by AU Ka Man 區嘉雯, CHAK Pui Kwan 翟珮君, CHENG Hang Fai 鄭珩暉,
CHENG Wing Yin 鄭穎妍, WAI Ka Heng Ophelia 衛嘉鏗
Rancidity is the deterioration of fats and oils due to the formation of odorous short-chained fatty acids. Oxidative and hydrolytic rancidity are common kinds of rancidity when the oil reacts with atmosphere oxygen and water respectively. Degree of unsaturation changes as hydroperoxides, aldehydes, carboxylic acids and other compounds are produced, accompanied by unfavourable odor produced by aldehydes and carboxylic acid, viscosity also changes. Some of the products such as aldehydes are harmful. Thus, an investigation of rancidity is needed.
Five different experiments were carried out to investigate rancidity in terms of the amount of C=C double bond, hydroperoxide, aldehydes, concentration of aqueous hydrogen ions and viscosity at different stages of deterioration in 4 types of cooking oil ( peanut oil, corn oil, canola oil and olive oil). We aim at investigating the process of deterioration and comparing the deterioration rate of different cooking oil, both before and after frying. We assume any impurities and antioxidant has negligible effect on rancidity.
Owing to the constraint in time and availability of chemistry laboratory, it was decided to speed up the deterioration of cooking oils. Oil samples were put in a water bath at 60oC. Air was then bubbled through the oil samples. Every two hours, 60cm3 of each oil sample was pipetted out and quenched immediately in ice bath. Five experiments were carried out as followed:
Degree
of unsaturation
In this experiment, C=C bonds in the oil reacted with excess iodine monochloride. Unreacted ICl then reacted with excess I- to form I2. Concentration of I2 was then found by titration with standard sodium thiosulphate solution.
The general trend of the degree of unsaturation decreased for the first three data and increased from the fourth datum for all uncooked oils. Moreover, olive oil which has the greatest decrease in amount of C=C bond and corn oils which has the least. This showed that corn oil was relatively less susceptible to oxidative rancidity. All types of fried oil except corn oil had a relatively lower degree of unsaturation compared to the uncooked oils which showed corn oil is more difficult to oxidize.
Oil molecules break down to give volatile compounds with C=C bonds and escape. Thus, degree of unsaturation drops. The increase in degree of unsaturation is due to formation of enol when acidity increases. Upon frying many C=C bonds break down as deterioration is speeded up so that fried oils are less easily to oxidize.
Hydroperoxide
The amount of hydroperoxide in oil was found by titration with sodium thiosulphate solution. Generally, the amount of hydroperoxide in oil increased and then decreased. Hydroperoxide was formed when oil was in contact with oxygen. After a period of time, the hydroperoxide decomposed to form carbonyl compounds and less C=C bond was left which leaded to a decreasing amount of hydroperoxide. The amount of hydroperoxide in all types of fried oil was more or less constant because high temperature speeded up the reaction. Thus, formation and decomposition of hydroperoxide had reached equilibrium at an earlier stage.
Unstable hydroperoxides breakdown to form carbonyl compounds including aldehyde and ketone which give unfavourable odour and taste. Fehling’s test was used to prove the presence of aldehyde due to oxidative rancidity. Aldehyde reduced copper (II) complex to copper (I) oxides and the copper (I) oxides was converted into copper (II) oxide by heating. Masses of copper (II) oxide found and compared to estimate the relative amount of aldehyde formed in rancidity.
The general trend of masses of copper (II) oxide formed increased at first and decreased later. This trend showed that at the beginning, little hydroperoxides formed to breakdown into aldehyde. Oil was still at early stage of deterioration so masses of copper (II) oxide obtained were small. Later, oil underwent more serious deterioration, a lot of hydroperoxides was produced and breakdown to form aldehyde. Hence, more copper (II) oxide was found. At last, amount of copper (II) oxide decreases significantly, because of oxidation of aldehyde to carboxylic acid or conversion to enol. Furthermore, volatile aldehyde escaped. Thus, masses of copper (II) oxide eventually drop.
Concentration
of acids
Acids in the oil layer were extracted by water. The concentration of acids was found by titrating the aqueous layer in extraction with sodium hydroxide solution.
There
is a trend that the concentration of acids increased with time due to the
formation of RCOOH by oxidative and hydrolytic rancidity. The concentration of
acids at the beginning of fried oil was higher than that of uncooked one. This
was because during frying, the high temperature and presence of water speeds up
the rate of hydrolytic and oxidative rancidity greatly.
However,
after frying, the rate of rancidity becomes slower than that of uncooked oil
because the aldehydes, which can decompose to form acid, are volatile and escape
during frying. As a result, there
are less aldehyde remained in the oil and hence the rate of formation of RCOOH
becomes slower.
Viscosity
Viscosity
was determined by measuring the time taken for 0.9ml of oil to flow through a
1ml pipette.
Results
showed that all uncooked oil increased in viscosity after bubbling. This
suggested that the effect of polymerization was great and masked the effect of
breakdown of molecules thus intermolecular force increased, and the cis C=C bond
had broken so that carbon chain can packed closer.
Moreover,
the viscosity of all fried oil was greater than fresh oil since the effect of
polymerization was great and masked the effect of breakdown of molecules.
However,
viscosity of fried oil decreased with bubbling time due to the dominant effect
of breaking down of oil molecules. Smaller
molecules have weaker intermolecular force, so, viscosity is lower.
Conclusion
It
is found that uncooked corn oil deteriorates at the slowest rate compared to
other three types. Based on our study, it is found that frying brings the oils
to later stage of deterioration. As a result, rate of deterioration of fried oil
is much slower than that of uncooked oils.
摘要
(前言)
食油常見的臭敗作用有氧化和水解臭敗，分別由食油和氧氣和水接觸產生。食油中較長的脂肪酸鍊被分解為較短的脂肪酸鍊，令素質轉差並發出噎味。當氫過氧化物、醛、羧酸及其他化合物產生時，碳雙鍵量會改變，凝結度增加，醛和羧酸更會產生難聞氣味，降解物如醛，對人有害。因此，有必要對此作深入研究。
我們進行五項實驗找出四種食油(花生油、栗米油、芥花籽油、橄欖油)在不同程度的氧化臭敗後的轉變：凝結度、碳雙鍵量、氫過氧化物量、醛量和氫離子濃度，以分析及比較四種食油的氧化臭敗過程及速度。我們亦比較未炸和炸過的油氧化臭敗情況。我們假設食油中的雜質及抗氧化物對實驗結果影響不大。
由於時間及實驗室資源設備不足，食油須被置於攝氏六十度水中，並插入氣泵加入空氣來加快氧化臭敗過程。每隔兩小時，我們抽出六十毫升的食油樣本並立刻用冰冷藏減慢任何化學作用，以便進行實驗。
不飽和程度
本實驗，我們加入過量的一氯化碘，油中的碳雙鍵可與一氯化碘產生化學作用，形成碳單鍵。未反應的一氯化碘會跟過量的碘離子產生碘。由於碘能跟硫代硫酸鈉化學作用，所以碘之濃度可用滴定方法找尋。
實驗的一般趨勢是先升後降。總的來說，橄欖油的碳雙鍵數量下降得最大，所以它形成自由基的機會最大;粟米及芥花籽油的碳雙鍵數量下降得最小，所以有較小可能形成自由基。除粟米油外，所有煮過一次的油都較未煮過的飽和，可見粟米油較難被氧化。
油分子分解會產生揮發性、含有碳雙鍵的化合物，所以不飽和程度減少。及後，由於酸度增加而產生烯醇，不飽和程度會增加。在煎炸過程中，臭敗過程加快，許多碳雙鍵因而分解。
氫過氧化物
本實驗透過硫代硫酸鈉及碘化鉀的滴定找出油的氫過氧化物含量。從整體趨向可見，油的氫過氧化物含量先升後降。當油與空氣接觸即產生氫過氧化物，之後，氫過氧化物亦會分解為碳醯基。碳鍊長度及碳雙鍵數目因此下降，減慢氫過氧化物的產生。炸過食物的油中氫過氧化物含量大致上不變，因高溫加快臭敗過程，令氫過氧化物的產生及分解在短時間內達到平行點。
醛量
不穩定的氫過氧化物分解成碳醯基，包括醛和酮。本實驗，我們利用費林氏測驗証明出氧化臭敗的過程中形成醛。醛把銅(II)絡合物還原為銅(I)絡合物。加熱後，銅(I)合物變成二氧化銅，醛的相對數量可從二氧化銅的質量估計。
從趨勢圖，可見二氧化銅的質量先升後降。因食油是在初期的氧化臭敗，只有少量氫過氧化物形成，產生不多的醛，所以一氧化銅不多。可是，當食油經過嚴重氧化臭敗便會產生許多氫過氧化物，再分解成醛。因此，一氧化銅亦達到最高水平。最後，我們發現少量一氧化銅，因為在氧化臭敗最後階段，醛會分解成羧酸及烯醇。再者，醛亦會因為其揮發性而逐漸減少，所以二氧化銅的質量減少。
氫離子濃度
油層裏的氫離子可用水來提取。以氫氧化鈉和油的水層進行滴定，可知道氫離子濃度。
從趨勢可見，氫離子濃度逐漸上升。因為油進行水解酸敗和氧化臭敗，產生有機酸化合物。至於炸油，開始時，其氫離子濃度較未煮過的高，因炸油時的高溫和水，令水解酸敗和氧化臭敗的速度加快，產生大量有機酸化合物。
但是，炸過的油，臭敗速度比未煮過的慢，因能分解成酸的醛於炸油時揮發，較少的酸能產生，令臭敗速度較慢。
黏度
黏度是以0.9毫升的油流出1.0毫升吸注管所需的時間來決定。
結果顯示，未煮過的油，經加熱和加氧後，黏度會增加。因為分子聚合的影響比分解的影響大，令分子間力增加。但煮過的油，經加熱和加氧後，黏度會減少。因分子聚合的影響比分解的影響小，小的分子間力較小。而煮過的油，黏度比未煮過的大，因分子聚合的影響比分解的影響大。
(總結)
研究顯示，栗米油的氧化臭敗速度最慢。我們亦發現炸可延長臭敗過程，所以炸過的油氧化臭敗作用較慢。