of the baked goods which develops upon baking. It is found that phosphate leavened cereal doughs can be prepared. whichcan be kept at their normal storage temperature (i.e., between about 7℃ and12℃) for periods up to 12 weeks or more, and which are free from visible phosphate crystal formation. Such a dough is made in the conventional manner, bymixing the ingredients such as flour, shortening, flavoring, water, the usual minor ingredients, and the chemical leavening agent, and then allowing the dough to develop, and shaping the dough as desired.

    The dough is then Placed in the consumer container and sealed, then allowed to proof rapidly so that dough temperature teaches 14℃ to 20℃ as quickly as possible. Proofing involves the reaction of the chemical leaveners to provide sufficient carbon dioxide to cause the dough to expand and fill the container so as to close the gas vents.

    The canned dough is cooled in a conditioning area, such as controlled temperature room, at a temperature above its freezing point but below O℃. 

    Normally the freezing point of such doughs is about -6℃. The time and temperature required to condition the dough will vary dependent upon can dimensions, net weight of the dough, formation of the dough, and refrigeration conditions used. However, it is important that the temperature of the conditioning room not be below the freezing point of the dough. The necessary time and temperature can be determined by withdrawing sample cans at intervals. opening them, and noting the temperature and condition of the dough. When microscopic crystals of disodium phosphate phosphate dodecahydrate are observed dispersed substantially uniformly throughout the dough, the dough is conditioned and will not exhibit visible phosphate crystals even after 12 weeks of storage. Once the time has been obtained for a given product under given cooling conditions, the same time of treatment can be used for subsequent production.

    Conventional refigerated doughs can be conditioned by being held at about -7℃ to -5℃ for about 48 hr. With a higher temperature it is recommended that the bolding time be increased to 4 days. At temperatures of about -1℃ a longer holding time of about 3 days is required to obtain proper conditioning of the dough.

    After the canned dough has been conditioned, the cans are removed from the cooling area and transferred to the usual storage area where the temperature is maintained at 4 to 10℃. The cans may then be transported and the dough used in the ordinary manner.

    Gas Storage of Fruits And Vegetables

    Controlled atmosphere (CA) storage

    Controlled atmosphere (CA) storage refers to the composition of the atmosphere altered from that of air in respect to the proportions of O2 and/or CO2. The proportioas are controlled; O2 usually is lower and CO2 [increased; nitrogen acts as an inert “filler,” other gases may be added in low concentrations.

    Modified atmosphere (MA) storage is similar in principle to CA storage. except control of gas concentrations is less precise. Respiratory CO2 or CO2 derived from dry ice accumulates and Oz decreases.

    Lipton (1975) believes that the modification of the O2 and/or CO2 concentration in the atmosphere surrounding fresh produce is justified if the vegetable or fruit will be more valuable after CA storage than after a similar storage period in the air. CA is used most commonly to slow ripening of fruits, but appropriate mixtures of O2 and CO2 also can retard the spread of certain diseases and lower the incidence of some disorders. This is usually not apparent when storage time is brief and/or when storage temperature is optimal.

    The use of CA can also prevent desirable ripening. induce severe physiological disorders and cause an increases in decay when misused. Each kind of vegetable and fruit has its own specific, unpredictable tolerance for atmosphere modification .

    Controlled atmosphere storage bas been commonly used to delay ripening of fruits, retard the spread of disease, lower the incidence of storage disorders and inhibit toughening and yellowing. An extension of this technique is the use of packaging films to develop a microcontrolled environment in retail packages. Modification of the storage environment by suitable packaging can provide storage benefits which exceed those observed with refrigeration and controlled atmosphere.

    Produce Package System

     A package of apples can be a dynamic s)-stem in which two processes, respiration and permeation, occur simultaneously. There is an uptake of O2 by the apples and evolution of CO2, C2H4, H20 and other volatiles, and, at the same time. specific restricted exchange of these gases through the packaging film. Variables that affect respiration are: weight of apples, stage of maturity, membrane permeability, temperature, 02 and CO2 partial pressures, ethylene concentration, light, etc. Variables affecting gas exchanges into and out of the package are; structure of the packaging film, thickness, area, temperature, O2, and CO2 concentrations .It has been demonstrated that steady state conditions are established withi
suck an intact packaging system; equilibrium concentrations of 02 and CO2 prevail, and the respiration rate is equal to the rate of gas exchange. 

    Any change in the system variables will affect the equilibrium or the time to establish steady state conditions. The packaging of fresh food in polymeric films is now frequently used in retail stores and about 40% of the produce is now distributed to retail stores in consumer packages. This packaging is designed for consumer appeal. Better use to regulate ripening and, thus, to prolong useful storage life is predicted by many people.

    食品低溫貯藏有明顯的好處。以往，隨著這一課題展開，就出現(xiàn)與之相平行的另外新的發(fā)展。新的課題雖然尚處在緩慢成熟的過程中)．、卻同樣有很大的潛力。這一領域的研究內容是生鮮食品的氣體保藏法。此事首、先出現(xiàn)在一個完全沒有意料。到的食物體系——冷藏生面團之中。本課我們將探討當前在發(fā)展中的氣體保藏技術的特點和趨勢。

    充氣包裝冷藏面團

    烹任是利用加熱方法使食物的風味、外觀、．嫩度和化學組成發(fā)生變化的食物制作技藝。焙烤是一種在烤爐中進行的烹證形式�？局频拿姘�不僅是人類制作的最古老食品之，而且也是世界上食用最廣泛的食品。真正的焙烤過程其實是焙烤制品生產(chǎn)中最后、也是最重要的一步，通過加熱的作用，一塊食而無味的面團就轉變成了疏松多孔、易于消化的美食制品。

    新烤制產(chǎn)品的生產(chǎn)效率和銷售效果都可以很好。但松軟的焙烤制品的貯藏期一般不超過一周。在貯藏期內，制品變硬并失去美味和質構。焙烤制品在移出烤爐之后，誘人的香氣便慢慢消失。通常，焙烤制品剛烤好就消費是最適宜的。

    最近幾十年里發(fā)展了一種巧妙的方法，向消費者提供便于焙烤的現(xiàn)成面團產(chǎn)品。這種產(chǎn)品的生產(chǎn)線稱為冷藏面團生產(chǎn)線。其基本的工藝包括面團的制備、面團包封在能夠保持氣體壓力為5atm的密封容器中、以及CO2最初有控制地促使釋放。在冷藏(略高于冰點)溫度條件下，這種產(chǎn)品可以有效地進行了生產(chǎn)和銷售，且在焙烤后可以獲得優(yōu)質的產(chǎn)品。

    馬茨(1968年)曾報道，在這種商業(yè)化冷藏面團的生產(chǎn)作業(yè)中，通常使甩一種作用緩慢、分子式大體為Na2H2P2O7的焦磷酸氫鈉與小蘇打的混合物作為膨松劑(產(chǎn)氣成分)。所有的配料要在非常嚴格控制的溫度下混合(注：從混合器出來的面團溫度在10~15℃)。調好的面團通過軋輥送出，壓成面片,再切割成面坯，如切成直徑約；5cm、厚1.2cm的圓形面坯。將這些切出來的面坯，撒上米粉或涂上油，以免粘在一起。然后把它們壘起來，裝入適當?shù)慕饘俟�。金屬罐對面團不透漏，但不是氣密性的。因此空氣和二氧化碳可以逸出，使面團有可能到達并堵住氣體逸出口。例如，罐裝容器在密封后1.5~3h內；餅干面坯會膨脹充滿整個容器，并堵住原有的泄氣口。由膨松材料產(chǎn)生的內部二氧化碳壓力將升到1atm左右。只要餅干面坯和罐子正常，且貯藏溫度在7~12℃之間，那么罐內的壓力將維持約8周以上。

    面團生產(chǎn)中使用膨松劑磷酸鹽的困難之一是生成肉眼看得見的磷酸鹽晶體(十二水磷酸氫二鈉)。這種晶體在貯藏溫度低于10℃時形成，而在7℃左右時常常大量產(chǎn)生。罐裝冷藏面團中這種結晶現(xiàn)象很普遍，其原因是它的貯藏期長(包括正常的運輸時間和在：商店及家庭中的存放時間)。這些肉眼可見的晶體致使產(chǎn)品難以為消費者所接受，因為晶體外觀象玻璃，并且焙烤時會在焙烤制品上形成不均勻的顏色。

    有人發(fā)現(xiàn)，用磷酸鹽作膨松劑制成的谷物面團可在正常的貯藏溫度(即在7~12℃)下存放12周或更長的時間，并且無明顯磷酸鹽晶體形成。這種面團用傳統(tǒng)方法制造，先把各種配料如面粉、起酥油、調味料、水、常用的微量配料以及化學膨松劑混合在一起，然后讓面團展開，并按所要求的形狀成型。

    接著將面團裝入小包裝容器并密封，讓其迅速醒發(fā)，使面團的溫度盡快達到14~20℃。醒發(fā)過程涉及到化學膨松劑的反應，產(chǎn)生足夠的二氧化碳，使面團膨發(fā)充滿容器，從而封住泄氣口。

    然后將罐裝面團放在溫控室之類的調理場所中，在高于面團冰點但低于0℃的溫度下進行冷卻。這類面團的冰點通常在-6℃左右。面團調理所要求的時間和溫度隨金屬罐尺寸、面團凈重、面團成形及所用制冷條件的不同而異。但是，重要的是調理室的溫度不得低于面團的冰點。面團調理所必需的時間和溫度可以通過定期取樣開罐、觀測面團的溫度和狀態(tài)來確定。當觀察到有顯微鏡可見的十二水焦磷酸二鈉微晶體明顯均勻地分散在整個面團中時，那么此時面團已處于正常良好狀態(tài)，便不會在甚至貯存12周以后出現(xiàn)肉眼可見的磷酸鹽晶體。給定產(chǎn)品在給定冷卻條件下的調理時間一旦確定，便可將同樣的時間應用于隨后的生產(chǎn)中。

    傳統(tǒng)冷藏面團的調理可在約-7~-5℃下保持約48小時。若采用更高的溫度，則建議將保持時間延長到4天。在-1℃左右的溫度下，為使面團獲得適當?shù)恼{理，要求將保持時間再延長3天左右。

    罐裝面團經(jīng)調理后，可從冷卻場所移到溫度保持在4~10℃的普通貯藏場所，這時便可按常規(guī)方式運送罐裝面團和使用面團。
  
    水果和蔬菜的氣體保藏法

    氣調貯藏(CA貯藏)

    氣調(CA)貯藏是指貯藏環(huán)境大氣的組成在02或C02(或兩者)的含率上是由改變空氣組成而來的貯藏方法。各氣體的含率受到控制，通常做法將O2的含率降低，將CO2的含率提高，而氮氣起著惰性“填充劑”的作用，還可以加入一些低濃度的其它氣體。變氣(MA)貯藏與氣調(CA)貯藏的原理相似，只是對氣體濃度的控制沒那么精確而已。由呼吸作用產(chǎn)生的CO2和來自干冰的C02累積起來，降低了O2的含量。李卜頓(1975)認為：如果蔬菜或水果經(jīng)CA貯藏后比在空氣中貯藏相同時間后有更高的價值，那么改變新鮮農(nóng)產(chǎn)品周圍氣體中02或C02 (或兩者)的濃度是合理的。CA用于水果的緩慢后熟最為普遍，但混合適當?shù)?2和C02也能減少某些植物病害的蔓延，從而降低腐壞事故出現(xiàn)的機會。當貯藏時間很短或貯藏溫度適宜時，通常不易見到這種不良的事故。

    如果使用不當，CA也會妨礙正當?shù)暮笫�，誘發(fā)嚴重的生理紊亂和造成腐爛率的增加。每一種蔬菜和水果都有它自己特殊的難以預測的對氣體調整的承受限度。

    氣調貯藏已波普遍地用于減慢水果后熟，減少病害傳播，降低貯藏腐壞事故和防止變老、變韌、變黃，這種技術的一種推廣應用是零售包裝中使用包裝薄膜；以形成小范圍的控制環(huán)境。因適當包裝而改變的貯藏環(huán)境可以給貯藏帶來勝過在致冷氣調貯藏中能見到的好處。
農(nóng)產(chǎn)品包裝系統(tǒng)一包蘋果可以是一個動態(tài)體系，在該體系中，同時進行著呼吸和滲透這兩種過程；蘋果吸取02，放出C02、乙烯、水份和其它揮發(fā)性物質，與此同時,這些氣體透過包裝薄膜進行著特別有節(jié)制的交換。影響呼吸作用的變量有：蘋果重量、成熟期、薄膜的可滲透性、溫度、O2和C02的分壓、乙烯濃度、光照等。影響進出蘋果包氣體交換的變量有：包裝薄膜的構造b厚度、面積、溫度、O2和C02的濃度。

    已經(jīng)由實驗證明：在這樣完整無損的包裝體系內部，穩(wěn)定狀態(tài)的條件巳建立，02和C02的平衡濃度占主要地位，并且呼吸速率等于氣體交換速率。體系中任何變量的改變都將影響這個平衡，即影響建立穩(wěn)定狀態(tài)條件的時間。目前，零售商店中常常用高分子薄膜包裝新鮮食品，并且，大約有40％的這類產(chǎn)品是以小包裝的形式批發(fā)到零售商店的。這種包裝是根據(jù)顧客要求設計的。許多人斷言：這種包裝可以更好地用來調節(jié)產(chǎn)品的后熟，從而延長有效的貯藏壽命。