Investigating the effect of cooking and storage time on the amount of nitrate, nitrite and ascorbate salts of some vegetables

Dissertation for receiving the master degree "M.Sc"

Treatment: Food Science and Industry

Abstract

To carry out this research, samples of spinach (Damghan), tomato species (Dezful), lettuce (Varamin), celery (Tehran), zucchini (Gorgan), eggplant (Qasarshirin) were purchased from Damghan market in June 2013. Nitrate, nitrite and ascorbic acid concentrations were measured in the edible parts of each sample. The test was repeated three times on each sample. According to this experimental design, the method of multivariate analysis (Univariate Analysis) and then Duncan's test was used with the help of SPSS version 21 software to examine both the individual effects of the independent variables and their reciprocal effects on the dependent variables.  In general, the amount of ascorbic acid decreased over time, but no specific trends were observed in nitrate and nitrite, but from the regression between all three salts, it was concluded that the decrease in the amount of ascorbic acid is associated with the increase in nitrite. The lowest amount of ascorbic acid was related to celery, and the highest amount of ascorbic acid was in lettuce.  Nitrate in spinach was more than other vegetables and its decrease was also higher, the lowest amount of nitrate was in tomato. The highest amount of nitrite was observed in tomato and the lowest amount was observed in eggplant. Steaming reduced the amount of ascorbic acid, but no specific trend was seen in nitrate and nitrite.

Key words: ascorbic acid; vegetables; nitrate and nitrite

Chapter 1

Introduction

1-1- Preface

Today, due to excessive use of chemical fertilizers containing nitrogen to accelerate vegetative growth, many vegetables, especially leafy vegetables, have a high percentage of nitrates, which in many cases exceed the standards is determined more. Of course, nitrate (NO3) itself is not considered a toxic substance for humans, but nitrite (NO2) resulting from its reduction can combine with amines and form nitrosamines, which is considered a carcinogenic substance for the body (Tabatabai, 1384). They supply the majority of nitrates entering the body. (Cross et al. 1992). Sodium nitrite is a food additive that is used as a preservative (Agency of Toxic Substances and Disease Registry, [1] 2001; World Health Organization [2], 2006) through contaminated drinking water and foods containing preservatives, especially processed meat, such as hot dogs and instant meats (Agency of Toxic Substances and Disease Registry, 2001; Reinick et al. [3], 2005).

Sometimes when storing vegetables in storage or performing operations on them, nitrate turns into nitrite, and people who are fed with nitrite-containing foods are at risk of methaemoglobinemia [4] (Kashi, 2013). Also, in canned vegetables, a large amount of nitrate in storage for a few months causes tin to be released from them (Khaldebrin and Islamzadeh, 1380). According to research, 50% of the nitrate in the human body comes from the consumption of vegetables (Blomxarnastra [5] 1986). Nitrate reducing activity is affected by a wide range of microorganisms, including resistant bacteria in the stomach tube and intestine of mammals, and then it can react with amines, amides, and amino acids and produce N-nitroso compounds (Hagesh and Shilvah [6], 1982). Nitrite is transformed into nitrate under oxidative metabolism in tissues and blood, and the mechanism of oxidation is the reaction of oxyhemoglobin (Fe2+) as a result of the formation of methemoglobin complex (Fe3+) and finally enzymatic reduction to nitrate. The reaction speed between nitrite and hemoglobin depends on different species; In humans, the reaction speed is slower than in ruminant animals, but faster than in pigs (Smith and Beutler [7], 1966).

There are different standards for the maximum nitrate concentration in vegetables, for example, the European Union has recommended the maximum nitrate concentration for lettuce in spring crops in water culture about 3500 mg/kg and in autumn crops about 4500 mg/kg. This amount is about 2500 mg/kg for field crops (CESCF [8], 1992).

Given the high consumption of vegetables and the prevalence of storing them in the refrigerator, in this research, the effect of cooking and storage time on the amount of nitrate, nitrite and ascorbate salts of vegetables is investigated. 

1-2- Hypotheses

Raw or cooked green leafy vegetables changes the nitrate, nitrite and ascorbate salts in it.

Keeping green leafy vegetables in the refrigerator increases the amount of nitrate, nitrite and ascorbate salts.

Amount Nitrite, nitrate and ascorbate in green leafy vegetables are not more than allowed.

1-3- Research objectives

Investigating the storage time of raw and cooked green leafy vegetables in the refrigerator on the content of nitrate, nitrite and ascorbate content

Investigating the effect of cooking on the amount of salts Nitrate, nitrite and ascorbate in green leafy vegetables

Determination of nitrate, nitrite and ascorbate in green leafy vegetables

Nitrate and nitrite

Nitrite is also used as a preservative against Clostridium and other spoilage bacteria. Nitrite has the ability to prevent the growth of microorganisms, especially Clostridium. Adding nitrite to meat products not only has antimicrobial effects, but also stabilizes the color of the product, prevents the oxidation of lipids, creates a pleasant taste and improves the texture of the product. Nitrate is used when slow release of nitrite in the oven is desired (Moorcroft et al. [9], 2001).

Nitrate is a naturally occurring compound that is part of the nitrogen cycle, and is also approved as a food additive. Nitrate plays an important role in plant nutrition and performance.  Nitrate is an important component of vegetables because of its ability to accumulate.  Nitrate can be affected by a number of living and non-living factors (Yovah et al. [10], 2009).

2-2- Nitrate accumulation rate in different plant organs

Nitrate accumulation in all kinds of vegetables depends on many factors, including the amount and type of fertilizer containing nitrogen, frequency of use, cultivar, light intensity, length of day, season, harvest time, traditional or Planting mechanization, air temperature, soil temperature and humidity, fertility and mineral elements in the soil such as molybdenum, manganese, copper, boron, potassium, zinc, chlorine, calcium and sulfur have a bearing (Santa Maria [11], 2006). In general, leafy vegetables have more nitrate accumulation than fruit and seed vegetables, and among leafy vegetables, spinach is the most prominent. In this plant, the highest amount of nitrate accumulates in the petiole (Brown [12], 1966).

Using suitable cultivars, ammonium-based fertilizers, optimal use of fertilizer, adding organic fertilizers and harvesting in the evening are methods to reduce nitrate accumulation in vegetables (Dezfouli et al., 2009).

2-3- Effective factors in nitrate accumulation

Nitrate accumulation in plants is a natural phenomenon and occurs when the accumulation of nitrate in the plant is more than its reduction due to absorption and decomposition. This process in the plant can be affected by the following three factors:

2-3-1- Plant characteristics

Nitrate accumulation in the plant can depend on the species, variety, age and different parts of the plant.