To comprehends fully the principles of food sanitation. One must understand the role of microorganisms in food spoilage and poisoning. Microorganisms are found throughout the natural environment. These microorganisms (also called microbes) cause food spoilage through ingestion of food, which contains microorganism’s food borne infection because certain disease con microorganisms of public health concern. Microorganisms are also important in connection with food sanitation because certain disease-producing microbes may be transmitted through food. The importance of sanitation practices is to combat the proliferation and activity of food spoilage and food poisoning microorganisms.
Microorganisms in food

    The microorganisms most common to food are bacteria and fungi. The fungi, which are less common than bacteria, consist of two major microorganisms, molds and yeasts. 

    Molds are multicellular microorganisms with mycelial (filamentous) morphology. These microbes are also characterized by their display of a variety of colors and generally recognized by their mildewy or fuzzy, cotton like appearance. Molds can develop numerous tiny spores that are t found in the air and can be spread by air currents. These spores can produce new mold growth if they are transferred to a location that hears conditions conductive to germination. Mold generally withstands greater fluctuation in PH than bacteria and yeasts and can frequently tolerate more temperature fluctuation.

    Although molds thrive best at or near a PH of 7.0,a PH range of from 2.0 to 8.0 can be tolerated at ambient temperature than in a colder environment, even though an acid to neutral PH is preferred. Molds thrive better at ambient temperature than in a colder environment ,even though  growth  can occur below 0℃.Although mold growth is optimal at a water activity (Aw) of approximately 0.85,growth can and does occur below 0.80 .at an Aw of 0.90 or higher, bacteria and yeasts grow more effectively and normally utilize available nutrients for growth at the expense of molds. When the Aw goes below 0.90 molds grow more growth. 

    Yeasts are generally unicellular and differ from bacteria in their larger cell size and morphology, and because they produce buds during the process of reproduction by division.Like molds, yeasts can be spread through the air, or other means, and alights on the surface of foodstuffs. Yeast yeasts prefer an Aw of 0.90-0.94,but can grow below 0.90. These microorganisms grow best in the intermediate acid range; a PH of from 4.0 to 4.5 foods that highly contaminated with yeasts will frequently have a slightly fruity odor. 

    Bacteria are unicellular microorganisms that are approximately 1 \m in diameter with morphology variation from short and elongated rods (bacilli) to spherical or ovoid forms, Cocci are spherically shaped bacteria. Individual bacteria closely combine in various forms according to genera. Some sphere-shaped bacteria occur in clusters similar to a bunch of grapes (i.e., staphylococci). Other bacteria (rod shaped of sphere shaped) are linked together to form chains (i.e., streptococci). Certain genera of sphere-shaped bacteria also are formed together in pairs (diploid formation; i.e., pneumococci) or as a group of four (tetrad formation; i.e., sarcinia), while other genera appear as an individual bacterium. Other bacteria possess flagella and are motile. 

    Bacteria produce various pigments, which range from shades of yellow to dark pigments such as brown or black. Certain bacteria have pigmentation of intermediate colors such as red, pink, orange, blue green, or purple. These bacteria cause food discoloration, especially among foods with unstable color pigments such as meat. Some bacteria also cause discoloration by slime formation. 

    Some species of bacteria also produce spores. The properties of which vary considerably. Certain spores are resistant of heat, chemicals, and other adverse environmental conditions. Many of these spore-forming bacteria are thermophilic microorganisms, which produce a toxin that will cause food poisoning. 

    Methods of Killing Microorganisms

    Before assessment of methods available for d3estruction of microorganisms, it is important to define death as applied to microorganisms. 

    Microorganisms are considered dead when they cannot multiply, even after being in a suitable growth medium under favorable environmental conditions. This concept differs from dormancy, especially of bacterial spores, since dormant microbes have not lost permanently the ability to reproduce as evidenced by eventual multiplication after prolonged incubation, transfer to a different growth medium, or some form of activation.
Regardless of the cause of death, microorganisms follow a logarithmic rate of death. 

    This pattern suggests that the population of microbial cells is dying at a relatively constant rate. Deviations from this death rate can occur due to accelerated effects from a lethal agent (i.e., sanitizers), effects due to a population mixture of sensitive and resistant cells, of with chain-or clump forming microflora with uniform resistance to the environment.

    Heat 

    Application of heat has historically been the most widely used method of killing spoilage and pathogenic bacteria in foods. Heat processing has been considered a way to cook food products and destroy spoilage and pathogenic microorganisms. Therefore, extensive studies have been conducted to determine optimal heat treatment for destruction of microorganisms. A measurement of time required to sterilize completely a suspension of bacterial cells of spores at a given temperature is thermal death time (TDT). The value of TDT will depend upon the nature of the subject microorganism, number of cells, and factors related to the nature of the growth medium. 

    Another measurement of microbial destruction is decimal reduction time (D value), which is the time in minutes required to destroy 90% of the cells at a given temperature. Again the value depends on the nature of the microorganism, characteristics of the medium, and calculation method for determining the D value. This value is calculated for a period of exponential death of microbial cells (following the logarithmic order of death). The D value can be determined through an experimental survivor curve. 

    A thermal resistance curve (phantom thermal death time curve) may be plotted from D values or TDT values at different heating temperatures. The slope of this curve is the Z value. The Z value is defined as the number of degrees that the temperature must be increased to cause a log (90%)reduction in D. The ordinate of this thermal death time curve may also be TDT values. Therefore, the destruction rate of a specific strain of bacteria (or its spores) in a foodstuff is best described by its D and Z values. 

    Chemicals 

    Many chemical compounds that destroy microorganisms are not appropriate for killing bacteria in or on a foodstuff. Those chemicals, which can be used, are applied as sanitizing agents for equipment and utensils that can contaminate food.             The survivor curve of microorganisms treated with chlorine reveals a deviation from the logarithmic order of death that is normally obtained from heat treatment of sanitizing with other bactericidal agents by being S shaped. This deviation cannot be fully explained, but the cell or the necessity of inactivating multiple sites within the cell before death results. 

    Radiation 

    When microorganisms in foods are irradiated with high-sped electrons (beta rays) have with X-rays (or gamma rays), the log of the number of survivors is directly proportional to the radiation dose. The relative sensitivity of a specific strain of microorganism subjected to specific conditions is normally expressed as the slope of the survivor curve. The log10 of survivors from radiation is plotted against the radiation dosage, and the radiation D or D10, which is comparable with the thermal D value, is obtained. The D10 is defined as the amount of radiation in rads (ergs of energy per 100g of material) required reducing the microbial population by 1 log (90%). 

    The destruction mechanism of radiation is not fully understood. It appears that death is caused by inactivation lf components within the cell through that death is caused by inactivation of components within the cell through energy absorbed within the cell. Inactivation by radiation results in an inability of the cell to divide and produce visible outgrowth.

    Methods of Inhibiting Microbial Growth 

    Most methods used to kill microorganisms may be applied in a milder treatment to inhibit microbial growth. Milder treatment of microbial cells through sublethal heating, irradiation, or toxic chemicals frequently causes injury and impaired growth without death. Injury is reflected through an increased lag phase, bitory conditions. Synergistic combinations of inhibitory agents such as irradiation and heat and chemicals can increase microbial sensitivity to inhibitory conditions. Injured cells appear to require synthesis of some essential cell materials, that is, ribonucleic acid or enzymes before recovery is accomplished. The major methods for microbial inhibition will be discussed under the topics that follow.

    Refrigeration 

    Freezing and subsequent thawing will kill some of the microflora. Those that survive freezing will not proliferate during frozen storage. Yet, this is not a practical method of reducing the microbial load. Also, microorganisms that survive frozen storage will grow on thawed foods at a rate similar to those, which have not been frozen. Refrigerated storage can be used in conjunction With other methods of inhibition, that is, preservatives, heat, and irradiation. 

    Chemicals 

    Chemicals that increase osmotic pressure with reduced Aw below the level that permits growth of most bacteria can be used as bacteriostats. Examples include salt and sugar. Nitrite, which is used in cured meats, also functions as a bacteriostat.

    Dehydration 

    Reduction of microbial growth by dehydration is another method of reducing the Aw to a level that prevents microbial proliferation. Some dehydration techniques restrict the types of microorganisms that may multiply and cause spoilage. Dehydration is most effective when combined with other methods of controlling microbial growth such as salting and refrigeration. 

    Fermentation 

    In addition to desirable flavors produced from fermentation, this technique can control microbial growth. Fermentation functions through anaerobic metabolism of sugars by acid-producing bacteria that lower the pH of the substrate, which would be the foodstuff. A pH below 5.0 restricts growth of spoilage microorganisms. Acid products that result from fermentation contribute to a lower may be packed in hermetically sealed containers to prevent spoilage by aerobic growth of yeasts and molds 

    为了深刻理解食品卫生的原理，就必须了解微生物在食物腐败和食物中毒中的作用。微生物在整个自然环境中到处存在。这些’微生物致使食物因色、香、味品质下降而败坏，致使人体因摄入的食物含有与公共卫生有关的微生物而罹致食物传染病。微生物还与食品卫生有密切的联系，因为某些致病微生物能通过食物传播。卫生作业的重要性就在于与食物腐败性和食物中毒性微生物的繁殖和活动进行斗争。

    食物中的微生物 

    食品中最普通的微生物是细菌和真菌。真菌不如细菌普遍，它包括两种主要微生物，霉菌和酵母菌。

    霉菌：霉菌是带有菌丝(丝状)形态的多细胞微生物。这些微生物的另外特征是显示出各种各样的颜色，并且常常靠它们发霉(即绒毛状)的象棉花一样的外观来识别。霉菌可以产生大量很小的孢子，这些孢子可在空气中找到，并会被气流传播开来。这些孢子如果被传播到发芽条件有利的地方，便产生新长出的霉菌。霉菌常常比细菌和酵母能耐受更大的pH波动，还常常能耐受更大的温度波动。尽管霉菌在pH 7.0或接近7.0时生长旺盛，但它也能耐受pH范围从2.0到8.0的变化，不过pH由酸性到中性更好。霉菌在常温下比在较冷环境下要生长得更好，在0℃以下也能生长。霉菌生长的最适水分活度(Aw约为0.85但低于0.80时也能生长。在Aw大于或等于0.90时，细菌和酵母的生长比较旺盛，并且常常以牺牲霉菌生长为代价耗用可利用的营养物质来生长。  直到Aw低于0.90时，霉菌生长才比较旺盛。这就是为什么象糕点、奶酪和坚果之类水份含量低的食物容易由于霉菌的生长而腐败。

    酵母：酵母一般为单细胞，它与细菌不同地方不仅在于它细胞较大和形态不同，而且还由于在分裂繁殖过程中产生芽体。象霉菌一样，酵母也可以通过空气或其它途径传播，并落在食物表面上。酵母菌落表面通常是潮湿或粘呼呼的，呈乳白色。酵母生长的适宜水份活度Aw为0.90、0.94，但也能在低于0．90时生长。这些微生物在中等酸性范围(pH4．0、4.5)内生长最好。有严重酵母污染的食物常常会有轻度的水果气味。

    细菌：细菌是单细胞微生物，直径约为1um，形状从长长短短的杆状(杆菌)直到球形或卵球形。球菌是球形细菌。单个细菌按其菌属以各种形式紧连在一起。有些球形细菌以团块形式出现，类似一串葡萄(即葡萄球菌)。其它细菌(杆状或球状)连在一起形成链(即链球菌)。球形细菌的某些菌属还以成对在一起呈形(形成二倍体，即肺炎双球菌)或以四个一组呈形(形成四联体，即四联球菌），而另一些菌属则呈单个细菌的形式。还有些细菌具有鞭毛，而且能运动。

    细菌会产出多种色素，从浓淡不等的黄色色素直到棕色或黑色的深色色素。某些细菌有着中间色的色素沉积，如红、粉红、橙黄、蓝、绿或紫色。这些细菌引起食品变色，尤其是像肉类等含有不稳定色素的食品。，有些细菌还会因粘液形成引起变色。

    杀灭微生物的方法

    在评价消灭微生物的有效方法之前，对适用于微生物的“死亡”作出定义是很重要的。微生物即使在有利环境条件下处于适宜生长培养基中以后也不能增殖时，便认为该微生物是死亡的。这个概念有别于休眠，尤其是细菌芽抱的休眠，因为休眠微生物并没有永远失去再生的能力，正如经过长时间培养或向另一生长培养基转移或某种形式活化之后仍能繁殖起来所证明的。

    不论死亡的原因如何，微生物的死亡遵循对数致死率。这种死亡方式意味着微生物细胞群体以相对恒定的比率在死亡。但偏离这一致死率有可能发生，原因是某种致死物质(如消毒剂)的加速作用，也可能是敏感细胞和稳定细胞混合菌群或者与对环境有一致抵抗力的成链或成簇菌丛在一起所引起的作用。

    加热法

    加热历来是杀灭食物中腐败菌和致病菌最广泛使用的方法。一般认为热处理是煮熟食品消灭腐败微生物及致病微生物的一种手段。因此，人们已经进行了广泛的研究，以确定为杀灭微生物所需的最适热处理程度。在指定温度下为某细菌细胞(或芽孢)悬浮液完全灭菌所需的时间测定值就是“热致死时间”(TDT)。TDT值将取决于对象微生物的特性、细胞的数量和与生长培养基性质有关的因素。

    有关微生物死亡的另一种量度就是“十减余一时间”(D值)， 这是在给定温度下为杀灭90％细、脑所需要的以分钟计的时间。同样，此值也取决于微生物的特性、培养基的性质和确定D值时的计算方法。此D值是针对微生物细胞的一段指数死它时间(按对数死亡量级)推算出来的，也可通过实验的存活曲线来确定。

    从不同加热温度下的D值或TDT值出发，可以标绘出一条耐热性曲线(假热死时间曲线)，  我们将以分钟为单位的D的对数值对加热温度进行标绘，这条线的斜率是Z值。Z值的定义为D值减少一个对数周期(90％)所必须升高的温度数。此热死时间曲线的纵坐标也可以是TDT值。因此，  对食物中特定菌株的细菌(或其芽抱)、，最好用D值和Z值表达它的致死率。

    化学法

    许多能杀灭微生物的化合物并不适宜于杀灭食物内部或其表面上的微生物。而可用的化学物质又都是作为设备和器皿(可能会污染食品)的卫生消毒剂使用。由于热力消毒的能量费用愈来愈高，所以使用化学消毒剂便愈来愈多。用氯处理的微生物的存活曲线呈S型，说明与通常由热处理法或其他杀菌剂消毒法所得到的对数量级死亡有一定的偏差。这种偏差不能得到完满的解释，但一般假定：氯的消毒作用可能是由于氯渗入细胞很慢造成的，换言之，在细胞死亡之前细胞内部要有多个失活的部位。

    照射法

    当食品中的微生物受到高速电子(β射线)或X—线(γ射线)照射时，微生物存活数的对数值与照射剂量成正比。一般用存活曲线的斜率表示具体微生物菌株在特定条件下的相对敏感性。若将照射中存活微生物数目的log10值对照射剂量进行标绘，则可求出辐照D值(即D10值)，此值与热力D值相当。D10值的意义是微生物总数减少一个对数周期(90％)所必需的照射拉德数(每100克物质得到的能量尔格数)。

    还没有完全弄清照射致死的机理。看来死亡是由于细胞内部的构成部分因胞内吸收的能量而钝化引起的。照射所产生的钝化作用使得细胞无力分裂生成明显的分支。

抑制微生物生长的方法

    大多数用来杀灭微生物的方法都可以以温和的处理方式用来抑制微生物的生长。微生物细胞常通过亚致死的加热照射或毒性化学物质的温和处理而受到杀伤和生长障碍但不致死亡。微生物生长迟滞期的延长，它对环境条件抗抵力的减弱以及它对其它抑制条件的更加敏感都是这种杀伤作用的反映。几种抑制因素的协同配合，如照射与加热、加热与化学药品等均能增强微生物对抑制条件的敏感性。受杀伤的细胞似乎要合成一些必不可少的细胞物质，如核糖核酸或酶类，然后才能达到完全复原。下面将分题讨论一下抑制微生物的几种主要方法：
   
    冷藏法

    冷冻和其后的解冻会杀死部分区系的微生物。冷冻时存活下来的微生物在冻结贮藏期间不会增殖。但这不是降低微生物数量的实用方法。此外，冻藏中存活的微生物也会在解冻食物上生长，其生长速率类似于它在未被冻结食品上的生长速率。可以把冷藏法与其它抑制方法，如加防腐剂、加热和照射等结合起来使用。

    化学药品法

    凡是使渗透压升高从而使Aw下降至大多数细菌能生长的水平以下的化学物质， 都可以用作抑菌剂。例如盐和搪。用于俺肉的亚硝酸盐也具有抑菌剂的功能。

    脱水法

    用脱水法减慢微生物生长是又一种降低Aw值到阻止微生物增殖水平的方法。 有些脱水方法可以抑制若干类固增殖而导致腐败的微生物。脱水如果与其它控制微生物生长方法如盐渍和冷藏结合起来是最有效的。

    发酵法

    发酵法徐了产生必需的风味以外，还能控制微生物的生长。发酵是通过产酸菌对糖类的厌氧代谢活动降低底物(可能就是食物)的pH值而起作用的。pH值低于5．0便抑制腐败微生物的生长。由发酵作用形成的酸性产物是pH值降低、微生物活动减慢的主要原因。经酸化和加热过的食品可以包装在密封的容器中、以阻止因好气性酵母和霉菌的生长而引起的腐败。