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Abstract [Objectives] The aim is to provide a theoretical basis for people to store and process vegetables more scientifically.
[Methods] Changes in the nitrate content of three kinds of leafy vegetables （cabbage， lettuce and celery） cooked and processed by different methods were studied.
[Results] After the three kinds of leafy vegetables were processed by different methods， the nitrate and vitamin C content of the leafy vegetables reduced， and their reduction was above 50% after they were boiled. After they were salted for 5 min， nitrate content decreased by 20%-40%， and changed slightly with time. Salting had a small effect on Vc content. After lettuce and celery were soaked in 0.05% NaCl solution for 30 min， the effect was the best. CaCl 2 had an obvious effect on the reduction of nitrate content in lettuce.
[Conclusions] The study is of great significance to safety production and evaluation of edible value of agricultural products.
Key words Leafy vegetables; Nitrate content; Vc content
Nitrates are harmless or has a relatively low toxicity to the human body， but various factors that contribute to the growth and reproduction of certain reducing bacteria （such as temperature， humidity， pH， etc.） can promote the reduction of nitrates to nitrites. On the one hand， it can cause hypoxic poisoning reaction in human blood， which can cause oxidation of divalent iron ions in hemoglobin to form methemoglobin， reduce oxygen delivery capacity， produce methemoglobinemia， and even lead to death in severe cases[1-2]. On the other hand， under acidic conditions， nitrite can combine with secondary amines in other foods， pharmaceuticals， and residual pesticides taken by humans to form a strong carcinogen nitrite， thereby causing canceration in the human digestive system[3-4]. While providing abundant nutrients for humans ， vegetables are also a kind of crops that are easy to concentrate nitrates[5]. In actual production， improper fertilization often cause soil and groundwater pollution from nitrates， and then lead to high residues of nitrates and nitrites in vegetables， which has a great negative impact on human health. 81.2% of nitrate intake of the human body comes from vegetables. Studies have shown that the effects of nitrogen fertilizer on vegetable yield， nitrate and Vc content were very obvious， and the degree and direction of the influence varied with the amount， type， morphology and ratio of nitrogen fertilizer[6-8]. The nitrate content of vegetables is as follows： leafy vegetables>root vegetables>fruit vegetables>fruits. Vitamin C， an essential nutrient for the human body， is mainly obtained from fresh vegetables and fruits. Due to the special structure and nature of vitamin C， it is easily destroyed during harvesting， processing， cooking and storage. If vegetables are improperly processed during cooking， a lot of vitamin C will be lost. The World Health Organization （WHO） and the Food and Agriculture Organization of the United Nations established a limited standard for nitrates in foods in 1973， proposed that nitrate accumulation in vegetables is divided into four levels[9]. Level 1： nitrate content≤432 mg/kg， raw vegetables can be eaten. Level 2： nitrate content≤785mg/kg， raw vegetables are not suitable for eating， and salted or cooked vegetables can be eaten. Level 3： nitrate content≤1 440 mg/kg， cooked vegetables can be eaten. Level 4： nitrate content≤3 100 mg/kg， vegetables are not edible. WHO stipulates that the acceptable daily intake （ADI） of nitrates is 0-0.37 mg/kg body weight， but according to the eating habits of daily life， the nitrate intake of people always exceeds the standard[10].
Many studies have shown that water washing and blanching[11]， and soaking[12] will effectively reduce nitrate and nitrite content. The processing and cooking of vegetables will inevitably lead to the loss and destruction of nutrients. Vitamin C is easy to lose and damage during processing and cooking. The longer the hot frying and stewing are， the greater the loss of vitamin C content is[13]. In this paper， the nitrate and vitamin C content of three kinds of leafy vegetables （cabbage， lettuce and celery） cooked and processed by different methods were studied， and rational methods for storage and cooking of vegetables were discussed to reduce the unnecessary loss of vitamin C. Targeted control measures for nitrate content were taken. The aim is to provide a theoretical basis for people to store vegetables more scientifically and safely and to use reasonable methods to cook vegetables. It is of great significance to safety production and evaluation of edible value of agricultural products.
Experimental materials
Experimental materials were three common vegetables （cabbage， lettuce and celery） sold in Yongchuan District， Chongqing City.
Reagents
Reagents included phenol disulfonic acid and ammonia water （AR）. Extracting agent oxalic acid： 2% solution （2 g of oxalic acid was dissolved in 100 ml of water. Oxalic acid： 1% solution （1 g of oxalic acid was dissolved in 100 ml of water）.
Standard ascorbic acid solution： 10 mg of pure ascorbic acid （it should be white， and can not be used if it is yellow ） was dissolved in 1% oxalic acid solution， diluted to 100 ml， stored in a brown bottle， and refrigerated. It is best to prepare before use. 2，6 dichloroindophenol solution： 52 mg of sodium bicarbonate was dissolved in 200 ml of hot distilled water. Afterwards， 50 mg of 2，6 dichloroindophenol was dissolved in the above sodium bicarbonate solution. After the solution was cooled， distilled water was added to it until the volume was up to 250 ml. It was filtered into a brown bottle and refrigerated.
Instruments and equipment
The main instruments included electronic balance （Merlett Toledo Instrument Shanghai Co.， Ltd.）， 754 UV spectrophotometer （Shanghai Shunyu Hengping Scientific Instrument Co.， Ltd.）， electronic thermostat universal electric furnace （Tianjin Taisite Co.， Ltd.）， water bath （Shanghai Yuejin Medical Instrument Factory）， high speed desktop microcentrifuge （Hermle labortechnik）， Philips mixer （Zhuhai Special Economic Zone Philips Home Appliances Co.， Ltd.）.
Experimental methods
Effects of boiling on nitrate and vitamin C content in the leafy vegetables
About 1 000 g of cabbage， lettuce （leaf） and celery were selected each， from which rotted leaves and yellow leaves were removed. They were washed， cut， and divided into two groups each， and the weight was equivalent. Nitrate and vitamin C content were measured 0， 0.5， 1， 3， and 5 min after boiling to 100 ℃. Each samples were measured three times.
Effects of salting on nitrate and vitamin C content in the leafy vegetables
Around 800 g of cabbage （stem）， lettuce （stem） and celery were selected each， from which rotted leaves and yellow leaves were removed. They were washed， cut， and divided into two groups each， and the weight was equivalent. They were completely immersed in 0.89 mol/L brine （20 g of NaCl was dissolved in 1 L of water）. After 0， 5， 10 and 20 min， nitrate and vitamin C content were measured. Each sample was measured three times， and the three data were averaged.
Effects of soaking on nitrate and vitamin C content in the leafy vegetables
About 1 500 g of lettuce and celery were selected each， from which rotted leaves and yellow leaves were removed. They were washed， cut， and divided into two groups each， and the weight was equivalent. After lettuce and celery were soaked in different concentrations of NaCl and CaCl 2 solution （0.05% and 0.5%） for 30 or 60 min， nitrate content was measured. Each sample was measured three times， and the three data were averaged.
Analytical methods
Some of fresh vegetables purchased in the market were taken to determine nitrate content， and some were stored， boiled， salted and soaked to determine nitrate content. Nitrate content in the leafy vegetables was measured by phenol disulfonic acid colorimetric method[14]， and vitamin C content in the leafy vegetables was measured by 2，6 dichlorophenol method. Results and Analysis
Effects of boiling on nitrate and vitamin C content in the leafy vegetables
According to Table 1， after the leafy vegetables were boiled， both nitrate and Vc content decreased， and the reduction increased with the extension of time. After 5 min， the nitrate content of cabbage， lettuce （leaf） and celery decreased by 1 415， 509 and 1 597 mg/kg respectively compared with the fresh leafy vegetables， namely reducing by 30%-56%， 18%-61% and 33%-57% respectively. In particular， the reduction of lettuce was the largest， reaching 61.3%. The Vc content of cabbage， lettuce （leaf） and celery reduced by 192， 27.5 and 72.6 mg/kg respectively， namely decreasing by 14%-52%， 14%-43% and 43%-74% respectively. The reduction of Vc content in lettuce was the largest， up to 74%.
Effects of salting on nitrate and vitamin C content in the leafy vegetables
After cabbage （stem）， lettuce （stem） and celery were salted for 5 min， the nitrate content of the leafy vegetables lowered by 1 550 ， 421 and 686 mg/kg， namely reducing by 36.9%， 38.8% and 23.3% respectively. There was no obvious change in the reduction with the continuous extension of time. Salting affected the Vc content of the leafy vegetables slightly. In comparison with the fresh vegetables， the Vc content of cabbage （stem）， lettuce （stem） and celery decreased by 9.7%-11.4%， 4.9%-17.7% and 1.6%-6.2% respectively.
Effects of soaking on nitrate content in the leafy vegetables
Seen from Table 3， after lettuce and celery were soaked in different concentrations of NaCl and CaCl 2 solution （0.05% and 0.5%） for 30 min or 60 min， the nitrate content of the leafy vegetables reduced. After lettuce and celery were soaked in NaCl solution， the nitrate content of celery and lettuce decreased by 9.5%-37.8% and 1.1%-37.8% respectively. When lettuce and celery were soaked in 0.05% NaCl solution for 30 min， the decreases were the largest. After lettuce and celery were soaked in CaCl 2 solution， the nitrate content of celery and lettuce reduced by 14.5%-22.0% and 17.8%-39.4% respectively. There were small differences between various CaCl 2 treatments in terms of the reduction in nitrate content. 0.05% CaCl 2 solution had an obvious effect on the nitrate content of lettuce， and the reduction reached about 35%.
Conclusions and Discussion
After the leafy vegetables were cooked and processed by different methods， there were big differences between various treatments in terms of the reduction in nitrate and vitamin C content. After cabbage was boiled for 1 min， its nitrate content was the lowest. After lettuce was salted or soaked in 0.5% CaCl 2 solution， the decrease of nitrate content was large， and nitrate content reduced with time. Vitamin C content was less affected by salting. After celery was boiled for 0.5 min， the decrease of nitrate content was the largest. Through this experiment， the leafy vegetables can be cooked by boiling， salting and soaking. Therefore， it is recommended that after leafy vegetables are boiled for 3-5 min， the boiling water is discarded， and then they are boiled again or fried. References
[1] HSU J， ARCOT J， LEE NA. Nitrate and nitrite quantification from cured meat and vegetables and their estimated dietary intake in Australians[J]. Food Chemistry，2009，115（1）： 334-339.
[2] JAWORSKA G. Effect of technological measures and storage time on the level of nitrates， nitrites，and oxalates in frozen and canned products of spinach and New Zealand spinach[J]. Food Chemistry，2005，93（3）：395-401.
[3] WANG YB， HUANG W， LIU DY. Nitrite content of common vegetables in Wuhu City[J]. Chinese Journal of Applied Ecology，2005，16（1）： 189-192. （in Chinese）.
[4] YE YF， XU XQ， ZHU Y， et al. Study on determination and storage conditions of nitrate and nitrite in vegetables[J]. Auhui Agricultural Science Bulletin，2007（21）： 82-83. （in Chinese）.
[5] WANG XL，CHU CS， LU LX， et al. Comparison of determination methods for nitrate content in vegetables[J]. Food Science and Technology，2010，35（4）：273-276. （in Chinese）.
[6] SI T， LIU Q， ZHANG YP， et al. Effects of combined application of organic fertilizer with inorganic fertilizer on yields and quality of lettuce[J]. Hunan Agricultural Sciences，2012（1）：50-53. （in Chinese）.
[7] JIAO XG， DENG Y， ZHAO WL， et al. Effects of different fertilization treatments on quality and yield of pakchoi[J]. Journal of Heilongjiang Hydraulic Engineering College，2011，2（2）：70-73. （in Chinese）.
[8] ZHENG WL， QU CD， WANG FZ， et al. The effect of organic and inorganic compound fertilizer on the yield and quality of Chinese cabbage[J]. Liaoning Agricultural Sciences，2012（2）：43-45. （in Chinese）.
[9] LIU N， SHEN MH. Food toxicology[M]. Beijing： China Light Industry Press， 2005： 8. （in Chinese）.
[10] BOINK A， SPEIJERS G. Health effects of nitrates and nitrites[J]. Acta Horticulture， 2001， 563：29-36.
[11] JAWRKSKA G. Effect of technological measures and storage time on the level of nitrates and oxalates in frozen and canned products of spinach and New Zealand spinach[J]. Food Chemistry，2005，93：395-401.
[12] LIU YQ， GUI Z， WU JJ， et al. Content variation of nitrite in differently treated vegetables[J]. Journal of Tianjin Agricultural University，2006，13（2）：21-23. （in Chinese）.
[13] LIU JK， LEI NF， WU HF， et al. Study on the content of vitamin C in common vegetables[J]. Trace Elements Science，2006，13（4）：75-95. （in Chinese）.
[14] HE SP， JIN YZ， ZHANG XM. Determination on nitrate of foliar vegetable with phenol two sulfonic acid method[J]. Journal of Heilongjiang August First Land Reclamation University，2005，17（2）： 37-40. （in Chinese）.
[Methods] Changes in the nitrate content of three kinds of leafy vegetables （cabbage， lettuce and celery） cooked and processed by different methods were studied.
[Results] After the three kinds of leafy vegetables were processed by different methods， the nitrate and vitamin C content of the leafy vegetables reduced， and their reduction was above 50% after they were boiled. After they were salted for 5 min， nitrate content decreased by 20%-40%， and changed slightly with time. Salting had a small effect on Vc content. After lettuce and celery were soaked in 0.05% NaCl solution for 30 min， the effect was the best. CaCl 2 had an obvious effect on the reduction of nitrate content in lettuce.
[Conclusions] The study is of great significance to safety production and evaluation of edible value of agricultural products.
Key words Leafy vegetables; Nitrate content; Vc content
Nitrates are harmless or has a relatively low toxicity to the human body， but various factors that contribute to the growth and reproduction of certain reducing bacteria （such as temperature， humidity， pH， etc.） can promote the reduction of nitrates to nitrites. On the one hand， it can cause hypoxic poisoning reaction in human blood， which can cause oxidation of divalent iron ions in hemoglobin to form methemoglobin， reduce oxygen delivery capacity， produce methemoglobinemia， and even lead to death in severe cases[1-2]. On the other hand， under acidic conditions， nitrite can combine with secondary amines in other foods， pharmaceuticals， and residual pesticides taken by humans to form a strong carcinogen nitrite， thereby causing canceration in the human digestive system[3-4]. While providing abundant nutrients for humans ， vegetables are also a kind of crops that are easy to concentrate nitrates[5]. In actual production， improper fertilization often cause soil and groundwater pollution from nitrates， and then lead to high residues of nitrates and nitrites in vegetables， which has a great negative impact on human health. 81.2% of nitrate intake of the human body comes from vegetables. Studies have shown that the effects of nitrogen fertilizer on vegetable yield， nitrate and Vc content were very obvious， and the degree and direction of the influence varied with the amount， type， morphology and ratio of nitrogen fertilizer[6-8]. The nitrate content of vegetables is as follows： leafy vegetables>root vegetables>fruit vegetables>fruits. Vitamin C， an essential nutrient for the human body， is mainly obtained from fresh vegetables and fruits. Due to the special structure and nature of vitamin C， it is easily destroyed during harvesting， processing， cooking and storage. If vegetables are improperly processed during cooking， a lot of vitamin C will be lost. The World Health Organization （WHO） and the Food and Agriculture Organization of the United Nations established a limited standard for nitrates in foods in 1973， proposed that nitrate accumulation in vegetables is divided into four levels[9]. Level 1： nitrate content≤432 mg/kg， raw vegetables can be eaten. Level 2： nitrate content≤785mg/kg， raw vegetables are not suitable for eating， and salted or cooked vegetables can be eaten. Level 3： nitrate content≤1 440 mg/kg， cooked vegetables can be eaten. Level 4： nitrate content≤3 100 mg/kg， vegetables are not edible. WHO stipulates that the acceptable daily intake （ADI） of nitrates is 0-0.37 mg/kg body weight， but according to the eating habits of daily life， the nitrate intake of people always exceeds the standard[10].
Many studies have shown that water washing and blanching[11]， and soaking[12] will effectively reduce nitrate and nitrite content. The processing and cooking of vegetables will inevitably lead to the loss and destruction of nutrients. Vitamin C is easy to lose and damage during processing and cooking. The longer the hot frying and stewing are， the greater the loss of vitamin C content is[13]. In this paper， the nitrate and vitamin C content of three kinds of leafy vegetables （cabbage， lettuce and celery） cooked and processed by different methods were studied， and rational methods for storage and cooking of vegetables were discussed to reduce the unnecessary loss of vitamin C. Targeted control measures for nitrate content were taken. The aim is to provide a theoretical basis for people to store vegetables more scientifically and safely and to use reasonable methods to cook vegetables. It is of great significance to safety production and evaluation of edible value of agricultural products.
Experimental materials
Experimental materials were three common vegetables （cabbage， lettuce and celery） sold in Yongchuan District， Chongqing City.
Reagents
Reagents included phenol disulfonic acid and ammonia water （AR）. Extracting agent oxalic acid： 2% solution （2 g of oxalic acid was dissolved in 100 ml of water. Oxalic acid： 1% solution （1 g of oxalic acid was dissolved in 100 ml of water）.
Standard ascorbic acid solution： 10 mg of pure ascorbic acid （it should be white， and can not be used if it is yellow ） was dissolved in 1% oxalic acid solution， diluted to 100 ml， stored in a brown bottle， and refrigerated. It is best to prepare before use. 2，6 dichloroindophenol solution： 52 mg of sodium bicarbonate was dissolved in 200 ml of hot distilled water. Afterwards， 50 mg of 2，6 dichloroindophenol was dissolved in the above sodium bicarbonate solution. After the solution was cooled， distilled water was added to it until the volume was up to 250 ml. It was filtered into a brown bottle and refrigerated.
Instruments and equipment
The main instruments included electronic balance （Merlett Toledo Instrument Shanghai Co.， Ltd.）， 754 UV spectrophotometer （Shanghai Shunyu Hengping Scientific Instrument Co.， Ltd.）， electronic thermostat universal electric furnace （Tianjin Taisite Co.， Ltd.）， water bath （Shanghai Yuejin Medical Instrument Factory）， high speed desktop microcentrifuge （Hermle labortechnik）， Philips mixer （Zhuhai Special Economic Zone Philips Home Appliances Co.， Ltd.）.
Experimental methods
Effects of boiling on nitrate and vitamin C content in the leafy vegetables
About 1 000 g of cabbage， lettuce （leaf） and celery were selected each， from which rotted leaves and yellow leaves were removed. They were washed， cut， and divided into two groups each， and the weight was equivalent. Nitrate and vitamin C content were measured 0， 0.5， 1， 3， and 5 min after boiling to 100 ℃. Each samples were measured three times.
Effects of salting on nitrate and vitamin C content in the leafy vegetables
Around 800 g of cabbage （stem）， lettuce （stem） and celery were selected each， from which rotted leaves and yellow leaves were removed. They were washed， cut， and divided into two groups each， and the weight was equivalent. They were completely immersed in 0.89 mol/L brine （20 g of NaCl was dissolved in 1 L of water）. After 0， 5， 10 and 20 min， nitrate and vitamin C content were measured. Each sample was measured three times， and the three data were averaged.
Effects of soaking on nitrate and vitamin C content in the leafy vegetables
About 1 500 g of lettuce and celery were selected each， from which rotted leaves and yellow leaves were removed. They were washed， cut， and divided into two groups each， and the weight was equivalent. After lettuce and celery were soaked in different concentrations of NaCl and CaCl 2 solution （0.05% and 0.5%） for 30 or 60 min， nitrate content was measured. Each sample was measured three times， and the three data were averaged.
Analytical methods
Some of fresh vegetables purchased in the market were taken to determine nitrate content， and some were stored， boiled， salted and soaked to determine nitrate content. Nitrate content in the leafy vegetables was measured by phenol disulfonic acid colorimetric method[14]， and vitamin C content in the leafy vegetables was measured by 2，6 dichlorophenol method. Results and Analysis
Effects of boiling on nitrate and vitamin C content in the leafy vegetables
According to Table 1， after the leafy vegetables were boiled， both nitrate and Vc content decreased， and the reduction increased with the extension of time. After 5 min， the nitrate content of cabbage， lettuce （leaf） and celery decreased by 1 415， 509 and 1 597 mg/kg respectively compared with the fresh leafy vegetables， namely reducing by 30%-56%， 18%-61% and 33%-57% respectively. In particular， the reduction of lettuce was the largest， reaching 61.3%. The Vc content of cabbage， lettuce （leaf） and celery reduced by 192， 27.5 and 72.6 mg/kg respectively， namely decreasing by 14%-52%， 14%-43% and 43%-74% respectively. The reduction of Vc content in lettuce was the largest， up to 74%.
Effects of salting on nitrate and vitamin C content in the leafy vegetables
After cabbage （stem）， lettuce （stem） and celery were salted for 5 min， the nitrate content of the leafy vegetables lowered by 1 550 ， 421 and 686 mg/kg， namely reducing by 36.9%， 38.8% and 23.3% respectively. There was no obvious change in the reduction with the continuous extension of time. Salting affected the Vc content of the leafy vegetables slightly. In comparison with the fresh vegetables， the Vc content of cabbage （stem）， lettuce （stem） and celery decreased by 9.7%-11.4%， 4.9%-17.7% and 1.6%-6.2% respectively.
Effects of soaking on nitrate content in the leafy vegetables
Seen from Table 3， after lettuce and celery were soaked in different concentrations of NaCl and CaCl 2 solution （0.05% and 0.5%） for 30 min or 60 min， the nitrate content of the leafy vegetables reduced. After lettuce and celery were soaked in NaCl solution， the nitrate content of celery and lettuce decreased by 9.5%-37.8% and 1.1%-37.8% respectively. When lettuce and celery were soaked in 0.05% NaCl solution for 30 min， the decreases were the largest. After lettuce and celery were soaked in CaCl 2 solution， the nitrate content of celery and lettuce reduced by 14.5%-22.0% and 17.8%-39.4% respectively. There were small differences between various CaCl 2 treatments in terms of the reduction in nitrate content. 0.05% CaCl 2 solution had an obvious effect on the nitrate content of lettuce， and the reduction reached about 35%.
Conclusions and Discussion
After the leafy vegetables were cooked and processed by different methods， there were big differences between various treatments in terms of the reduction in nitrate and vitamin C content. After cabbage was boiled for 1 min， its nitrate content was the lowest. After lettuce was salted or soaked in 0.5% CaCl 2 solution， the decrease of nitrate content was large， and nitrate content reduced with time. Vitamin C content was less affected by salting. After celery was boiled for 0.5 min， the decrease of nitrate content was the largest. Through this experiment， the leafy vegetables can be cooked by boiling， salting and soaking. Therefore， it is recommended that after leafy vegetables are boiled for 3-5 min， the boiling water is discarded， and then they are boiled again or fried. References
[1] HSU J， ARCOT J， LEE NA. Nitrate and nitrite quantification from cured meat and vegetables and their estimated dietary intake in Australians[J]. Food Chemistry，2009，115（1）： 334-339.
[2] JAWORSKA G. Effect of technological measures and storage time on the level of nitrates， nitrites，and oxalates in frozen and canned products of spinach and New Zealand spinach[J]. Food Chemistry，2005，93（3）：395-401.
[3] WANG YB， HUANG W， LIU DY. Nitrite content of common vegetables in Wuhu City[J]. Chinese Journal of Applied Ecology，2005，16（1）： 189-192. （in Chinese）.
[4] YE YF， XU XQ， ZHU Y， et al. Study on determination and storage conditions of nitrate and nitrite in vegetables[J]. Auhui Agricultural Science Bulletin，2007（21）： 82-83. （in Chinese）.
[5] WANG XL，CHU CS， LU LX， et al. Comparison of determination methods for nitrate content in vegetables[J]. Food Science and Technology，2010，35（4）：273-276. （in Chinese）.
[6] SI T， LIU Q， ZHANG YP， et al. Effects of combined application of organic fertilizer with inorganic fertilizer on yields and quality of lettuce[J]. Hunan Agricultural Sciences，2012（1）：50-53. （in Chinese）.
[7] JIAO XG， DENG Y， ZHAO WL， et al. Effects of different fertilization treatments on quality and yield of pakchoi[J]. Journal of Heilongjiang Hydraulic Engineering College，2011，2（2）：70-73. （in Chinese）.
[8] ZHENG WL， QU CD， WANG FZ， et al. The effect of organic and inorganic compound fertilizer on the yield and quality of Chinese cabbage[J]. Liaoning Agricultural Sciences，2012（2）：43-45. （in Chinese）.
[9] LIU N， SHEN MH. Food toxicology[M]. Beijing： China Light Industry Press， 2005： 8. （in Chinese）.
[10] BOINK A， SPEIJERS G. Health effects of nitrates and nitrites[J]. Acta Horticulture， 2001， 563：29-36.
[11] JAWRKSKA G. Effect of technological measures and storage time on the level of nitrates and oxalates in frozen and canned products of spinach and New Zealand spinach[J]. Food Chemistry，2005，93：395-401.
[12] LIU YQ， GUI Z， WU JJ， et al. Content variation of nitrite in differently treated vegetables[J]. Journal of Tianjin Agricultural University，2006，13（2）：21-23. （in Chinese）.
[13] LIU JK， LEI NF， WU HF， et al. Study on the content of vitamin C in common vegetables[J]. Trace Elements Science，2006，13（4）：75-95. （in Chinese）.
[14] HE SP， JIN YZ， ZHANG XM. Determination on nitrate of foliar vegetable with phenol two sulfonic acid method[J]. Journal of Heilongjiang August First Land Reclamation University，2005，17（2）： 37-40. （in Chinese）.