WO2019085734A1 - 冻干食品的蒸汽巴氏灭菌方法 - Google Patents

冻干食品的蒸汽巴氏灭菌方法 Download PDF

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Publication number
WO2019085734A1
WO2019085734A1 PCT/CN2018/110353 CN2018110353W WO2019085734A1 WO 2019085734 A1 WO2019085734 A1 WO 2019085734A1 CN 2018110353 W CN2018110353 W CN 2018110353W WO 2019085734 A1 WO2019085734 A1 WO 2019085734A1
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temperature
food
steam
sterilization
pasteurization
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PCT/CN2018/110353
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English (en)
French (fr)
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王远溪
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王一田
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Priority to CA3080235A priority Critical patent/CA3080235C/en
Priority to US16/758,355 priority patent/US10849341B2/en
Publication of WO2019085734A1 publication Critical patent/WO2019085734A1/zh

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    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23L—FOODS, FOODSTUFFS, OR NON-ALCOHOLIC BEVERAGES, NOT COVERED BY SUBCLASSES A21D OR A23B-A23J; THEIR PREPARATION OR TREATMENT, e.g. COOKING, MODIFICATION OF NUTRITIVE QUALITIES, PHYSICAL TREATMENT; PRESERVATION OF FOODS OR FOODSTUFFS, IN GENERAL
    • A23L3/00—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs
    • A23L3/16—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs by heating loose unpacked materials
    • A23L3/165—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs by heating loose unpacked materials in solid state
    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23B—PRESERVING, e.g. BY CANNING, MEAT, FISH, EGGS, FRUIT, VEGETABLES, EDIBLE SEEDS; CHEMICAL RIPENING OF FRUIT OR VEGETABLES; THE PRESERVED, RIPENED, OR CANNED PRODUCTS
    • A23B4/00—General methods for preserving meat, sausages, fish or fish products
    • A23B4/005—Preserving by heating
    • A23B4/0053—Preserving by heating with gas or liquids, with or without shaping, e.g. in form of powder, granules or flakes
    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23B—PRESERVING, e.g. BY CANNING, MEAT, FISH, EGGS, FRUIT, VEGETABLES, EDIBLE SEEDS; CHEMICAL RIPENING OF FRUIT OR VEGETABLES; THE PRESERVED, RIPENED, OR CANNED PRODUCTS
    • A23B4/00—General methods for preserving meat, sausages, fish or fish products
    • A23B4/03—Drying; Subsequent reconstitution
    • A23B4/037—Freeze-drying, i.e. cryodessication, lyophilisation; Apparatus therefor
    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23B—PRESERVING, e.g. BY CANNING, MEAT, FISH, EGGS, FRUIT, VEGETABLES, EDIBLE SEEDS; CHEMICAL RIPENING OF FRUIT OR VEGETABLES; THE PRESERVED, RIPENED, OR CANNED PRODUCTS
    • A23B7/00—Preservation or chemical ripening of fruit or vegetables
    • A23B7/005—Preserving by heating
    • A23B7/0053—Preserving by heating by direct or indirect contact with heating gases or liquids
    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23B—PRESERVING, e.g. BY CANNING, MEAT, FISH, EGGS, FRUIT, VEGETABLES, EDIBLE SEEDS; CHEMICAL RIPENING OF FRUIT OR VEGETABLES; THE PRESERVED, RIPENED, OR CANNED PRODUCTS
    • A23B7/00—Preservation or chemical ripening of fruit or vegetables
    • A23B7/02—Dehydrating; Subsequent reconstitution
    • A23B7/024—Freeze-drying, i.e. cryodessication or lyophilisation
    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23B—PRESERVING, e.g. BY CANNING, MEAT, FISH, EGGS, FRUIT, VEGETABLES, EDIBLE SEEDS; CHEMICAL RIPENING OF FRUIT OR VEGETABLES; THE PRESERVED, RIPENED, OR CANNED PRODUCTS
    • A23B9/00—Preservation of edible seeds, e.g. cereals
    • A23B9/02—Preserving by heating
    • A23B9/025—Preserving by heating with use of gases
    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23L—FOODS, FOODSTUFFS, OR NON-ALCOHOLIC BEVERAGES, NOT COVERED BY SUBCLASSES A21D OR A23B-A23J; THEIR PREPARATION OR TREATMENT, e.g. COOKING, MODIFICATION OF NUTRITIVE QUALITIES, PHYSICAL TREATMENT; PRESERVATION OF FOODS OR FOODSTUFFS, IN GENERAL
    • A23L3/00—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs
    • A23L3/003—Control or safety devices for sterilisation or pasteurisation systems
    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23L—FOODS, FOODSTUFFS, OR NON-ALCOHOLIC BEVERAGES, NOT COVERED BY SUBCLASSES A21D OR A23B-A23J; THEIR PREPARATION OR TREATMENT, e.g. COOKING, MODIFICATION OF NUTRITIVE QUALITIES, PHYSICAL TREATMENT; PRESERVATION OF FOODS OR FOODSTUFFS, IN GENERAL
    • A23L3/00—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs
    • A23L3/015—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs by treatment with pressure variation, shock, acceleration or shear stress or cavitation
    • A23L3/0155—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs by treatment with pressure variation, shock, acceleration or shear stress or cavitation using sub- or super-atmospheric pressures, or pressure variations transmitted by a liquid or gas
    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23L—FOODS, FOODSTUFFS, OR NON-ALCOHOLIC BEVERAGES, NOT COVERED BY SUBCLASSES A21D OR A23B-A23J; THEIR PREPARATION OR TREATMENT, e.g. COOKING, MODIFICATION OF NUTRITIVE QUALITIES, PHYSICAL TREATMENT; PRESERVATION OF FOODS OR FOODSTUFFS, IN GENERAL
    • A23L3/00—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs
    • A23L3/36—Freezing; Subsequent thawing; Cooling
    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23L—FOODS, FOODSTUFFS, OR NON-ALCOHOLIC BEVERAGES, NOT COVERED BY SUBCLASSES A21D OR A23B-A23J; THEIR PREPARATION OR TREATMENT, e.g. COOKING, MODIFICATION OF NUTRITIVE QUALITIES, PHYSICAL TREATMENT; PRESERVATION OF FOODS OR FOODSTUFFS, IN GENERAL
    • A23L3/00—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs
    • A23L3/40—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs by drying or kilning; Subsequent reconstitution
    • A23L3/44—Freeze-drying
    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
    • A23L—FOODS, FOODSTUFFS, OR NON-ALCOHOLIC BEVERAGES, NOT COVERED BY SUBCLASSES A21D OR A23B-A23J; THEIR PREPARATION OR TREATMENT, e.g. COOKING, MODIFICATION OF NUTRITIVE QUALITIES, PHYSICAL TREATMENT; PRESERVATION OF FOODS OR FOODSTUFFS, IN GENERAL
    • A23L3/00—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs
    • A23L3/015—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs by treatment with pressure variation, shock, acceleration or shear stress or cavitation

Definitions

  • the invention relates to a method for steam sterilization of foods, in particular to a steam pasteurization method for freeze-dried foods.
  • Microbial contamination of low-moisture foods has become a new food safety issue in recent years, and food-borne diseases and food recall problems associated with it have occurred many times, including foods such as dried fruits, spices and pet dry foods.
  • the most common pathogens are Salmonella, followed by Listeria monocytogenes and E. coli O157:H7, all of which exhibit increased heat tolerance at low water activities.
  • Water activity refers to the ratio of the saturated vapor pressure of a food to the saturated vapor pressure of pure water at the same temperature, and is a means of expressing the level of free water in a food.
  • Low-water activity foods generally refer to foods with a water activity of less than 0.7.
  • Foodborne pathogens such as Salmonella cannot grow and reproduce in low-altitude foods, but they can survive for a long time. Sterilization of low-water activity foods often requires heat treatment at higher temperatures or longer to achieve a sufficient reduction in the pathogen population, but high treatment temperatures or long heating times can affect the quality or yield of the product, resulting in loss of nutrients, Deformation, discoloration, poor taste, increased energy consumption, and low production efficiency.
  • the low-moisture food is heated in a baking oven to mix humid air with saturated steam to wet heat the water activity and temperature. At the same time, the product quality is reduced by 5 log units. During the heating process, the food temperature is higher than the dew point and no moisture absorption occurs.
  • Napasol North America LLC http://www.napasol.com
  • the steam sterilization unit of Revtech Process Systems http://www.revtech-process-systems.com
  • Revtech Process Systems http://www.revtech-process-systems.com
  • the product inside the spiral tube is in contact with the tube wall and is pre-vibrated under vibration. After the heat is passed, steam is sterilized in the tube, and finally the cooling air is introduced into the spiral tube in the rear stage to cool the product, and the whole process is continuously operated.
  • the application number is CN200880023032.6
  • the Chinese patent entitled “Method for Surface Pasteurization and Disinfection of Food Blocks” proposes a surface pasteurization method suitable for oily seed foods: preheating the food to The evaporation temperature of the pasteurization is approached to minimize moisture absorption of the food, heat treatment is carried out for 1 to 30 minutes, and the condensed water on the surface of the food is removed by further vacuum drying under a further reduced pressure.
  • freeze-drying In low-moisture foods, the foods treated by vacuum freeze-drying (referred to as freeze-drying) are mostly porous and loose structures, which are beneficial to the diffusion of steam inside foods, but the moisture of freeze-dried foods compared with foods treated by other drying methods. The content is lower, generally between 2% and 5%, and the killing of pathogenic bacteria in food is more difficult.
  • the water activity of the food itself is also an important factor affecting the sterilization effect. It is essential to effectively eliminate pathogenic bacteria such as Salmonella, which have improved heat resistance at low water activity, in a suitable time, and to greatly increase the water activity of freeze-dried foods.
  • pathogenic bacteria such as Salmonella
  • the existing steam sterilization method has limited improvement on the water activity of the food, so that the freeze-dried food with extremely low water activity can reach 5 logarithmic units under the premise of ensuring product quality and in a suitable time. The above sterilization effect is almost impossible.
  • an object of the present invention is to provide a steam pasteurization method suitable for a low-water activity freeze-dried food, which uses a saturated steam to increase the temperature of the food while greatly increasing the temperature of the food. Improve the water activity of the food to achieve a pasteurization effect of 5 log units or more in a suitable time.
  • the present invention provides a steam pasteurization method for a freeze-dried food, the method comprising:
  • step 2) cooling the food subjected to step 1) to a cooling temperature corresponding to the type of the food;
  • the cooling temperature in the step 2) is determined as follows:
  • T 1 T 2 -(x 2 -x 1 )*(h 1 -h 2 )/c p
  • c p is the constant pressure specific heat capacity of the food
  • T 1 is a cooling temperature
  • T 2 is the sterilization temperature
  • x 1 is the initial moisture content of the lyophilized food
  • x 2 is the moisture content of the food obtained by using the isothermal moisture absorption line of the food at the sterilization temperature and the target water activity;
  • h 1 is the enthalpy of saturated water vapor at a temperature of T 1 ;
  • h 2 is the enthalpy of saturated water at the sterilization temperature.
  • the sterilization temperature determined in the step 1) is 60 to 90 ° C, and the target water activity is 0.70 to 0.85.
  • the absolute pressure in the sterilization chamber is lowered to 100 to 3000 Pa, and then the vacuum is stopped.
  • the water vapor introduced into the sterilization chamber in the step 4) is a negative pressure saturated water vapor.
  • the pressure of the steam source is equal to or slightly higher than the saturation pressure of the water corresponding to the sterilization temperature.
  • the negative pressure saturated vapor pressure in the step 4) corresponds to a saturation temperature higher than the pasteurization temperature by 0 to 10 °C.
  • the specific sterilization time in the step 4) is from 10 min to 90 min.
  • the step 5) is specifically: connecting the sterilization chamber to a cold trap, and the steam is condensed or desublimed on the surface of the cold trap, and then the non-condensable gas is extracted by the vacuum pump.
  • the surface temperature of the cold trap is -40 ° C to 10 ° C.
  • the steam pasteurization method of the present invention mainly includes “pre-cooling-vacuum”
  • pre-cooling-vacuum The four stages of steam-steaming, this steam pasteurization method is particularly suitable for the sterilization of freeze-dried foods:
  • the structure of the freeze-dried food is porous and loose, which is beneficial to the diffusion of steam inside the food, and the sterilization process is uniform and rapid.
  • Cooling and cooling of the food before sterilization can increase the water content of the food after contact with water vapor. Significantly increase water activity.
  • the water on the surface of the food and the condensed water in the pores is evaporated by the steam trapping process, and the food is cooled to ensure the quality of the food.
  • the sterilization method adopted by the invention is: pre-cooling the temperature of the lyophilized food to a suitable temperature, and then condensing the low-pressure saturated steam in a vacuum environment on the surface and pores of the food at a lower temperature to raise the temperature of the food. At the same time, the moisture content increases and the water activity rises rapidly, allowing the food to be sterilized for a period of time at a suitable pasteurization temperature and a higher water activity.
  • the method of reducing the pressure by (collecting steam) causes the condensed water to evaporate, and the temperature and moisture content of the food are lowered, so that the food is in an appropriate state of preservation.
  • the sterilization method comprises the steps of:
  • the temperature range is 60 ° C to 90 ° C; the target water activity at the sterilization temperature, the target water activity range is 0.7 to 0.85.
  • the temperature T 1 that the food needs to reach before sterilization can be calculated by the following method: According to the principle of energy balance, the heat required for the temperature rise of the material is equal to the heat released by the steam condensation, regardless of the heat capacity of the carrying container. In the case, the energy balance relationship is:
  • c p is the constant pressure specific heat capacity of the food
  • T 1 is the temperature that needs to be reached before the food is sterilized
  • T 2 is the sterilization temperature
  • x 1 is the initial moisture content of the lyophilized food
  • x 2 is the moisture content of the food obtained by using the isothermal moisture absorption line of the food at the sterilization temperature and the target water activity;
  • h 1 is the enthalpy of saturated water vapor at a temperature of T 1 ;
  • h 2 is the enthalpy of saturated water at the sterilization temperature.
  • the temperature before the food is sterilized is the same.
  • T 1 T 2 -(x 2 -x 1 )*(h 1 -h 2 )/c p (2)
  • the set value of the target water activity is appropriately adjusted, and the sterilization effect is achieved by prolonging the sterilization time.
  • the pressure of the steam source is equal to or slightly higher than the saturation pressure of the water corresponding to the pasteurization temperature. After the temperature of the food reaches the sterilization temperature, the steam flow rate is adjusted, and the food is stopped at the sterilization temperature for a period of time, and the water vapor is stopped. The time for maintaining the sterilization temperature is 10 minutes to 90 minutes.
  • the sterilization chamber is connected to a cold trap having a low surface temperature (temperature range of -40 ° C to 10 ° C), and the vapor is condensed or dehydrated on the surface of the cold trap, and then the non-condensable gas is extracted by a vacuum pump.
  • a cold trap having a low surface temperature (temperature range of -40 ° C to 10 ° C)
  • the vapor is condensed or dehydrated on the surface of the cold trap, and then the non-condensable gas is extracted by a vacuum pump.
  • the sterilization method of the present invention will be further explained by taking a sterilization process in which the moisture content is 2.8% freeze-dried chicken as an example.
  • the pasteurization temperature to be set is 75 ° C and the target water activity is 0.75.
  • the isothermal moisture absorption line of the material was examined, and the water activity at a temperature of 25 ° C and a water content of 2.8% was 0.06, and the corresponding moisture content was 8.4% at a temperature of 75 ° C and a water activity of 0.75.
  • the specific heat capacity of the freeze-dried chicken is 1.7 kJ/(kg ° C), and the temperature of the material before steam sterilization is 3 ° C calculated by the above formula (2).
  • the sterilization chamber is connected to the cold trap, the refrigerant is passed through the cold trap, the evaporation temperature is maintained at about -20 ° C, the steam is condensed on the surface of the cold trap, and when the pressure in the sterilization chamber drops to 3000 Pa, the vacuum pump is turned on to extract the non-condensable gas.
  • the condensed water on the surface of the material and in the pores evaporates, and the water vapor freezes on the surface of the cold trap, and the temperature of the material drops.
  • the water content of the material after sterilization was 3.6%, and the number of inoculated Enterococcus faecalis was reduced to 4.0 ⁇ 10 3 CFU/g, which decreased by 5.2 log units.
  • the freeze-dried chicken was examined for an isothermal moisture absorption line at 25 ° C, and the water activity at a water content of 3.6% was 0.18. Although the water content and water activity of the material after sterilization treatment increased slightly, it had little effect on the quality and shelf life of the product.

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  • Engineering & Computer Science (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Chemical & Material Sciences (AREA)
  • Polymers & Plastics (AREA)
  • Food Science & Technology (AREA)
  • Nutrition Science (AREA)
  • Health & Medical Sciences (AREA)
  • Wood Science & Technology (AREA)
  • Zoology (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Freezing, Cooling And Drying Of Foods (AREA)
  • Apparatus For Disinfection Or Sterilisation (AREA)
  • Food Preservation Except Freezing, Refrigeration, And Drying (AREA)

Abstract

一种冻干食品的蒸汽巴氏灭菌方法，包括：1)根据需要灭菌的冻干食品的种类对应确定灭菌温度，以及在灭菌温度下的目标水活度；2)对步骤1)的食品冷却降温至与食品的种类对应的冷却温度；3)用真空泵抽除食品所在的灭菌室内的空气；4)往灭菌室通入水蒸气，在食品温度达到灭菌温度后，调节蒸汽流量，使食品温度维持在灭菌温度一段时间后停止通入水蒸气；5)对所述灭菌室进行捕汽降压；6)在灭菌室内的压力达到或接近灭菌前冷却温度所对应的水的饱和压力后，向灭菌室内充入洁净空气直至真空状态解除，取出食品即完成。

Description

冻干食品的蒸汽巴氏灭菌方法 技术领域
本发明涉及食品蒸汽灭菌的方法，特别是一种冻干食品的蒸汽巴氏灭菌方法。
背景技术
低水分食品的微生物污染近些年来已成为新的食品安全问题，与之相关的食源性疾病和食品召回问题多次发生，涉及的食品包括干果、香料和宠物干燥食品等。这其中最常出现的致病菌是沙门氏菌，其次是单核细胞增生李斯特菌和大肠杆菌O157：H7，这些致病菌均表现出了在低水活度下耐热性增加的特点。
水活度指食品的饱和蒸气压与相同温度下纯水的饱和蒸气压的比值，是食品中自由水含量高低的一种表示方式。低水分活度食品一般指水活度小于0.7的食品，沙门氏菌等食源性病原菌在低水活度食品中无法生长繁殖，但却可以长期存活。低水活度食品的灭菌往往需要更高温度或更长时间的热处理，以达到病原菌种群的充分降低，但是处理温度高或加热时间长会影响产品的品质或产量，产生诸如营养成份损失、变形变色、口感变差、耗能增加、生产效率低等问题。
低水活度食品采用蒸汽巴氏灭菌是一种很好的解决方案，一方面热蒸汽的穿透能力强，能进入食品的孔隙内，水分子的存在有助于破坏维持蛋白质结构的氢键和其他相互作用弱键，更易使蛋白质变性。另一方面巴氏灭菌的温度通常在60～90℃，既可杀死致病菌又能保持食品中营养物质成份和风味不变。
由荷兰Duyvis Wiener公司(http://duyviswiener.com)开发的Log5系统，低水分食品在烘烤炉中被加热，将潮湿空气与饱和蒸汽混合，以湿热处理水活度和温度，在避免影响产品质量的同时实现5个对数单位的细菌减少，加热过程中食品温度高于露点，不发生吸湿。
美国的Napasol North America LLC(http://www.napasol.com)将坚果、种子和干果预热后在静态或旋转式高压釜中，使用饱和蒸汽和真空，在相对较低的温度下进行巴氏杀菌，然后将产品转移至冷却室。
法国的Revtech Process Systems(http://www.revtech-process-systems.com)的蒸汽灭菌单元是基于外表面电加热的振动螺旋管，螺旋管内的产品与管壁接触并在振动下被预热，之后管内通入蒸汽进行灭菌，最后在后段螺旋管内通入冷却空气对产品进行冷却，整个流程连续运行。
申请号为CN200880023032.6，发明名称为“用于食品块的表面巴氏灭菌和消毒的方法”的中国专利提出了适用于含油种籽食品的表面巴氏灭菌方法：将食品预热到接近巴氏灭菌的蒸发温度，以使食品的水分吸收被最小化，热处理进行1～30min，在进一步降低的压力下借助后续的真空干燥除去食品表面的凝结水。
在低水分食物中，经真空冷冻干燥(简称冻干)过程处理的食品多为多孔疏松结构，有利于蒸汽在食品内部的扩散，但相比用其它干燥方式处理的食品，冻干食品的水分含量更低，一般在2％～5％之间，食品中致病菌的灭杀难度也更大。
我们采用申请号为CN200880023032.6的专利所公开的所述的方法对一种冻干鸡肉块进行灭菌实验，将含水率为2.8％、接种了粪肠球菌(ATCC 8459)的冻干鸡块预热到68℃，之后在真空环境中通入饱和蒸汽灭菌，物料温度上升到75℃后维持在该温度下进行蒸汽灭菌60min,结果发现粪肠球菌的数量从灭菌前的2.2×10 8CFU/g减少到灭菌后的1.75×10 7CFU/g，仅减少了1.1个对数单位，距离达到5个对数单位的目标减少量相距甚远。经测试，所述含水率2.8％的冻干鸡块在25℃和75℃下的水活度分别仅为0.06和0.39。实验表明采用申请号为CN200880023032.6公开的方法对这么低水活度的冻干食品进行灭菌的效果并不理想。
除了一定的巴氏灭菌温度及饱和蒸汽环境外，食品自身的水活度也是影响其灭菌效果的重要因素。在合适的时间内要有效地灭除诸如沙门氏菌等在低水活度下耐热性提升的致病菌，大幅度地提高冻干食品的水活度是关键。而现有的蒸汽灭菌方法对食品水活度的提升很有限，要使水活度极低的冻干 食品在保证产品品质的前提下，并在合适的时间内，达到5个对数单位以上的灭菌效果几乎是不可能的。
发明内容
鉴于现有技术存在的上述问题，本发明的目的在于提供一种适用于低水活度的冻干食品的蒸汽巴氏灭菌方法，灭菌过程中利用饱和蒸汽提高食品温度的同时大幅度地提升食品的水活度，以在合适时间内达到5个对数单位以上的巴氏灭菌效果。
为了实现上述目的，本发明提供的一种冻干食品的蒸汽巴氏灭菌方法，该方法包括：
1)根据需要灭菌的冻干食品的种类确定对应灭菌温度，并确定在灭菌温度下的目标水活度；
2)对进行步骤1)的食品进行冷却降温至与食品的种类对应的冷却温度；
3)用真空泵抽除食品所在的灭菌室内的空气；
4)在灭菌室通入水蒸气，在食品温度达到灭菌温度后，调节蒸汽流量，使食品温度维持在灭菌温度一段特定灭菌时间后停止通入水蒸气；
5)对所述灭菌室进行捕汽降压处理；
6)在灭菌室内的压力到或接近灭菌前冷却温度所对应的水的饱和压力后，向灭菌室内充入洁净空气直至真空状态解除，取出食品即完成。
作为优选，所述步骤2)中的冷却温度通过如下方式确定：
T 1＝T 2-(x 2-x 1)*(h 1-h 2)/c p
式中c p为食品的定压比热容；
T 1为冷却温度；
T 2为灭菌温度；
x 1为冻干食品的初始水分含量；
x 2为利用食品的等温吸湿线得到的食品在灭菌温度和目标水活度下的水分含量；
h 1为在T 1温度下的饱和水蒸气的焓值；
h 2为在灭菌温度下的饱和水的焓值。
作为优选，所述步骤1)中确定的灭菌温度为60～90℃，目标水活度为0.70～0.85。
作为优选，所述步骤3)中在抽除灭菌室内的空气时，使灭菌室内的绝对压力下降到100～3000Pa后停止抽真空。
作为优选，所述步骤4)中向灭菌室内通入的水蒸气为负压饱和水蒸气。
作为优选，步骤4)中向灭菌室内通入的水蒸气为负压饱和水蒸气时，蒸汽源的压力等于或稍高于灭菌温度所对应的水的饱和压力。
作为优选，所述步骤4)中的负压饱和蒸汽压力对应的饱和温度比巴氏灭菌温度高0～10℃。
作为优选，所述步骤4)中的特定灭菌时间为10min～90min。
作为优选，所述步骤5)具体为：将灭菌室与一冷阱连通，蒸汽在冷阱表面凝结或凝华，再通过真空泵抽取不凝性气体。
作为优选，所述冷阱的表面温度为-40℃～10℃。
本发明达到的有益效果是：
与现有的蒸汽灭菌方法主要包括“真空-蒸汽-真空”或“预热-真空-蒸汽-冷却”的流程不同，本发明所述的蒸汽巴氏灭菌方法主要包括“预冷-真空-蒸汽-捕汽”这四个阶段，这种蒸汽巴氏灭菌方法特别适用于冻干食品的灭菌：
冻干食品结构上多孔疏松，有利于蒸汽在食品内部的扩散，灭菌过程均匀、快速。
对于低含水量的冻干食品，水分含量的少量变化就会引起水活度大幅度的变化，食品在灭菌之前的冷却降温可以使食品在与水蒸气接触后更多地增加含水量，以大幅度地提高水活度。
食品在灭菌后通过蒸汽捕集过程使食品表面及孔隙内凝结的水分蒸发，并对食品降温，保证了食品的品质。
具体实施方式
以下对本发明的具体实施方案做进一步详细的说明。
本发明采用的灭菌方法是：先将冻干食品温度预先冷却到一个合适的温度，然后使低压饱和蒸汽于真空环境下在较低温度的食品的表面及孔隙内部凝结，使食品温度升高，同时水分含量增加，水活度迅速上升，让食品在合 适的巴氏灭菌温度和较高的水活度下维持一段时间进行灭菌。灭菌结束后通过(捕集蒸汽)降低压力的方法使凝结水分蒸发，食品的温度和水分含量下降，使食品处于适宜的保存状态。
在本发明的一些实施方式中，该灭菌方法包括以下步骤：
(1)首先选取适合的冻干食品的巴氏灭菌温度，该温度范围为60℃～90℃；在该灭菌温度下的目标水活度，该目标水活度范围为0.7～0.85。
(2)对要灭菌的冻干食品进行冷却降温。食品在灭菌前需要达到的温度T 1可利用下述方法计算得到：根据能量守衡的原理，物料温度上升所需的热量与蒸汽凝结释放的热量相等，在不考虑承载容器的热容的情况下，能量平衡关系式为：
c p*(T 2-T 1)＝(x 2-x 1)*(h 1-h 2)  (1)
式中c p为食品的定压比热容；
T 1为食品灭菌前需要达到的温度；
T 2为灭菌温度；
x 1为冻干食品的初始水分含量；
x 2为利用食品的等温吸湿线得到的食品在灭菌温度和目标水活度下的水分含量；
h 1为在T 1温度下的饱和水蒸气的焓值；
h 2为在灭菌温度下的饱和水的焓值。
得到食品灭菌前的温度为
T 1＝T 2-(x 2-x 1)*(h 1-h 2)/c p(2)
通过上述计算如果得到的T 1值太低，实现起来困难，则适当调整目标水活度的设定值，通过延长灭菌时间来达到需求的灭菌效果。
(3)用真空泵抽除食品所处的灭菌室内的空气，使灭菌室绝对压力下降到100～3000Pa后停止抽真空。
(4)往灭菌室内通入负压饱和水蒸气，蒸汽源的压力等于或稍高于巴氏灭菌温度所对应的水的饱和压力。在食品温度达到灭菌温度后，调节蒸汽流量，食品在该灭菌温度下维持一段时间后停止通入水蒸气，这段维持灭菌温度的时间在10min～90min。
(5)将灭菌室与一个低表面温度(温度范围为-40℃～10℃)的冷阱连通，蒸汽在冷阱表面凝结或凝华，再通过真空泵抽取不凝性气体。当灭菌室内压力低于食品温度对应的水的饱和压力时，食品中凝结的水分蒸发，食品温度下降。
(6)灭菌室压力达到或接近灭菌前冷却温度T 1所对应的饱和压力后，停止抽真空后并往灭菌室内充入干净空气，直到灭菌室真空状态解除，取出食品，灭菌过程结束。
实施例1
以下以一种含水量为2.8％冻干鸡肉的灭菌过程为例，对本发明的灭菌方法做进一步的解释。
(1)参数设定与计算
设定要采用的巴氏灭菌温度为75℃，目标水活度0.75。查该物料的等温吸湿线，在温度为25℃和含水量为2.8％时的水活度为0.06，在温度为75℃和水活度为0.75时对应的水分含量为8.4％。该冻干鸡肉的定压比热容为1.7kJ/(kg℃)，由前述公式(2)计算得到物料在蒸汽灭菌前的温度为3℃。
(2)抽真空
在冻干鸡肉中取数块接种粪肠球菌(ATCC 8459)，接种数量为6.3×10 8CFU/g；将冻干鸡肉在冷库内冷却到-1℃后取出，放到灭菌室内用旋片式真空泵抽真空至1000Pa，在此过程中食品温度回升到3℃。
(3)蒸汽灭菌
停止抽真空，往灭菌室内充入78℃的饱和蒸汽，蒸汽在物料表面及孔隙内凝结，物料温度在10分钟左右从3℃升高到了74℃，调节蒸汽量，使物料温度稳定在75±1℃，并维持45min。
(4)蒸汽捕集
将灭菌室与冷阱接通，冷阱内通制冷剂，蒸发温度维持在约-20℃，蒸汽在冷阱表面凝结，等灭菌室压力下降到3000Pa时开启真空泵抽取不凝性气体。在降压过程中，物料表面及孔隙内的凝结水蒸发，水蒸汽在冷阱表面结霜，物料温度下降。
(5)解除真空
灭菌室内压力降至700Pa时，物料温度下降到20℃。解除灭菌室真空，取出物料。
(6)测试结果
经测试，物料在灭菌后的含水量为3.6％，接种的粪肠球菌数量降低为4.0×10 3CFU/g，下降了5.2个对数单位。查该冻干鸡肉在25℃的等温吸湿线，含水量为3.6％时的水活度为0.18。虽然灭菌处理后物料的含水量及水活度略有增加，但对产品的品质及保存期影响很小。

Claims (10)

  1. 一种冻干食品的蒸汽巴氏灭菌方法，其特征在于，该方法包括：
    1)根据需要灭菌的冻干食品的种类对应确定灭菌温度，并确定在灭菌温度下的目标水活度；
    2)对进行步骤1)的食品进行冷却降温至与食品的种类对应的冷却温度；
    3)用真空泵抽除食品所在的灭菌室内的空气；
    4)在灭菌室通入水蒸气，在食品温度达到灭菌温度后，调节蒸汽流量，使食品温度维持在灭菌温度一段特定灭菌时间后停止通入水蒸气；
    5)对所述灭菌室进行捕汽降压处理；
    6)在灭菌室内的压力到或接近灭菌前冷却温度所对应的水的饱和压力后，向灭菌室内充入洁净空气直至真空状态解除，取出食品即完成。
  2. 根据权利要求1所述的冻干食品的蒸汽巴氏灭菌方法，其特征在于，所述步骤2)中的冷却温度通过如下方式确定：
    T 1＝T 2-(x 2-x 1)*(h 1-h 2)/c p
    式中c p为食品的定压比热容；
    T 1为冷却温度；
    T 2为灭菌温度；
    x 1为冻干食品的初始水分含量；
    x 2为利用食品的等温吸湿线得到的食品在灭菌温度和目标水活度下的水分含量；
    h 1为在T 1温度下的饱和水蒸气的焓值；
    h 2为在灭菌温度下的饱和水的焓值。
  3. 根据权利要求1所述的冻干食品的蒸汽巴氏灭菌方法，其特征在于，所述步骤1)中确定的灭菌温度为60～90℃，目标水活度为0.70～0.85。
  4. 根据权利要求1所述的冻干食品的蒸汽巴氏灭菌方法，其特征在于，所述步骤3)中在抽除灭菌室内的空气时，使灭菌室内的绝对压力下降到100～3000Pa后停止抽真空。
  5. 根据权利要求1所述的冻干食品的蒸汽巴氏灭菌方法，其特征在于，所述步骤4)中向灭菌室内通入的水蒸气为负压饱和水蒸气。
  6. 根据权利要求1所述的冻干食品的蒸汽巴氏灭菌方法，其特征在于，步骤4)中向灭菌室内通入的水蒸气为负压饱和水蒸气时，蒸汽源的压力等于或稍高于灭菌温度所对应的水的饱和压力。
  7. 根据权利要求5所述的冻干食品的蒸汽巴氏灭菌的方法，其特征在于，所述步骤4)中的负压饱和蒸汽压力对应的饱和温度比巴氏灭菌温度高0～10℃。
  8. 根据权利要求1所述的冻干食品的蒸汽巴氏灭菌的方法，其特征在于，所述步骤4)中的特定灭菌时间为10min～90min。
  9. 根据权利要求1所述的冻干食品的蒸汽巴氏灭菌的方法，其特征在于，所述步骤5)具体为：将灭菌室与一冷阱连通，蒸汽在冷阱表面凝结或凝华，再通过真空泵抽取不凝性气体。
  10. 根据权利要求8所述的冻干食品的蒸汽巴氏灭菌的方法，其特征在于，所述冷阱的表面温度为-40℃～10℃。
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