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 

    為了深刻理解食品衛(wèi)生的原理，就必須了解微生物在食物腐敗和食物中毒中的作用。微生物在整個(gè)自然環(huán)境中到處存在。這些’微生物致使食物因色、香、味品質(zhì)下降而敗壞，致使人體因攝入的食物含有與公共衛(wèi)生有關(guān)的微生物而罹致食物傳染病。微生物還與食品衛(wèi)生有密切的聯(lián)系，因?yàn)槟承┲虏∥⑸�物能通過食物傳播。衛(wèi)生作業(yè)的重要性就在于與食物腐敗性和食物中毒性微生物的繁殖和活動(dòng)進(jìn)行斗爭。

    食物中的微生物 

    食品中最普通的微生物是細(xì)菌和真菌。真菌不如細(xì)菌普遍，它包括兩種主要微生物，霉菌和酵母菌。

    霉菌：霉菌是帶有菌絲(絲狀)形態(tài)的多細(xì)胞微生物。這些微生物的另外特征是顯示出各種各樣的顏色，并且常�？克鼈儼l(fā)霉(即絨毛狀)的象棉花一樣的外觀來識(shí)別。霉菌可以產(chǎn)生大量很小的孢子，這些孢子可在空氣中找到，并會(huì)被氣流傳播開來。這些孢子如果被傳播到發(fā)芽條件有利的地方，便產(chǎn)生新長出的霉菌。霉菌常常比細(xì)菌和酵母能耐受更大的pH波動(dòng)，還常常能耐受更大的溫度波動(dòng)。盡管霉菌在pH 7.0或接近7.0時(shí)生長旺盛，但它也能耐受pH范圍從2.0到8.0的變化，不過pH由酸性到中性更好。霉菌在常溫下比在較冷環(huán)境下要生長得更好，在0℃以下也能生長。霉菌生長的最適水分活度(Aw約為0.85但低于0.80時(shí)也能生長。在Aw大于或等于0.90時(shí)，細(xì)菌和酵母的生長比較旺盛，并且常常以犧牲霉菌生長為代價(jià)耗用可利用的營養(yǎng)物質(zhì)來生長。  直到Aw低于0.90時(shí)，霉菌生長才比較旺盛。這就是為什么象糕點(diǎn)、奶酪和堅(jiān)果之類水份含量低的食物容易由于霉菌的生長而腐敗。

    酵母：酵母一般為單細(xì)胞，它與細(xì)菌不同地方不僅在于它細(xì)胞較大和形態(tài)不同，而且還由于在分裂繁殖過程中產(chǎn)生芽體。象霉菌一樣，酵母也可以通過空氣或其它途徑傳播，并落在食物表面上。酵母菌落表面通常是潮濕或粘呼呼的，呈乳白色。酵母生長的適宜水份活度Aw為0.90、0.94，但也能在低于0．90時(shí)生長。這些微生物在中等酸性范圍(pH4．0、4.5)內(nèi)生長最好。有嚴(yán)重酵母污染的食物常常會(huì)有輕度的水果氣味。

    細(xì)菌：細(xì)菌是單細(xì)胞微生物，直徑約為1um，形狀從長長短短的桿狀(桿菌)直到球形或卵球形。球菌是球形細(xì)菌。單個(gè)細(xì)菌按其菌屬以各種形式緊連在一起。有些球形細(xì)菌以團(tuán)塊形式出現(xiàn)，類似一串葡萄(即葡萄球菌)。其它細(xì)菌(桿狀或球狀)連在一起形成鏈(即鏈球菌)。球形細(xì)菌的某些菌屬還以成對(duì)在一起呈形(形成二倍體，即肺炎雙球菌)或以四個(gè)一組呈形(形成四聯(lián)體，即四聯(lián)球菌），而另一些菌屬則呈單個(gè)細(xì)菌的形式。還有些細(xì)菌具有鞭毛，而且能運(yùn)動(dòng)。

    細(xì)菌會(huì)產(chǎn)出多種色素，從濃淡不等的黃色色素直到棕色或黑色的深色色素。某些細(xì)菌有著中間色的色素沉積，如紅、粉紅、橙黃、藍(lán)、綠或紫色。這些細(xì)菌引起食品變色，尤其是像肉類等含有不穩(wěn)定色素的食品。，有些細(xì)菌還會(huì)因粘液形成引起變色。

    殺滅微生物的方法

    在評(píng)價(jià)消滅微生物的有效方法之前，對(duì)適用于微生物的“死亡”作出定義是很重要的。微生物即使在有利環(huán)境條件下處于適宜生長培養(yǎng)基中以后也不能增殖時(shí)，便認(rèn)為該微生物是死亡的。這個(gè)概念有別于休眠，尤其是細(xì)菌芽抱的休眠，因?yàn)樾菝呶⑸�物并沒有永遠(yuǎn)失去再生的能力，正如經(jīng)過長時(shí)間培養(yǎng)或向另一生長培養(yǎng)基轉(zhuǎn)移或某種形式活化之后仍能繁殖起來所證明的。

    不論死亡的原因如何，微生物的死亡遵循對(duì)數(shù)致死率。這種死亡方式意味著微生物細(xì)胞群體以相對(duì)恒定的比率在死亡。但偏離這一致死率有可能發(fā)生，原因是某種致死物質(zhì)(如消毒劑)的加速作用，也可能是敏感細(xì)胞和穩(wěn)定細(xì)胞混合菌群或者與對(duì)環(huán)境有一致抵抗力的成鏈或成簇菌叢在一起所引起的作用。

    加熱法

    加熱歷來是殺滅食物中腐敗菌和致病菌最廣泛使用的方法。一般認(rèn)為熱處理是煮熟食品消滅腐敗微生物及致病微生物的一種手段。因此，人們已經(jīng)進(jìn)行了廣泛的研究，以確定為殺滅微生物所需的最適熱處理程度。在指定溫度下為某細(xì)菌細(xì)胞(或芽孢)懸浮液完全滅菌所需的時(shí)間測(cè)定值就是“熱致死時(shí)間”(TDT)。TDT值將取決于對(duì)象微生物的特性、細(xì)胞的數(shù)量和與生長培養(yǎng)基性質(zhì)有關(guān)的因素。

    有關(guān)微生物死亡的另一種量度就是“十減余一時(shí)間”(D值)， 這是在給定溫度下為殺滅90％細(xì)、腦所需要的以分鐘計(jì)的時(shí)間。同樣，此值也取決于微生物的特性、培養(yǎng)基的性質(zhì)和確定D值時(shí)的計(jì)算方法。此D值是針對(duì)微生物細(xì)胞的一段指數(shù)死它時(shí)間(按對(duì)數(shù)死亡量級(jí))推算出來的，也可通過實(shí)驗(yàn)的存活曲線來確定。

    從不同加熱溫度下的D值或TDT值出發(fā)，可以標(biāo)繪出一條耐熱性曲線(假熱死時(shí)間曲線)，  我們將以分鐘為單位的D的對(duì)數(shù)值對(duì)加熱溫度進(jìn)行標(biāo)繪，這條線的斜率是Z值。Z值的定義為D值減少一個(gè)對(duì)數(shù)周期(90％)所必須升高的溫度數(shù)。此熱死時(shí)間曲線的縱坐標(biāo)也可以是TDT值。因此，  對(duì)食物中特定菌株的細(xì)菌(或其芽抱)、，最好用D值和Z值表達(dá)它的致死率。

    化學(xué)法

    許多能殺滅微生物的化合物并不適宜于殺滅食物內(nèi)部或其表面上的微生物。而可用的化學(xué)物質(zhì)又都是作為設(shè)備和器皿(可能會(huì)污染食品)的衛(wèi)生消毒劑使用。由于熱力消毒的能量費(fèi)用愈來愈高，所以使用化學(xué)消毒劑便愈來愈多。用氯處理的微生物的存活曲線呈S型，說明與通常由熱處理法或其他殺菌劑消毒法所得到的對(duì)數(shù)量級(jí)死亡有一定的偏差。這種偏差不能得到完滿的解釋，但一般假定：氯的消毒作用可能是由于氯滲入細(xì)胞很慢造成的，換言之，在細(xì)胞死亡之前細(xì)胞內(nèi)部要有多個(gè)失活的部位。

    照射法

    當(dāng)食品中的微生物受到高速電子(β射線)或X—線(γ射線)照射時(shí)，微生物存活數(shù)的對(duì)數(shù)值與照射劑量成正比。一般用存活曲線的斜率表示具體微生物菌株在特定條件下的相對(duì)敏感性。若將照射中存活微生物數(shù)目的log10值對(duì)照射劑量進(jìn)行標(biāo)繪，則可求出輻照D值(即D10值)，此值與熱力D值相當(dāng)。D10值的意義是微生物總數(shù)減少一個(gè)對(duì)數(shù)周期(90％)所必需的照射拉德數(shù)(每100克物質(zhì)得到的能量爾格數(shù))。

    還沒有完全弄清照射致死的機(jī)理�？磥硭劳鍪怯捎诩�(xì)胞內(nèi)部的構(gòu)成部分因胞內(nèi)吸收的能量而鈍化引起的。照射所產(chǎn)生的鈍化作用使得細(xì)胞無力分裂生成明顯的分支。

抑制微生物生長的方法

    大多數(shù)用來殺滅微生物的方法都可以以溫和的處理方式用來抑制微生物的生長。微生物細(xì)胞常通過亞致死的加熱照射或毒性化學(xué)物質(zhì)的溫和處理而受到殺傷和生長障礙但不致死亡。微生物生長遲滯期的延長，它對(duì)環(huán)境條件抗抵力的減弱以及它對(duì)其它抑制條件的更加敏感都是這種殺傷作用的反映。幾種抑制因素的協(xié)同配合，如照射與加熱、加熱與化學(xué)藥品等均能增強(qiáng)微生物對(duì)抑制條件的敏感性。受殺傷的細(xì)胞似乎要合成一些必不可少的細(xì)胞物質(zhì)，如核糖核酸或酶類，然后才能達(dá)到完全復(fù)原。下面將分題討論一下抑制微生物的幾種主要方法：
   
    冷藏法

    冷凍和其后的解凍會(huì)殺死部分區(qū)系的微生物。冷凍時(shí)存活下來的微生物在凍結(jié)貯藏期間不會(huì)增殖。但這不是降低微生物數(shù)量的實(shí)用方法。此外，凍藏中存活的微生物也會(huì)在解凍食物上生長，其生長速率類似于它在未被凍結(jié)食品上的生長速率�？梢园牙洳胤ㄅc其它抑制方法，如加防腐劑、加熱和照射等結(jié)合起來使用。

    化學(xué)藥品法

    凡是使?jié)B透壓升高從而使Aw下降至大多數(shù)細(xì)菌能生長的水平以下的化學(xué)物質(zhì)， 都可以用作抑菌劑。例如鹽和搪。用于俺肉的亞硝酸鹽也具有抑菌劑的功能。

    脫水法

    用脫水法減慢微生物生長是又一種降低Aw值到阻止微生物增殖水平的方法。 有些脫水方法可以抑制若干類固增殖而導(dǎo)致腐敗的微生物。脫水如果與其它控制微生物生長方法如鹽漬和冷藏結(jié)合起來是最有效的。

    發(fā)酵法

    發(fā)酵法徐了產(chǎn)生必需的風(fēng)味以外，還能控制微生物的生長。發(fā)酵是通過產(chǎn)酸菌對(duì)糖類的厭氧代謝活動(dòng)降低底物(可能就是食物)的pH值而起作用的。pH值低于5．0便抑制腐敗微生物的生長。由發(fā)酵作用形成的酸性產(chǎn)物是pH值降低、微生物活動(dòng)減慢的主要原因。經(jīng)酸化和加熱過的食品可以包裝在密封的容器中、以阻止因好氣性酵母和霉菌的生長而引起的腐敗。