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§ 114.90 Methodology.
Methods that may be used to determine pH or acidity for acidified foods include, but are not limited to, the following:
?。╝） Potentiometric method for the determination of pH —（1） Principles. The term “pH” is used to designate the intensity or degree of acidity. The value of pH, the logarithm of the reciprocal of the hydrogen ion concentration in solution, is determined by measuring the difference in potential between two electrodes immersed in a sample solution. A suitable system consists of a potentiometer, a glass electrode, and a reference electrode. A precise pH determination can be made by making an electromotive force （emf） measurement of a standard buffer solution whose pH is known, and then comparing that measurement to an emf measurement of a sample of the solution to be tested.
?。?） Instruments. The primary instrument for use in pH determination is the pH meter or potentiometer. For most work, an instrument with a direct-reading pH scale is necessary. Battery and line-operated instruments are available commercially. If the line voltage is unstable, line-operated instruments should be fitted with voltage regulators to eliminate drifting of meter-scale readings. Batteries should be checked frequently to ensure proper operation of battery operated instruments. An instrument using an expanded unit scale or a digital readout system is preferred since it allows more precise measurements.
?。?） Electrodes. The typical pH meter is equipped with a glass membrane electrode and a reference electrode or a single probe combination electrode. Various types of electrodes designed for specific uses are available. The most commonly used reference electrode is the calomel electrode, which incorporates a salt bridge filled with saturated potassium chloride solution.
?。╥） Care and use of electrodes. Calomel electrodes should be kept filled with saturated potassium chloride solution or other solution specified by the manufacturer because they may become damaged if they are allowed to dry out. For best results, electrodes should be soaked in buffer solution, distilled or deionized water, or other liquid specified by the manufacturer for several hours before using and kept ready by storing with tips immersed in distilled water or in buffer solution used for standardization. Electrodes should be rinsed with water before immersing in the standard buffers and rinsed with water or the solution to be measured next between sample determinations. A lag in meter response may indicate aging effects or fouling of the electrodes, and cleaning and rejuvenation of the electrodes may be necessary and may be accomplished by placing the electrodes in 0.1 molar sodium hydroxide solution for 1 minute and then transferring them to 0.1 molar hydrochloric acid solution for 1 minute. The cycle should be repeated two times, ending with the electrodes in the acid solution. The electrodes should then be thoroughly rinsed with water and blotted with soft tissue before proceeding with the standardization.
?。╥i） Temperature. To obtain accurate results, a uniform temperature should be maintained for the electrodes, the standard buffer solutions, and the samples. Tests should be made at a temperature between 20° and 30 °C, the optimum being 25 °C. Any temperature determinations made without meter compensation may affect pH values. An automatic temperature compensator may be used.
?。╥ii） Accuracy. The accuracy of most pH meters is stated to be approximately 0.1 pH unit, and reproducibility is usually ±0.05 pH unit or less. Some meters permit the expansion of any pH unit range to cover the entire scale and have an accuracy of approximately ±0.01 pH unit and a reproducibility of ±0.005 pH units.
?。?） General procedure for determining pH. When operating an instrument, the operator should use the manufacturer's instructions and should observe the following techniques for pH determinations:
（i） Switch the instrument on and allow the electronic components to warm up and stabilize before proceeding.
?。╥i） Standardize the instrument and electrodes with commercially prepared standard 4.0 pH buffer or with freshly prepared 0.05 molar potassium acid phthalate buffer solution prepared as outlined in “Official Methods of Analysis of the Association of Official Analytical Chemists” （AOAC）, 13th Ed. （1980）, section 50.007（c）, under “Buffer Solutions for Calibration of pH Equipment—Official Final Action,” which is incorporated by reference. Copies may be obtained from the AOAC INTERNATIONAL, 481 North Frederick Ave., suite 500, Gaithersburg, MD 20877, or may be examined at the National Archives and Records Administration （NARA）. For information on the availability of this material at NARA, call 202–741–6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html. Note the temperature of the buffer solution and set the temperature compensator control at the observed temperature （room temperature is near 25 °C）.
?。╥ii） Rinse the electrodes with water and blot, but do not wipe, with soft tissue.
（iv） Immerse the tips in the buffer solution and take the pH reading, allowing about 1 minute for the meter to stabilize. Adjust the standardization control so that the meter reading corresponds to the pH of the known buffer （for example, 4.0） for the temperature observed. Rinse the electrodes with water and blot with soft tissue. Repeat procedure with fresh portions of buffer solution until the instrument remains in balance on two successive trials. To check the operation of the pH meter, check the pH reading using another standard buffer such as one having a pH of 7.0, or check it with freshly prepared 0.025 molar phosphate solution prepared as outlined in the AOAC, 13th Ed. （1980）, section 50.007（e）, which is incorporated by reference. The availability of this incorporation by reference is given in paragraph （a）（4）（ii） of this section. Expanded scale pH meters may be checked with pH 3.0 or pH 5.0 standard buffers. Buffers and instruments can be further checked by comparison with values obtained with a second properly standardized instrument.
?。╲） Indicating electrodes may be checked for proper operation by first using an acid buffer and then a base buffer. First standardize the electrodes using a pH 4.0 buffer at or near 25 °C. Standardization control should be adjusted so that the meter reads exactly 4.0. Electrodes should be rinsed with water, then blotted and immersed in a pH 9.18 borax buffer prepared as outlined in the AOAC, 13th Ed. （1980）, section 50.007（f）, which is incorporated by reference. The availability of this incorporation by reference is given in paragraph （a）（4）（ii） of this section. The pH reading should be within ±0.3 units of the 9.18 value.
（vi） The pH meter can be tested for proper operation by shorting the glass and reference electrode inputs, thereby reducing the voltage to zero. In some meters this shorting is done by switching the instrument to standby, and in other instruments by use of a shorting strap. With the instrument shorted out, standardization control should be turned from one extreme to another. This operation should produce a deflection greater than ±1.5 pH unit from center scale.
?。?） Determining pH on samples. （i） Adjust the temperature of the sample to room temperature （25 °C）, and set the temperature compensator control to the observed temperature. With some expanded scale instruments, the sample temperature must be the same as the temperature of the buffer solution used for the standardization.
?。╥i） Rinse and blot the electrodes. Immerse the electrodes in the sample and take the pH reading, allowing 1 minute for the meter to stabilize. Rinse and blot the electrodes and repeat on a fresh portion of sample. Oil and grease from the samples may coat the electrodes; therefore, it is advisable to clean and standardize the instrument frequently. When oily samples cause fouling problems, it may become necessary to rinse the electrodes with ethyl ether.
?。╥ii） Determine two pH values on the well-mixed sample. These readings should agree with one another to indicate that the sample is homogeneous. Report values to the nearest 0.05 pH unit.
?。?） Preparation of samples. Some food products may consist of a mixture of liquid and solid components that differ in acidity. Other food products may be semisolid in character. The following are examples of preparation procedures for pH testing for each of these categories:
?。╥） Liquid and solid component mixtures. Drain the contents of the container for 2 minutes on a U.S. standard No. 8 sieve （preferably stainless steel） inclined at a 17- to 20-degree angle. Record weight of the liquid and solid portions and retain each portion separately.
（ a ） If the liquid contains sufficient oil to cause electrode fouling, separate the layers with a separatory funnel and retain the aqueous layer. The oil layer may be discarded. Adjust the temperature of the aqueous layer to 25 °C and determine its pH.
?。?b ） Remove the drained solids from the sieve, blend to a uniform paste, adjust the temperature of the paste to 25 °C and determine its pH.
（ c ） Mix aliquots of solid and liquid fractions in the same ratio as found in the original container and blend to a uniform consistency. Adjust the temperature of the blend to 25 °C and determine the equilibriated pH. Alternatively, blend the entire contents of the container to a uniform paste, adjust the temperature of the paste to 25 °C, and determine the equilibriated pH.
?。╥i） Marinated oil products. Separate the oil from the solid product. Blend the solid in a blender to a paste consistency; it may become necessary to add a small amount of distilled water to some samples to facilitate the blending. A small amount of added water will not alter the pH of most food products, but caution must be exercised concerning poorly buffered foods. No more than 20 milliliters of distilled water should be added to each 100 grams of product. Determine the pH by immersing electrodes in the prepared paste after adjusting the temperature to 25 °C.
?。╥ii） Semisolid products. Food products of a semisolid consistency, such as puddings, potato salad, etc., may be blended to a paste consistency, and the pH may be determined on the prepared paste. If more fluidity is required, 10 to 20 milliliters of distilled water may be added to 100 grams of product. Adjust the temperature of the prepared paste to 25 °C and determine its pH.
?。╥v） Special product mixtures. For special product mixtures such as antipasto, pour off the oil, blend the remaining product to a paste, and determine the pH of the blended paste. If more fluidity is required, add 10 to 20 milliliters of distilled water to each 100 grams of product and blend. Adjust the temperature of the prepared paste to 25 °C and determine its pH.
?。?） Process pH determination. Obtain sample portions of material for pH determination.
?。╥） For process liquids, adjust the temperature of the liquid to 25 °C and determine the pH by immersing the electrodes in the liquid.
（ii） Drain solid materials on a sieve and blend to a workable paste. Adjust the temperature of the prepared paste to 25 °C and determine its pH.
?。╥ii） If enough solid materials are available to make a paste, blend representative aliquots of liquid and solid materials to a workable paste. Adjust the temperature of the prepared paste to 25 °C and determine the equilibrated pH. Alternatively, blend the entire contents of the container to a uniform paste, adjust the temperature of the paste to 25 °C, and determine the equilibrated pH.
?。╞） Colorimetric methods for the determination of pH. This method may be used in lieu of the potentiometric method if the pH is 4.0 or lower.
?。?） Principle. The colorimetric method for pH involves the use of indicator dyes in solutions that gradually change color over limited pH ranges. An indicator that has the greatest color change at approximately the pH of the sample being tested is selected. The pH is determined by the color of the indicator when exposed to the sample under test.
?。?） Indicator solutions. Most indicator solutions are prepared as a 0.04 percent solution of the indicator dye in alcohol. In testing, a few drops of indicator solution are added to 10-milliliter portions of the sample solution. Colors should be compared using a bright background. Approximate determinations can be made on white porcelain spot plates, the test colors being compared thereon with a set of color standards. More accurate colorimetric tests can be made using a comparator block fitted with sets of tubes of standard indicator solutions of known pH.
（3） Indicator paper. A paper tape treated with indicator dye is dipped into the sample solution. Depending upon the pH of the solution, the tape will change color and an approximate pH can be determined by comparison with a standard color chart.
?。╟） Titratable acidity. Acceptable methods for determining titratable acidity are described in the AOAC, 13th Ed. （1980）, section 22.060, under “Titratable Acidity—Official Final Action,” for “Indicator Method,” and section 22.061 for “Glass Electrode Method—Official Final Action,” which is incorporated by reference. The availability of this incorporation by reference is given in paragraph （a）（4）（ii） of this section. The procedure for preparing and standardizing the sodium hydroxide solution is described in the AOAC, 13th Ed. （1980）, sections 50.032–50.035, under “Sodium Hydroxide—Official Final Action” by the “Standard Potassium Hydroxide Phthalate Method,” which is also incorporated by reference and available as set forth in paragraph （a）（4）（ii） of this section.
[44 FR 16235, Mar. 16, 1979, as amended at 47 FR 11822, Mar. 19, 1982; 49 FR 5609, Feb. 14, 1984; 54 FR 24892, June 12, 1989; 63 FR 14035, Mar. 24, 1998]
§114.90方法
用于測(cè)定酸化食品的pH或酸度方法如下（不僅限于此）：
?。╝）用電位法測(cè)定pH：
?。?）原理 pH這一符號(hào)是用來(lái)表示酸的強(qiáng)度或酸度。pH是溶液氫離子濃度倒數(shù)的對(duì)數(shù)，它是用測(cè)量浸沒(méi)在樣品溶液中兩個(gè)電極之間的電位差來(lái)測(cè)定的。一個(gè)合適的測(cè)量系統(tǒng)是由一個(gè)電位計(jì)、一個(gè)玻璃電極和一個(gè)參照電極組成。可以用下述方法精確測(cè)定pH：首先測(cè)量一種已知pH的標(biāo)準(zhǔn)緩沖溶液的電動(dòng)勢(shì)，然后將這一電動(dòng)勢(shì)與從需要測(cè)試的樣品溶液中測(cè)得的電動(dòng)勢(shì)作比較。
?。?）儀器 用以測(cè)定pH的主要儀器是pH表或電位計(jì)。在一般情況下，只需一只帶有pH（讀數(shù)）刻度尺的儀表。電池供電和線路供電的儀表都可以用。如果線路電壓不穩(wěn)定，線路供電儀表應(yīng)配有穩(wěn)壓器，以防止刻度表上的讀數(shù)偏差。電池要經(jīng)常更換，以確保電池供電儀表的正常工作。最好采用帶有放大的刻度尺或數(shù)字讀數(shù)系統(tǒng)的儀表，因?yàn)檫@樣可以測(cè)量得更精確。
?。?）電極 典型的pH表裝有一個(gè)玻璃護(hù)膜電極和一個(gè)參照電極或一個(gè)單一的探測(cè)組合電極。為特殊使用而設(shè)計(jì)的各種電極都有供應(yīng)。最常用的參照電極是甘汞電極，甘汞電極中裝有一個(gè)灌滿飽和氯化鉀溶液的鹽橋。
（i）電極的使用和注意事項(xiàng)：甘汞電極應(yīng)該保持充滿飽和的氯化鉀溶液或者制造廠規(guī)定的其它溶液。因?yàn)槿绻芤焊珊?，電極可能損壞。為了達(dá)到最好的效果，電極在使用前應(yīng)在緩沖溶液、蒸餾水或去離子水或制造廠規(guī)定的其它溶液中浸泡幾小時(shí)，然后將頂端浸入蒸餾水或標(biāo)準(zhǔn)化的緩沖溶液中貯存?zhèn)溆谩Ｔ诮霕?biāo)準(zhǔn)緩沖溶液前，電極要用水沖洗。在測(cè)量下一個(gè)樣品之前，電極要用水或溶液沖洗，儀表反應(yīng)滯后可能表示電極老化作用或者有污垢，有必要對(duì)電極進(jìn)行清洗和復(fù)原處理。為此可將電極放入0.1mol/L NaOH溶液中1min，再移至0.1mol/L HCl中1min，這一過(guò)程需重復(fù)兩次，結(jié)束時(shí)電極在酸溶液中。然后將電極用水徹底洗凈，再用軟質(zhì)物品吸干，使其恢復(fù)標(biāo)準(zhǔn)化。
（ii）溫度：為了獲得正確的結(jié)果，電極、標(biāo)準(zhǔn)緩沖溶液和樣品應(yīng)該保持同一溫度。測(cè)試溫度應(yīng)為20～30℃，最佳溫度為25℃。如果溫度表不作補(bǔ)償調(diào)整，測(cè)得的溫度會(huì)影響pH?？梢圆捎米詣?dòng)溫度補(bǔ)償器。
?。╥ii）精確度：pH表的精確度約為0.1pH單位，復(fù)制的通常為±0.05pH單位或更小的偏差。有的表將所有pH范圍標(biāo)注在整個(gè)刻度尺上，這種表的精確度約為±0.01pH單位，偏差為±0.005pH單位。
?。?）測(cè)定pH的一般方法 操作人員應(yīng)該按照制造商的說(shuō)明使用儀表，并應(yīng)按下述方法測(cè)定pH：
?。╥）打開(kāi)儀表開(kāi)關(guān)，在進(jìn)入測(cè)定前使電子元件預(yù)熱起來(lái)，并達(dá)到穩(wěn)定。
（ii）用市售的標(biāo)準(zhǔn)pH 4.0緩沖溶液或現(xiàn)配的0.05mol/L（濃度）的鄰苯二甲酸氫鉀緩沖溶液將儀表和電極標(biāo)準(zhǔn)化。鄰苯二甲酸氫鉀緩沖溶液的配制見(jiàn)《公職分析化學(xué)家協(xié)會(huì)法定分析法》（AOAC），第13版（1980），50.007（c）節(jié)，題為"用于pH設(shè)備標(biāo)定的緩沖溶液--法定最終操作"。該資料已列入?yún)⒖嘉墨I(xiàn)，文本可按下列地址索?。篈ssociation of Official Analytical Chemists International, 481 North Frederick Ave., suite 500, Gaithersburg, MD 20877，或在下列地址查詢：Office of the Federal Register， 800 North Capitol Street，NW.， Suite 700， Washington，DC.。注意緩沖溶劑的溫度并將溫度補(bǔ)償器的調(diào)節(jié)控制設(shè)定在觀察到的溫度上（室溫接近25℃）。
（iii）用水沖洗電極并用軟質(zhì)物品吸干，不要揮動(dòng)。
?。╥v）將頂端浸入緩沖溶液約1min，使其達(dá)到穩(wěn)定，記下pH讀數(shù)。調(diào)節(jié)標(biāo)準(zhǔn)化操作，使表在所測(cè)得溫度下的讀數(shù)與已知緩沖劑的pH（如4.0）讀數(shù)一致。用水沖洗電極并用軟物品吸干。用新鮮緩沖溶液重復(fù)上述過(guò)程，直至在連續(xù)兩次實(shí)驗(yàn)中儀表保持平衡。檢查pH表的性能，用另一份標(biāo)準(zhǔn)緩沖劑，如pH為7.0的緩沖劑校對(duì)pH讀數(shù)，或者用新配制的0.025mol/L磷酸鹽溶液進(jìn)行校對(duì)。磷酸鹽溶液的配制見(jiàn)AOAC第13版（1980）50.007（e）節(jié)。該資料已列入?yún)⒖嘉墨I(xiàn)，文本的索取或查詢見(jiàn)本節(jié)（a）（4）（ii）。增長(zhǎng)刻度的pH表可以用pH為3.0或5.0的標(biāo)準(zhǔn)緩沖劑校對(duì)。緩沖劑和儀表可以用與從第二次標(biāo)準(zhǔn)化的儀表獲得的值作比較的方法進(jìn)一步驗(yàn)證。
?。╲）校正指示電極是否正常可以先用酸性緩沖劑，然后用堿性緩沖劑。先用pH為4.0的緩沖劑在25℃左右校準(zhǔn)電極。并將其調(diào)整到pH為4.0。電極應(yīng)用水沖洗，然后吸干并浸在pH為9.18的硼砂緩沖劑中，硼砂緩沖劑的配制見(jiàn)AOAC第13版，（1980）50.007（f）節(jié)。該資料已列入?yún)⒖嘉墨I(xiàn)。參考文獻(xiàn)文本的索取或查閱見(jiàn)本節(jié)（a）（4）（ii）。pH的讀數(shù)應(yīng)該為9.18±0.3。
?。╲i）測(cè)試pH表是否正常的方法是將玻璃電極和參照電極的輸入短路，使電壓減到零。對(duì)于某些表，短路的方法是將儀表的開(kāi)關(guān)旋到待用，有的儀表則用短路帶。儀表短路之后，標(biāo)準(zhǔn)控制從一端轉(zhuǎn)到另一端。這一操作應(yīng)產(chǎn)生一個(gè)從中心刻度起±1.5pH單位的偏轉(zhuǎn)。
（5）測(cè)定樣品的pH
?。╥）將樣品的溫度調(diào)整到室溫（25℃），將溫度補(bǔ)償控制定在所觀察到的溫度。采用擴(kuò)大刻度的儀表，樣品溫度必須與用于標(biāo)準(zhǔn)化的緩沖溶液相同。
?。╥i）沖洗并吸干電極。將電極浸入樣品中并記下pH讀數(shù)，要讓表有1min的穩(wěn)定時(shí)間。沖洗并吸干電極，在另一份樣品中重復(fù)測(cè)試。樣品中的油或油脂可能覆蓋在電極上，因此，建議經(jīng)常清洗并校正儀表。如果沾染了油性樣品，可能需要用乙醚沖洗電極。
（iii）在均勻混合的樣品中測(cè)pH兩次。兩次的讀數(shù)應(yīng)該是一致的，這就表明樣品是均質(zhì)的。記下最接近pH 0.05的值。
?。?）樣品的準(zhǔn)備 有些食品產(chǎn)品是由不同酸度的液體和固體成分混合組成的。另一些食品產(chǎn)品或呈半固體狀。對(duì)于上述各種食品準(zhǔn)備pH測(cè)試樣品的方法舉例如下：
?。╥）液體和固體成分的混合物： 將容器的內(nèi)容物在美國(guó)標(biāo)準(zhǔn)8號(hào)篩（最好是不銹鋼的）上瀝干2min，篩子傾斜17°～20°角。記下液體和固體部分的重量，液體和固體分別保留。
（A）如果液體中含有太多的油，將沾染電極，用一只分液漏斗將油層和水層分開(kāi)，保留水層。油層可以丟棄。將水層部分溫度調(diào)節(jié)至25℃，測(cè)定它的pH。
?。˙）從篩中取出瀝干的固體，混合成漿狀，使?jié){的溫度調(diào)整到25℃，測(cè)定其pH。
?。–）按原來(lái)容器內(nèi)的比例將整個(gè)等分液量的固體和液體部分混合成同一稠度。將混合物調(diào)節(jié)到25℃，測(cè)量其平衡的pH。也可用另一種方法：將容器內(nèi)的全部?jī)?nèi)容物混合成均勻的漿，使?jié){的溫度調(diào)節(jié)至25℃，測(cè)定漿的平衡pH。
?。╥i）浸泡在油中的產(chǎn)品：將油從固體產(chǎn)品中分離出。將固體放在混合器中混合成漿狀稠度。樣品中可能需要加少量的蒸餾水以利于混合。加入少量的水將不改變大多數(shù)食品的pH，但是對(duì)于緩沖性差的食品要注意，在100g產(chǎn)品中加入的蒸餾水不應(yīng)該超過(guò)20mL。將準(zhǔn)備好的漿狀物溫度調(diào)整至25℃后，插入電極測(cè)定pH。
?。╥ii）半固體產(chǎn)品：半固狀稠度的食品如布丁、土豆沙拉等，可以將它們混合成漿狀稠度，用這種漿測(cè)定pH。如果需要將其流動(dòng)性增大一點(diǎn)，可在100g產(chǎn)品中加10～20mL的蒸餾水。將配制好的漿料調(diào)整到25℃，測(cè)定其pH。
（iv）特殊產(chǎn)品的混合物： 對(duì)于特殊產(chǎn)品的混合物，如antipasto，將油倒去，留下的產(chǎn)品混合成漿狀，測(cè)定漿狀混合物的pH。如果需要增加流動(dòng)性，在100g漿狀混合物中加10～20mL的蒸餾水。將配制好的漿調(diào)節(jié)到25℃，然后測(cè)量其pH。
?。?）加工過(guò)程中pH的測(cè)定 取出部分物料作為測(cè)定pH的樣本（樣品）。
?。╥）對(duì)于加工液體，將液體溫度調(diào)至25℃，將電極插入液體中測(cè)得pH。
（ii）在篩上瀝干的固體物料，將其混合成可測(cè)的漿狀。將制備好的漿料調(diào)至25℃，測(cè)定其pH。
?。╥ii）如果有足夠的固體物料制成漿料，將整等分液量的液體和固體物料混合成可測(cè)漿料。將這漿料調(diào)節(jié)至25℃，測(cè)定其平衡pH。另一種方法是將容器內(nèi)全部物料混合成均質(zhì)的漿料，將其溫度調(diào)至25℃，然后測(cè)得平衡pH。
（b）用比色法測(cè)定pH：如果pH等于或小于4.0，可用此法代替電位法。
?。?）原理 用比色法測(cè)定pH是利用指示劑在溶液中著色的原理，這種指示劑在規(guī)定的pH范圍內(nèi)漸漸變色。要選擇對(duì)測(cè)試樣品的pH具有最大色澤變化的指示劑。在測(cè)試時(shí)，根據(jù)指示劑在樣品中顯示的顏色確定pH。
?。?）指示劑溶液 大多數(shù)指示劑溶液是在乙醇中加0.04%指示劑染料溶液制成的。在測(cè)試時(shí)，在10mL樣品溶液中滴入幾滴指示劑溶液。應(yīng)用明亮的背景來(lái)比較其顏色。近似的測(cè)定可以用白瓷點(diǎn)滴板進(jìn)行，將測(cè)試的顏色與一組標(biāo)準(zhǔn)顏色作比較。更精確的比色測(cè)試可以用一個(gè)比較板，板上裝有一組管子，管內(nèi)盛有已知pH的標(biāo)準(zhǔn)指示劑溶液。
?。?）指示劑試紙 經(jīng)過(guò)指示劑染料處理過(guò)的試紙浸入樣品溶液中，根據(jù)溶液中的pH，試紙改變顏色。將試紙顏色與標(biāo)準(zhǔn)顏色圖表進(jìn)行比較而獲得近似的pH。
?。╟）可滴定酸度：測(cè)定可滴定酸度的方法見(jiàn)AOAC第13版（1980）22.00節(jié)，題為"可滴定酸度--法定最終操作"--"指示劑方法"和22.061節(jié)"玻璃電極法--法定最終操作"。這些資料都已列入?yún)⒖嘉墨I(xiàn)。參考文獻(xiàn)的索取或查詢見(jiàn)本節(jié)（a）（4）（ii）。配制和調(diào)整氫氧化鈉標(biāo)準(zhǔn)溶液的方法見(jiàn)AOAC（1980）50.032-50.035節(jié)，題為"氫氧化鈉--法定最終操作" ，用標(biāo)準(zhǔn)氫氧化鉀鄰苯二甲酸鹽法，該資料已列入?yún)⒖嘉墨I(xiàn)。參考文獻(xiàn)文本的索取或查詢見(jiàn)本節(jié)（a）（4）（ii）。
?。?4 FR 16235, Mar. 16, 1979以47 FR 11822, Mar. 19, 1982； 49 FR 5609, Feb. 14, 1984； 54 FR 24892, Jun. 12, 1989修訂］
更多關(guān)于美國(guó)FDA酸化食品HACCP法規(guī)，請(qǐng)?jiān)斠?jiàn)美國(guó)FDA酸化食品HACCP法規(guī)第113和114部分匯總
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編輯：foodfagui