Shanghai Kind-water Preservation Tech Co., Ltd., Shanghai 201108, China

Abstract　In recent ten years in China, the application research of hypobaric storage was extended to the non-freeze preservation of cooked food, fresh-cut vegetables, edible fungi and fruits, preliminary processing of growing area and vacuum cold fresh chain to the cold chain breakage for horticultural products, and also to the investigation of principle and theory but the investigation results would have some evident difference. This paper will simply summarize the basic conception, principle, two withdrawing air type hypobaric storage technologies, and introduce many examples as mentioned above extending in China. This paper thinks that the master mechanism of preservation of LP for fresh horticultural products effectively inhibits the activity change of manifold enzymes, which are PAL, POD, PPO, CAT, CAD, SOD et al., affecting the maturation and caducity of fresh horticultural products during LP storage for a short period, as a result, to reduce both risk of physiology obstacle and disease and to prolong their postharvest life; for the foreseeable future Burg’s hypobaric storage technology and hypobaric short period treatment technology will play an important role in the preservation on fresh agricultural products and food preservation in storage and transportation, the preliminary processing of growing area, fresh logistics, establishing the operating platform of perishable food and high business value, fresh e-business, and fresh delivery for scattered families, establishing the operating platform of fresh-cut vegetables including edible fungi and so on in China or in the world.

Key words　Hypobaric storage, Agriculture, Food, Current situation of research and application, China

1　Introduction

The hypobaric storage (LP) is a physical technology of non-freeze vacuum preservation for agricultural products and food. In 1967, an American scientist Stanley P. Burg founded the theory and invented the technology of LP, and have been researching and pushing forward the application of business up to now[1-4]. For past several decades, LP has been applied in storage and transportation for fresh agricultural products such as vegetables, fruits, edible fungi, flowers, meats, poultries and aquatic products. Some Chinese scientists considered that LP was the 3rd revolution after cold storage and controlled atmosphere cold storage (CA) in the preservation history of fruits and vegetables[5]. In recent ten years in China, the research of LP application has been extending to the free frozen preservation of cooked food[6], fresh-cut vegetables and fruits[7-14], preliminary processing of growing area[15-16]and LP or vacuum cold fresh chain to the cold chain breakage for horticultural products[7-20], and also to the investigation of principle and theory but investigation results would have some evident difference[5,21-30].

2　Hypobaric storage

2.1BasicconceptsIn 1980, Jamieson put[1]: "Hypobaric storage consists of placing the commodity in an environment in which pressure, air temperature, and humidity are precisely controlled. In addition, the rate at which the air in the storage environment is changed is closely regulated. Each of this four factors acts both individually and related with the other three to provide an excellent environment for the storage of perishable agricultural products. No gas is not needed to be supplied in a hypobaric system, whereas in modified-atmosphere and controlled atmosphere storage, other gases are required." In 2004, Burg put[2]: "The wet LP method requires a vacuum tank storing the commodity, a vacuum pump continuously withdrawing air from the tank, a pressure regulator leaking air into the tank at a rate sufficient to maintain the set pressure, a humidifier injecting enough moisture into the incoming low-pressure air to saturate the atmosphere in the vacuum tank, and a refrigerator controlling the cargo temperature. The dry LP method uses the same equipment, except that it replaces the humidifier with a device which controls the pumping speed." Briefly, LP built a storage environment of continuously uninterrupted "low pressure, high humidity and air-change" or "low pressure, low temperature, high humidity and air-change"[29], it was named "three continuation"[6]. That is the LP technology of continuously withdrawing air type from Burg. The element of the good preservation effects needs an appropriate LP equipment and it also needs an appropriate low pressure value. In 2004, Burg[2]pointed out: "Thirty-five years ago, to avoid low concentration of O2damage, the earliest hypobaric storage trials were carried out at 25-45 kPa. Eventually, it was found that better results occurred at 1.33-2.67 kPa, and it now appears that still lower pressures are feasible and may be beneficial." In 2007, Burg and Zheng[3]put forward definitely the optimum operation pressure was [(1.33-2.67) ± 0.067] kPa for fresh horticultural products and 0.6 kPa for meats, fishes and poultries. This paper and science research institutes in China used the hypobaric equipment under Burg’s LP technology to be manufactured by Shanghai Kind-water Preservation Tech Co., Ltd. and at 1.33-2.67 kPa of pressure and research results showed that the preservation effects assuredly were better[6,8-12,14,18-20,31-38]for hypobaric storage or hypobaric short period treatability storage of fresh vegetables, fruits and edible fungi.

An appropriate low pressure can evidently restrain the activity change of manifold enzymes influencing the maturation and caducity of fresh horticulture products and prolong their postharvest life. For example,Tricholomamatsutake[32]at 1.5 kPa and 0.8 kPa of pressure could evidently restrain the activity rising of polyphenol oxidase (PPO), keep the activity of peroxidase (POD), catalase (CAT) and superoxide dismutase (SOD) in a higher level, and delay the degradation of soluble protein and the increase of free amino acid content,etc. In summary, the LP technology can construct an environment of three elements which are continuously uninterrupted low pressure, high humidity and air-change, or four elements which are low pressure, low temperature, high humidity and air-change. Therefore it does not belong to Burg’s LP technology if it is only a low pressure.

2.2BasicprinciplesIn 2007, Burg and Zheng put[3]: "A hypobaric storage system controls the O2concentration by regulating the atmospheric pressure, but its technology and effect differ from CA entirely. At a low pressure the CO2, C2H4, and volatile contaminant concentrations present in the incoming air decrease by up to 99% when the air expands during entry into the vacuum storage chamber, and increased gaseous diffusion and respiratory inhibition causes different commodity types to tolerate 0.1%-0.2% O2. Increased gaseous diffusion also promotes the escape of CO2, NH3and C2H4from within the commodity, while continuous air-changes flush these gases from the storage chamber. During ultra-low pressure hypobaric storage at 1.33-2.67 kPa, a high relative humidity and the prevention of convective and radiant heat uptake by the commodity limit its water loss to the amount that can be evaporated in response to the small amount of respiratory heat it produces. O2is so scarce that a horticultural commodity’s respiratory heat production and ability to transfer heat by convection are inhibited by 90%, an ultra-low pressure prevents bacterial and mold growth, kills insects, prevents CO2-damage, inhibits ethylene production by >90%, inhibits toxic succinate formation, limits ascorbic acid loss, and inactivates ethylene forming enzyme. Therefore, the hypobaric equipment is the most beneficial for horticultural commodity storage." "The low concentration of CO2in LP causes dark stomata opening, dark stomata opening contributes to the storage differences noted in comparisons of LP, CA and MA (modified atmosphere) storage." "Commodities are preserved in better condition for much longer times in a flow-through ultra-low pressure LP system than they are in CA or a sealed higher-pressure hypobaric apparatus, and ultra-low pressure LP benefits fresh produce which does not respond favorably to CA."

The LP technology of "low pressure, high humidity and air-change" or "low pressure, low temperature, high humidity and air-change"[29]is the most beneficial for fresh horticultural product storage when especially the low pressure is lower than 1.33-2.67 kPa (0.275%-0.552% of O2concentration and 0.000 41%-0.000 83% of CO2concentration). Also Burg pointed out that[2]"the combined effects of enhanced diffusion and stomata opening can improve gas exchange up to 20 000-fold at a pressure of 1.33 kPa, allowing commodities to be stored at remarkably low O2tensions without inducing fermentation or low concentration of O2damage (P2)". In China, some scientists considered that LP is a method of integration with vacuum cold, CA, low temperature storage and hypobaric technology[22]; the temperature changing of LP is divided into two stages: the first decreases pressure cooling, the second is low pressure storage[22]; LP has the both function of low pressure and CA[22,26]; LP realizes the combining of vacuum pre-cooling and vacuum storage[26]; LP is a method of special CA to be developed further on the basis of cold storage and CA[22,39]; LP can create a condition of low O2or super low O2, serves as a similar function of CA[40]; the water loss of fruits and vegetables was very easy[5,21-23,39-40]; the fragrance loss of fruits and vegetables was very easy[5,21-23,39-40]; after ripening was bad[39]; adverse situation of LP could cause new physiology and obstacle or disease[21,39]and so on.

Through studying the literature of Burgetal. and our own research and practice, we considered that opinions above mentioned of some scientists are all not tenable but the misunderstandings even misrepresentings for LP. Among the opinions, the most have only the thesis or judgment but have no any bases of theory or practice for example LP is a method of integration with vacuum cold, CA, low temperature storage and hypobaric technology, or is a method of special CA, and adverse situation of LP could cause new physiology and obstacle or disease; the minorities could be from the experimental results due to an improper LP equipment or an improper operational parameter such as the water and fragrance lose easy. Above mentioned opinions are harmful rather than helpful to the research and application of hypobaric storage in China[29,30,41]. It needs to point out that in paper of Fan[28]the content on fresh-cut vegetables preservation is independent of its references[1-3,6-7,10-15]since this paper did know this paper publishing. With regard to the mechanism of preservation for fresh horticultural products, it needs further supplement to expound as follows.

Li and Ren[42]pointed out that the tissue of almost all higher plants can produce the microscale C2H4. Drought, water logging, ultra-temperature, chemistry injure and mechanical damage can stimulate C2H4increase (adverse situation C2H4). But the adverse situation ambience of low pressure caused by LP is able to inhibit the production of adverse C2H4effectively. Ke and Luo[43]pointed out that in general, the three of maturing proceeding, respiration and C2H4release in storage period of fruits and vegetables are in almost synchronization, any of which can control C2H4production and inhibit respiration and maturing proceeding; on the contrary, it promotes respiration and maturing proceeding. But LP can inhibit the both of C2H4production and respiration effectually. Besides, LP has the bacteria growth inhibition effects: the commendatory pressure from Burg is [(1.33±0.067)-(2.67±0.067)] kPa, the corresponding O2concentration is [(0.275%±0.014%)-(0.552%±0.014%)]. Li and Bi[44]pointed out that if O2concentration is lower than 1%, the spore germination rate ofRhizopusstoloniferandCladosporiumherbarumcan reduce 50%, if O2concentration is from 1% to 0.25%, the germination rate will further decrease. When O2concentration is lower than 0.25% or below, the spore germination rate ofAlternariaalternata,BotrytiscinereaandFusariumroseumwill decrease obviously. Therefore, the ultra low O2concentration under LP can play a role in inhibiting bacteria. Li[45]pointed out that all normal activity of plant can be carried out only in an existent condition of water, otherwise it will be hindered or even die. If water content of plant cellular protoplasm decreases and the cell will change from soliquid state to gel state, and the activity of cellular life is weak, if excessive water loss will cause colloid destroying and result in death. But LP can inhibit water loss of inner plant tissue availably. Also LP effectively inhibits the activity change of manifold enzymes influencing the maturation and caducity of fresh horticulture products such as POD, PAL, PPO, SOD, MDA, CAT,etc., thus reducing the risk of physiology obstacle or disease[29].

The vacuum pre-cooling or vacuum cooling or vacuum cold sometimes are confused with hypobaric storage[46]in China. "The principle of vacuum cooling is based on the rapid evaporation of moisture from the surface and within the products. When water evaporates, it needs to absorb heat in order to maintain higher energy level of molecular movement at gaseous state. The amount of heat required is called latent heat, which must be supplied from the product or from the surroundings that consequently are refrigerated."[47]That is vacuum cooling to have the effect of rapid cooling products only and it belongs to one of cooling technologies basically. Sometimes the technology of vacuum package is confused with hypobaric storage[48-50]in China. Zheng[29]analyzed deeply between essential difference of both LP and CA although both reduce O2concentration.

2.3TwowithdrawingairtypesofLPtechnologiesAlthough the low pressure storage environment of fresh agricultural product can prolong their preservation period, the appropriate low pressure value is the core of LP preservation technology and an appropriate low pressure method or type is the first element influencing preservation effects. There are two types of LP technology in China. One is the continuously withdrawing air type (CLP) as mentioned above from Burg, the other is the intermittent withdrawing air type (ILP) and has been using far and wide in scientific research. In CLP, when the pressure inside of chamber decreases to the set value the pump still operates and the pressure value is up and down slightly fluctuation under the set value, such as 1 500 Pa (set value) ±40 Pa (fluctuation value). In ILP, when the pressure inside of chamber decreases to the set value of lower limit, the pump stops and thereof the pressure value will rise slowly duo to vacuum system air leak, when the pressure value rises to the set value upper limit, the pump will operate again, for example, when it reaches 15 kPa (set value of lower limit), the pump stops if up to 20 kPa (set value upper limit) the pump operates again to decrease pressure, and so repeated cycle. The humidification and air-change also are intermittent. The comparison between CLP and ILP is shown in Table 1.

Table 1　The comparison between CLP and ILP

ItemCLPILPWater vapor169.12 (at 0.6 kPa )10.14 (at 10 kPa)78.06 (at 1.3 kPa )3.384 (at 30 kPa)O2168.71 (at 0.6 Pa )10.12 (at 10 kPa)77.88 (at 1.3 Pa )3.376 (at 30 kPa)To prolong preservation period compared with cold storage∥%30-50b100-1 000To prolong preservation period compared with CA∥%10-30b100-800cTo mix storage vegetables, fruits, meat, poultry and fishYesUnknownThe shelf-life after left LP equipment in uncontrolled temperature ambienceLongerShorter

Note: a.It is a theory value and at 2℃, and the gas diffusing rate will increase following the temperature to decrease; b.Li, Jiaqing, Zhang, Ping.PreservationHandbookofFruitandVegetables. China Light Industry Press, Beijing, 2003; c.Only the preservation period of apple is approximation in both LP and CA.

3　New equipment and preservation technologies of LP

3.1SomecompaniesmadetheLPequipmentanditsrepresentativeproductsSome LP equipments have entered scientific research institution and business application in China since the 21st century. (i) Shanghai Xianlu Vacuum Preservation Equipment Co., Ltd. (SHXL) at the beginning of the 21st century researched and made the intermittent withdrawing air type LP equipment. For experimental research, the chambers consisted of 4-6 of the plastic cylinders, volume of every chamber is about 20 L; for business application the chamber volume is of 6-12.5 m3and rectangular to be made from steel. The both chambers of equipment needed to place inside of cold warehouse. The pressure was shown and controlled by the electric contact vacuum gauge of disc type, and the guiding pressure indicated as minus MPa (-MPa) such as -0.06 MPa (≈40 kPa). There is not the function of humidifying and air-change. (ii) National Research Center of Agricultural Product Keeping Fresh Engineering and Technology (Tianjing), or TJNRC for short, at the beginning of the 21st century. It researched and made a simple-easy small hypobaric device[5,23,39-40], which used metal or nonmetal pipes and plastic film and by using their principle of structural mechanics and airtight consisting of a container system of low pressure and enduring vacuum. It was the intermittent withdrawing air type and needed to place inside of cold warehouse. (iii) Shanghai Kind-water Preservation Tech Co., Ltd. (SKW) researched and made the LP series products including two withdrawing air types as mentioned above. In 2008, JYL2 hypobaric cold warehouse[25]was made under the guidance of Stanley P. Burg. It was the LP equipment of first continuous withdrawing air type in China. It was a whole which consisted of the chamber of volume 2 m3and rectangular to be made from steel, insulation room, pumping system, refrigerating system, humidifying and air-change system, data acquisition system of pressureetc., automatic control systemetc. The pressure and temperature can be set up and controlled on the color monitor by the users, and when the pressure set value is 1 500 Pa, the fluctuation value is not greater than ±75 Pa. Afterwards, for scientific research R&D succeeding series products as follows: JYL0.2A the experimental machine of hypobaric cold storage; JYLJ0.03 five chambers of experimental machine of hypobaric cold storage (ILP) whose five chambers needed to place inside of cold warehouse; JYL0.1*4A four chambers of the experimental machine of hypobaric cold storage; JYL0.1*2A two chambers of the experimental machine of hypobaric cold storage; JYLL0.2A hypobaric cooling and cold storage machine,etc. For business application, the following products entered markets: JYLJ 0.2 hypobaric cold storage preservation cabinet; JYL50A hypobaric cold storage warehouse[4,15]; JYL20A hypobaric cold storage warehouse[16]. The range of chamber volume of the series products is from 30 L to 50 m3, the chambers all were rectangular to be made from steel. In 2016, the humidifying technology of CLP achieved important progress since relative humidity (RH) can be set and controlled on the monitor by the users and when the pressure set value is 1 000 Pa, the range of fluctuations value is ± (30-40) Pa. (iv) This paper first saw that a vacuum and 0℃ refrigerator was made by SIEMENS in Shanghai markets in 2010. The chamber was a rectangular plastic box and the volume was about 10 L, the withdrawing air type and operating pressure was not shown, the pressure could not be set and controlled by the users. By the way, some Chinese scientists[5,22-23,39-40]thought that in 1997, the first kilotons grade LP preservation warehouse in the world was born or built favorably in Baotou City in China, making this emerging industry stride forward the industrialization operation road, or march towards the operation road of industrialization manufacture. These opinions were spread by the others in China. However, actually the first kilotons grade LP preservation warehouse did not enter the industrialization operation road or industrialization manufacture up to now in China.

3.2Newtechnologies

3.2.1The preservation technology of short period hypobaric treatment. Since 2011, this paper has begun to carry out the research and application of preservation of non-freeze for the vegetables, fruits, fresh-cut vegetables and edible fungi by using preservation technology of hypobaric short period treatment, up to now it does not to see the information of analogous research or application in the world. Zheng[17]pointed out that: "Hypobaric short period treatment or hypobaric treatment means products placed in LP equipment environment in which suitable low pressure, relative humidity, air-change and keeping short period treatability storage. In a general way, the treatment period is 10-48 h, ≤2 500 Pa of pressure, ≥80% KH and continuously withdrawing air type." Zheng[51]further proposed "The fresh vegetables, fruits, and edible fungi by using hypobaric short period treatment had the preservation effects of the subsequent: the preservation period of cold storage can be prolonged; the preservation period of cold chain breakage in an environment of higher temperature can be prolonged; the cold damage in an environment of lower temperature can be lightened; the deterioration of original quality can be restrained; the growth of the mold can be restrained; the insects can be killed during treatability processing; the effect was better if the MAP was used after hypobaric treatment."

3.2.2The preliminary processing of growing area. The LP equipment can be an equipment for preliminary processing of growing area due to the preservation effects of the subsequent of hypobaric short period treatment[52]. The vegetables, fruits and edible fungi harvested in planting area are placed in a hypobaric cold warehouse for a short time as mentioned above thereof they have the preservation effects of the subsequent above mentioned. That is that if only vegetables, fruits and edible fungi were placed in a LP warehouse to carry out short treatment, their preservation period can be prolonged and keep original quality better. Thus this process operation will be propitious to following circulation of all preservation links of fresh chains.

3.2.3The vacuum cold fresh chain as the core of hypobaric treatment technology. In China, the cold chain on vegetables and fruits is defined in SB/T10728-2012. Zheng[17]defined the vacuum cold fresh chain "with the basis of vacuum technology and with the means of refrigeration technology, the system of circulation on the article during the process of circulation from production to that consumer can leave suitable the environment of low temperature at a short time and limit time." One of basic models is "hypobaric treatment +cold chain." For fresh horticultural products, one of the basic models is shown in Fig.1[51].

Fig.1Abasicmodelofvacuumcoldfreshchain

4　Research and application of LP

Since the end of the 20th century, Chinese scientists, educationists and engineers have carried out the research of experiment of preservation of LP and pushed forward the application of business or industry. The preservation research on agricultural products was over 100 kinds, cooked food close to 20 kinds, fresh cut-vegetables, fruits and edible fungi over 40 kinds in China since 2000.

4.1Hypobariccoldstorage

4.1.1Vegetables. (i) Green leafy vegetables. a.Burclover (Medicago hispida Gaertn)[6]. The extrinsic features (Fig.2) were perfect nearly which had no yellow or rotten leaves but wilt slightly under 25 d of storage at 1 350-1 750 Pa, 2-3.2℃ and RH 55%-70%. b. Green vegetables 2[6]. The leaf had a very small yellow spot (Fig.3, right) when 35 d of storage was at 1 250-1 400 Pa, 1-2.2℃ and near to 100% of RH. The experiment also showed that extrinsic feature was better than only LP (Fig.3, left) if first by vacuum cooling, and the weight loss rate was 8.82% under 30 d of storage. The vegetables had no pole or face of cut to rot when 64 d of LP storage whether or not by using vacuum cooling. c.Green vegetables 4[6]. The leaf only had minority yellowing and decaying under 41 d of storage at 1 450-1 550 Pa, 2-3℃ and RH 92%-95%, and the weight loss rate was 1.12%. d. Acerose green shallot. The leaf had minority yellow and decaying under 26 d of storage at 1 450-1 550 Pa, 2-3℃ and RH 92%-95%. After the LP equipment was put in refrigerator, an environment of higher temperature lasted for 16 d. The leaf had withered but did not get yellow and rot, the stalk kept white had no rot (Fig.4, left). The leaf was yellow (Fig.4, right). In a-d above-mentioned vegetables purchased from the vegetables markets in Shanghai, we did not know their picking time, and all the equipment used was JYL2 hypobaric cold warehouse to be made by SKW.

Fig.2Burcloverunder25dofLPstorage

Fig.3Greenvegetablesunder35dofLPstorage.Left:LPaftervacuumcooling.Right:onlyLP

Fig.4Acerosescallionunder25dofLPstorage+20dofatmosphere.Left:inhighertemperature.Right:inrefrigerator

This research showed that the rate of weight loss or water loss of the products first relates to the technology type of withdrawing air, the second to the HR, and the third to packaging material and methods, the fourth to code play rule of article inside of LP equipment. By the way, this research showed that a green vegetable like Fig.3 and the cabbage was not checked out nitrite by Food Quality Supervision and Testing Center of Ministry of Agriculture of the People’s Republic of China (Shanghai) under 54 d of hypobaric cold storage at 1 275-1 355 Pa and 2-4℃, the equipment was JYL2 hypobaric cold warehouse to be made by SKW. (ii) Snap bean or oil bean[53](PhaseolusvulgarisL.). The research of Jietal. showed that 23.9% of the snap bean outside surface had browning spots and a spot of roots were rot under 49 d of LP storage at 72-82 kPa, 9-11℃ and RH 87%-91%, but the surface developing into yellow and a majority of beans were rot under 49 d by using CA at 9-11℃, 5% of O2, less than 2% of CO2and about RH 91%, and 49.1% of surface had browning spots and the roots were rot under only 28 d of cold storage at 9-11℃ and about RH 78%. The LP equipment was made by researcher and it was the intermittent withdrawing air type. (iii) Lotus root[6]. The local outside surface had browning spots and around the nodusNelumbinisrhizomatishad white hyphostroma under 60 d of storage at 1 450-1 550 Pa, 2-3℃ and RH 92%-95%, and cut-surface was white. The weight loss rate was 2.45% during 82 d in JYL2 hypobaric cold warehouse mentioned above. (iv) Cane shoots or water bamboo shoot[54]. The research of Fangetal. indicated that LP could delay the degradation of total soluble sugar and Vc, suppress the increasing rate of POD, phenylalanine ammonia-lyase (PAL) and cinnamyl-alcohol dehydrogenase(CAD)activity, inhibit the decrease of water soluble pectin content and the rise of lignin content significantly,etc. during 60 d of LP storage. The experimental pressure was better at 70-80 kPa. The LP equipment was the low pressure, multi-chambers and differential pressure experimental equipment made by SHXL. (v) Thin skin green pepper[6]. The extrinsic feature was perfect during 46 d in JYL2 hypobaric cold warehouse above mentioned, at 1 450-1 550 Pa, 2-3℃ and RH 92%-95%. The peppers were not in wilt or dehiscence, and the weight loss rate was 0.66%. (vi) Potato[55]. The research of Suetal. showed that the rate of weigh loss of potato was 5.65% and 4.01% under LP storage 3.5 h at 1℃, RH 85%-90%, and 10 kPa and 55 kPa respectively. The LP equipment was researched and designed by researcher. (vii) Green asparagus[56]. The research of Lietal. showed that the LP storage life of 50 d was achieved, whereas only 25 d in the refrigerator and 6 d in room temperature. The keeping rate of Vc was 85% in LP, 74% in refrigerator and 33% in room temperature for 6 d respectively, and 66% in LP and about 34% in refrigerator for 25 d as well as exceeding 33% in LP for 50 d according to Fig.3. The decrease rate of chlorophyll was not more than 7% in LP, 22% in refrigerator and 41% in room temperature for 6 d respectively, and about 20% in LP and about 40% in refrigerator for 25 d as well as about 38% in LP for 50 d according to Fig.4. The LP parameter was 35-40 kPa, 2-4℃ and RH 85%-90%. Lietal. gave a diagrammatic sketch of LP device but did not show the manufacturing company. (viii) Lotus seed (Semennelumbinis). From September to October, 2017, this experiment indicated that the extrinsic feature of lotus seed was perfect under 20 d of storage at (1 300±40) Pa, 8-9℃ and RH 90%-95%, and then about 20 h put on an atmosphere environment of 21-28℃. The 20 h would be to say the cold chain breakage preservation period. The lotus seed came from Jiangxi Province and the time from plant area of Jiangxi to the place of experiment of Shanghai was about two days. They put in a horniness and white PVC box in which had some plastic water bottles, the temperature of lotus seed was 31.7℃ before they entered into the LP equipment which is JYL0.1*2A experimental machine of 2-chambers of hypobaric cold storage to be made by SKW.

4.1.2Edible fungi. (i)T.matsutake[34]. The research of Yangetal. showed that main volatile components checked out to have 8 species under 20 d of LP storage at 1 450-1 550 Pa and 2-4℃, but have 15 species the same as temperature and time in control of atmosphere pressure. From here it sees that hypobaric cold storage could inhibit the main volatile components to decline significantly and effectively to keep the flavor of freshT.matsutake. The LP equipment which was the experimental machine of 4-chambers of hypobaric cold storage was made by SKW. (ii) Button mushroom (Agaricusbisporus)[38]. The experiment of Zhengetal. indicated that the mushroom kept good freshness after 21 d of hypobaric cold storage and 2 d of cold chain breakage, or after 12 d of vacuum pre-cooling cold storage and 3 d under refrigerator cold storage. The organoleptic quality was as good as spotlessly white, no discolor, no opening umbrella and no wilting after 505 h of hypobaric cold storage. The hypobaric equipment was JYL2 hypobaric cold warehouse to be made by SKW and the parameters were of 1 269-1 460 Pa, 1.6-3.2℃ and RH 82.7%-90.9%. The mushrooms of hypobaric came from Shanghai and the time from plant area to the place of experiment was about 2.5 h.

By the way, this research showed that a kind of semi-transparent white fungus (Tremellafuciformis) to come from Gutian, Fujian Province, China did not check out nitrite by Food Quality Supervision and Testing Center of Ministry of Agriculture of the People’s Republic of China (Shanghai) under 33 d of hypobaric cold storage at 1 275-1 355 Pa and 2-4℃, the equipment was JYL2 hypobaric cold warehouse to be made by SKW.

4.1.4Meat, poultry, fish and shrimp. (i) Beef, pork and chicken[6]. The meat of beef and pork was purchased from the vegetables markets in Shanghai and we did not know their killing time, the time of chicken from killing to entering into the LP equipment was about 2 h, and the experimental equipment was JYL2 hypobaric cold warehouse to be made by SKW. The total number of bacteria colony or aerobic bacterial count was inspected by School of Medical Instruments and Food Engineering, University of Shanghai for Science and Technology. Before experiment, the aerobic bacterial count could not be inspected due to no setup time. Three test samples put in white and thin food bag were purchased from the supermarkets, and the time from experimental place to inspection place was about 2.5 h and no temperature was controlled on the passage. The parameter of LP was 450-600 Pa, 1-2.2℃ and nearly saturation of RH, the parameter of vacuum cooling was 0℃ and the equipment was ZLD 1.1 vacuum pre-cooling machine to be made by SKW. The total number of bacteria colony is shown in Table 2. In China, the standard NY5029-2008 and NY5044-2008 Pollution-free Food, the total number of bacteria colony must be ≤1×106cfu/g for the fresh meat of beef and pork, and ≤5×105cfu/g for fresh chicken meat in NY5034-2005 Pollution-free Food, Poultry Meat and Offals. From Table 2, the total number of bacteria colony of LP storage after vacuum cooling was lower than only LP storage.

Table 2　The total number of bacteria colony of LP storage (cfu/g,×105)

Note: a.After vacuum cooling then entering into LP; b.Only LP.

(ii) Poultry. This was this paper’s experiment. The chicken, duck and goose were killed in agricultural and trade market in Shanghai and the time from experimental place to inspection place was about 2 h and no temperature was controlled on passage. The LP equipment was JYL2 above-mentioned, and the parameter of LP was of 500-620 Pa, 0.5-1.7℃ and near to saturation of RH. The microorganism inspection of chicken, duck and goose was inspected by Food Quality Supervision and Testing Center of Ministry of Agriculture of the People’s Republic of China (Shanghai). The microorganism of inspection of chicken, duck and goose is shown in Table 3. In China, the standard NY/T753-2012 Green Food Poultry Meat stipulated the norm of microorganism. (iii) Fish. a. Crucian or golden carp[6]. The eyeball was fovea inferior under 21 d of storage, and the organoleptic quality was as good as at the beginning, no bad taste, under 26 d of storage at 450-600 Pa, 1-2.2℃ and nearly saturation of RH. The crucian, beef and pork were at one time put together in JYL2. There was 3-3.5 h from killing to entering LP equipment. b. Cilver carp. This paper’s experiment indicated that the organoleptic quality was as same good as at the beginning, no bad taste, and no eyeball fovea inferior under 14 d of storage at 500-600 Pa, 1-2.3℃ and nearly saturation of RH. The hypobaric equipment was JYL2 mentioned above. There was about 2 h from killing to entering LP equipment. (iv) Shrimp[6]. TheMacrobrachiumrosenbergiiwas as same good as the beginning color, no shell and head to disengage with its bodyhrimp, no bad taste under 26 d of storage. The shrimp and crucian were at same time put together in JYL2. At the beginning of the experiment, the shrimp had about half to be alive.

4.1.5Cooked food[6]. They were purchased from the noshery in Shanghai and there was about one hour from purchase to experiment. (i) Radish filling core pancakes (RFCP) and white sugar filling core pancakes (WSFCP) had no mold spot under 42 d of the RFCP and 45 d of the WSFCP during LP storage. The RFCP, WSFCP and the lotus root above-mentioned were at same time put together in JYL2. (ii) Steamed bun with meat stuffing (SBMS), steamed bun with vegetable stuffing (SBVS) and steamed bun with red been paste stuffing (SBRBS) had no mold spot under 70 d of the SBMS and SBVS, and 75 d of the SBRBS during LP storage. They and green vegetables 4 and thin skin green pepper above-mentioned were at one time put together in JYL2.

Table 3　Microorganism of inspection of chicken, duck and goose

4.1.6Bean products[6]. The tofu or bean curd, oil tofu fruit, dried bean curd and bean curd sheets were purchased from the markets in Shanghai and there was about one hour from purchase to experiment, they all were exposed and had not any package in the markets and in LP. The outside surface of them had no color changing, no mold spot, no sticky hand and bad odor under 22 d of tofu, 25 d of oil tofu fruit, 26 d of dried bean curd and bean curd sheets in LP storage at 1 250-1 400 Pa, 1-2.2 and near to saturation of RH respectively.

4.1.7Killing insects. The experiment of Zheng[6]etal. indicated that adzuki bean weevil was killed 97.5% when 12 d in LP and 100% when 14 d in LP at 1 450-1 550 Pa and 2-3℃, and the door of the vacuum chamber was opened 4 times and the time was about 4 h during storage. The hypobaric equipment was JYL2 mentioned above.

4.2Hypobaricshortperiodtreatment(HSPT)As mentioned earlier, the hypobaric storage means a long period storage or transportation, HSPT means short period treatability storage, such as 10-48 h storage. The HSPT is divided two methods that one is hypobaric short period cold storage and the other is only hypobaric short period storage but no cold. They are called by a joint name HSPT.

4.2.1Malantou of green leafy vegetables (one of the green leafy vegetables). The research of Hou[20]etal. showed that the good vegetable rate of Malantou was 79.8% by using hypobaric short period 22 h at 700-2 400 Pa and room temperature (JYL0.09*4A hypobaric storage cold storage experimental machine, the continuously withdrawing air type), then they were put in an un-control temperature room for 145 h, but the control of un-treatment was 70.8% in an un-control temperature room for 143 h, their color picture was shown in Fig.3[20].

4.2.2Edible fungi. (i)T.matsutake[32-34]. a.TheT.matsutake[32]was divided into three LP treatment groups: the treatment 1 at 1 500 Pa and (2±0.5)℃ for 36 h; the treatment 2 at 1 500 Pa and (2±0.5)℃ for 48 h; the treatment 3 at 800 Pa and (2±0.5)℃ for 48 h. Three treatments and controls were in cold storage 15 d of atmosphere. The LP equipment was JYL0.09*4A hypobaric storage cold experimental machine to be made by SKW. As compared with the controls, all LP treatments could inhibit the changing of activity of PPO, POD, CAT and SOD, delay the degradation of soluble protein and increase of free amino acid content, which was good for extending the freshness, the effect under treatment of 1 500 Pa and 48 h was better than under other LP treatments. b. Three treatment groups and LP equipment[33]were the same[32]but the cold storage period was 18 d. The rate of weight loss was about 10% below that of three LP treatments but was close to 20% of controls under 9 d. The treatment 2 (1 500 Pa and 48 h) was most effective in prolonging shelf life. After 12 d under three conditions, levels of soluble solids, cell membrane permeability and hardness value were 35%, 20% and 25% lower, respectively compared with controls. c. The research of Yang[34]etal. showed that main volatile components checked out to have 7 species under 20 d of cold storage after LP treatment for 48 h at 1 450-1 550 Pa and 2-4℃, but have 15 species with the same temperature and time as in controls of atmosphere pressure. From here it sees that hypobaric cold storage could inhibit the main volatile components to decline significantly and effectively to keep the flavor of freshT.matsutake. The LP equipment which was the experimental machine of 4-chambers of hypobaric cold storage was made by SKW. (ii) Morel[61]. The morel was divided into five treatment groups: the first clean to remove the clays of conglutination with morel then hypobaric 2.5 h at (800±50)Pa and (1±0.5)℃; the 2nd hypobaric 2.5 h at (800±50)Pa and (1±0.5)℃; the 3rd O3treatment 15 minutes; the 4th clean to remove the clays of conglutination with morel then O3treatment 15 minutes; the 5th pre-cooling 8 h in a cold warehouse at (1±0.5)℃. Five groups were stored at (2±0.5)℃ for 21 d. The rate of good mushroom was 100% of the first group, 90% of the 2nd group and 30% below others respectively under 11 d of cold storage.

4.2.3Fruits. (i) Honey peach[19]. The honey peach was divided into four groups: hypobaric cold storage 20 d (LP 20 d) then moving in (25±1)℃ for 5 d; hypobaric treatability cold storage 12 h (LPT 12 h) following cold storage 20 d and finally in (25±1)℃ for 5 d; hypobaric treatability cold storage 24 h (LPT 24 h) following cold storage 20 d and final in (25±1)℃ for 5 d; CK, cold storage 20 d then in (25±1)℃ for 5 d. The storage period of (25±1)℃ would be the period of cold chain breakage, or the self-life period of room temperature. The quality indexes were shown in Table 4. The LP experimental parameters were 1 450-1 550 Pa, (3±1)℃ and RH 93%-97%, and the equipment was made by SKW. Both the content of titratable acid and the soluble solids was the most important quality index of special flavor for honey peach.

Table 4　The quality indexes of honey peach

(ii) Chinese bayberry[36]. Chinese bayberry was divided into four groups: hypobaric cold storage 20 d (LP); first hypobaric treatability cold storage 12 h second using 1-MCP (LPT 12 h+1-MCP) following cold storage 20 d; hypobaric treatability cold storage 12 h (LPT 12h) following cold storage 20 d; CK, cold storage 20 d. The LP equipment was JYL0.09*4A hypobaric cold storage experimental machine and made by SKW, and the storage parameters were 1 450-1 550 Pa, (3±1)℃ and RH 93%-97%. The physiological and biochemical and pathological indexes were shown in Table 5. (iii) Blueberry[37]. The blueberry was divided into four groups: hypobaric cold storage 21 d (LP); after 1-MCP treatment then cold storage 21 d; hypobaric treatability cold storage 12 h (LPT 12 h) then cold storage 21 d; CK, cold storage 21 d. The LP equipment was JYL0.09-4A hypobaric cold storage experimental machine and made by SKW, and the storage parameters were 1 450-1 550 Pa, (4±1)℃and RH 85%-95%. The physiological and biochemical indexes were shown in Table 6.

Table 5　The physiological and biochemical and pathological indexes

Table 6　The physiological and biochemical indexes

(iv) Mango[20]. The research of Heetal. showed that the mango could reduce cold damage by using hypobaric short treatment 48 h at 2 000-2 300 Pa only (JYL0.09*4A hypobaric storage cold storage experimental machine, the continuously withdrawing air type) in winter in Shanghai. The mango is a cold sensitivity fruit, it will suffer cold damage if in 13℃ below. Fig.5 showed two pictures for exhibition of the affect difference of cold damage.

4.2.4Fresh cut-vegetables, fruits and edible fungi. Since 2011, Zheng[7-9]etal. have begun an experiment studying fresh-cut vegetables by using hypobaric short treatment technology. The products of fresh-cut of vegetables, fruits and edible fungi evidently can prolong the period of preservation in both cold storage and cold chain breakage in higher temperature environment, the deterioration of original quality can be restrained, the growth of the mold can be restrained, and the section brownness can be retarded,etc. When they are good fresh first by using HSPT then processed into fresh-cut products. It would be better after postharvest to carry out LP treatment immediately, otherwise it would have no evidently effects mentioned above.

(i) The preservation period of cold storage. The experiment of Zheng[7-9]etal. indicated that the preservation period of cold storage of most fresh-cut products would reach 96 h or above (Table 7)[12]. Zheng[12]etal. specially pointed out that He JS research indicated that when 49 d, the Vc content was 45.8 mg/100 g under 22 h of LP treatment and was only 3.4 mg/100 g of cold storage controls by using MAP. His research also showed that it will be the best effect under LP treatment then MAP.

Table 7　The preservation period of cold storage (PFC) of some fresh-cut vegetables and edible fungi　(Unit: h)

(ii) The preservation period of cold chain breakage (in an uncontrolled temperature environment). This experiment indicated that the preservation period of chain breakage of most fresh-cut products would reach 4 h or 24 h above. a.Green vegetable 1[8]was treated 54 h at 1 275-1 350 Pa, 0-1℃ and RH>90%, then washed and cut and put in the bags of plastic thin film following un-control environment (Cold chain breakage, March, in Shanghai) for 41 h, had good extrinsic feature, no yellow leaf, no brownness and rot, and when cold chain breakage 96 h, the cutting section had a little brownness and the leaf had a few rot, but the controls by vacuum pre-cooling 2℃ then cold chain breakage 46 h, the cutting section had brownness and the leaf had rot. The vacuum pre-cooling machine and LP equipment were made by SKW. LP equipment was the continuously withdrawing air type. b. Green vegetable 2 (Huawang-tsai)[18]was divided into three groups: the treatment 1 was the control stored in a refrigerator; the treatment 2 was a LP of intermittent withdrawing air type, which was JYLJ0.2 preservation cupboard of hypobaric cold storage and made by SKW; the treatment 3 was a LP of continuously withdrawing air type, which was JYL2 hypobaric cold warehouse and made by SKW. The three groups were under cold store 7 d then put in un-control environment for 24 h (May, in Shanghai), the control yellow and rot of some and were unedible. The treatment 2 had some rot of both leaf and stem but most could be eaten, the treatment 3 had good extrinsic feature, no yellow leaf, no rot, and edible rate was 100%. The treatment 1 had rot leaf when 121.5 h in refrigerator. c. Fresh-cut broccoli[9]kept fresh green, had no brownness, no yellow or drop of flower grain, and edible rate was 100% when 118 h, whose order was LP 24 h, cold storage 24 h, un-control environment for 24 h (August, in Shanghai), and cold storage 47 h again. d. Sword bean[18]with only low pressure (1 129-1 257 Pa of U disc recorded) and no low temperature (17-18.2℃ of U disc recorded. May, in Shanghai) was treated 23h then put in a plastic box of un-control temperature environment. The control of un-treated had brownness of some of cutting sections when 23 h in un-control temperature environment, but LP treated bean had no brownness of cutting section when 54 h in un-control temperature environment. The LP equipment was JYLL0.2A hypobaric cooling and cold storage experimental machine (the continuously withdrawing air type) and made by SKW. e. Towel gourd[11]was treated 47 h at four different low pressure then put in un-control temperature environment for 6 h, their brownness condition was not at 1 100 Pa, a little at 1 474 Pa and 1 717 Pa, and most at 1 838 Pa. f. Coriander[14]had no wilt, yellow, brownness and rot, when 6.5 h in 35-36℃, after LP treatment 24 h at 1 200 Pa and about 3℃ then cold storage 72 h. g.Green pepper[14]had no wilt, yellow, brownness and rot, when 5 h in 29-31.5℃, after LP treatment 24 h at 1 500 Pa and 6℃ then cold storage 72 h.

4.3Thedesignofvacuumcoldfreshchainsforfresh-cutvegetablesOn the one hand, the enzymatic browning is a physiology factor inhibiting development of fresh-cut fruit and vegetable industry and the rot because the microorganism is a pathological factor hindering development of fresh-cut fruit and vegetable industry[62], on the other hand, by using hypobaric short period treatment, it has the preservation effects of the subsequent including inhibition of the enzymatic browning and rot[51], also prolonging the preservation period (shelf life) of cold chain breakage in an environment of higher temperature and restraining the deterioration of original quality,etc. For a long time, MAP is its main stream preservation technology[63], but for fresh-cut broccoli, the technology of hypobaric short period treatment is better than MAP[12]. Therefore, the fresh-cut vegetable vacuum cold fresh chain as the core of hypobaric short period treatment technology would be shown in Fig.5. There is a need to point out that the model of Fig.5 will be not tenable if there is not the key hypobaric short period treatment.

Fig.5Abasicmodelofvacuumcoldfreshchainforfresh-cutvegetables

5　Discussions

Before 50 years, American scientist Stanley P. Burg founded the theory and invented the technology of LP, and many researches and some business applications have shown that LP technology has a lot of special superiorities, compared with other non-freeze technologies for non-freeze preservation of fresh agricultural products and non-freeze preservation of food as mentioned above, but up to now it is not used in business application of industry on large scale in the world. This paper thinks that due to the difficulty in realizing Burg’s LP technology, as well as the misunderstandings or misrepresentings or misleadings from some scientists[29-30,41], it as a result hinders the business application and development. Fortunately, some Chinese researchers and industrialists have been promoting LP technology research and application on agriculture and food industry, and are achieving effects. Since both technology of hypobaric storage and hypobaric short-period treatment have many special effects on preservation for fresh agricultural products and food, and the effects have been proved by the theory and practice and grasped by someone in China, both technologies will play an important role for the foreseeable future in the preservation of fresh agricultural products and food in storage and transportation, the preliminary processing of growing area, fresh logistics, and it is necessary to establish the operating platform of perishable foods with higher business value, fresh e-business, and fresh delivery for the scattered families, and establish the operating platform of fresh-cut vegetables including edible fungi and so on in China or in the world.