what happens if we fry spinach

Spinach is one of the highly consumed vegetable, with significant nutritional, and beneficial properties. This study revealed for the first time, the effects of high temperature frying on the carotenoids, chlorophylls, and tocopherol contents of spinach leaves. Spinach leaves were thermally processed in the sunflower oil for 15, 30, 45, and 60 min at 250°C. Reversed phase HPLC-DAD results revealed a total of eight carotenoids, four chlorophylls and α-tocopherol in the spinach leaves. Lutein, neoxanthin, violaxanthin, and β-carotene-5,6-epoxide were the major carotenoids, while chlorophyll a and b were present in higher amounts. Frying of spinach leaves increased significantly the amount of α-tocopherol, β-carotene-5,6-epoxide, luteoxanthin, lutein, and its Z-isomers and chlorophyll b isomer. There was significant decrease in the amounts of neoxanthin, violaxanthin, chlorophyll b, b and chlorophyll a with increase of frying time. The increase of frying time increased the total phenolic contents in spinach leaves and fried sunflower oil samples. Chemical characteristics such as peroxide values, free fatty acids, conjugated dienes, conjugated trienes, and radical scavenging activity were significantly affected by frying, while spinach leaves increased the stability of the frying oil. This study can be used to improve the quality of fried vegetable leaves or their products at high temperature frying in food industries for increasing consumer acceptability.

Frying of foods is one of the traditional methods of food preparation. Frying is usually carried out in edible oils or fats as medium of choice. During frying triglycerides components of the oil or fats are oxidized at varying degree and are degraded (Aniolowska et al., 2016). The primary products formed are oxidized triglycerides (Zeb, 2015), which upon further oxidation produces secondary oxidation compounds such as various aldehydes and ketones. These oxidized products are toxic and have been found to negatively contributing to the quality of foods. However, it has been found recently that phenolic compounds present in the frying medium trap these toxic aldehyde and thus reduces toxicity (Zamora et al., 2016). It is thus important that some natural antioxidants should be present or may be retained in order to decrease toxicity during frying (Sunil et al., 2015). Both natural and synthetic antioxidants when heated at frying temperature could protect against the oxidative stress in the obese individuals (Perez-Herrera et al., 2013). Tomato products (Zeb and Haq, 2016), sea buckthorn products (Zeb and Ullah, 2015) and plant leaf extracts (Esposto et al., 2015) were used as good scavenging agents for decreasing toxic properties in in vivo or in vitro model systems. Vegetable leaves are of the special interest in food preparation.

Spinach (Spinacia oleracea L.) is a green leafy vegetable widely consumed in food preparation or as a food itself. Spinach contributes to several properties of the foods such as chemical, rheological, nutritional as well as sensory characteristics of the food (Khan et al., 2015). Spinach leaves are rich in carotenoids such as lutein and β-carotene. The composition of major carotenoids such as lutein ranges from 37 to 53 μg/kg, neoxanthin from 10 to 22 μg/kg, violaxanthin ranged from 9 to 23 μg/kg, while β-carotene contents are present in the range of 18–31 μg/kg (Bunea et al., 2008). In vitro digestion revealed an increase in the liberation of carotenoids especially lutein and β-carotene (Eriksen et al., 2016). Spinach powder was used as natural food grade antioxidants for the preparation in deep fried products (Lee et al., 2002). However, thermal treatment and high pressure produce changes in color and chemical attributes of an oil based spinach sauce (Medina-Meza et al., 2015). There is a lack of information about the changes in the carotenoids, chlorophylls, and its interactions with lipid oxidation during deep frying of vegetable leaves such as spinach. This study was therefore aimed for the first time to evaluate the interactions of lipid oxidation and changes in the pigment contents during frying of spinach leaves in sunflower oil.

Lutein, β-carotene, chlorophyll a, BHT, and methanol were from Sigma-Aldrich (Steinheim, Germany). MTBE and ammonium acetate was purchased from Daejung Chemicals (Daejung, Korea). Ultrapure deionized double distilled water was prepared using Labtech distillation system (Namyangju, South Korea). All other chemicals and reagents were of analytical HPLC standard with highest purity. Luteoxanthin and mutatoxanthin standard was prepared from violaxanthin by acidification and subsequent TLC as reported previously (Gallardo-Guerrero et al., 2013). Similarly, lutein isomers were prepared using thermal stress and subsequent purification with TLC (Kimura and Rodriguez-Amaya, 2002).

Cooking spinach can lead to a reduction in certain water-soluble vitamins, such as vitamin C and folate, due to heat exposure and leaching into cooking water. However, cooking spinach actually increases the bioavailability of other nutrients, such as iron and calcium.

Effects of frying on total phenolic contents

Figure showed the changes in the total phenolic contents of frying oil and spinach leaves fried at 250°C. The TPC of the unfried spinach leaves was 585.6 mg/g of GAE, which increased significantly (p < 0.05) till 45 min of frying, while a significant decrease was observed at 60 min (1075.5 mg/g GAE). In case of frying oil, TPC increased significantly at 45 and 60 of frying, however, the amount of TPC in oil was very small as compared to the spinach leaves. In a study of Turkmen et al. (2005), the TPC in the spinach showed an increase retention with different cooking methods. The initial increase may be due to the liberation of phenolic compounds from the leave matrix. The decrease of TPC at 60 min of frying may be due to the degradation of phenolic compounds as well as leaching to the frying medium. Ismail et al. (2004), showed that TPC significantly decreased in the water boiled spinach, cabbage, kale and shallots leaves. The authors suggested that the degradation of phenolic compounds during boiling may be reason of loss in TPC. In case of sunflower oil, the TPC decreased significantly (p < 0.05) at 15 min and increased further with increase of frying time. The increase in the TPC was highly correlated (R2 = 0.8435) with a decrease in the TBARS values of the sunflower oil. The present results are in agreement with the Silva et al. (2001), who showed that the addition of phenolic compounds increased the stability of sunflower oil. These authors showed that caffeic acid derivatives showed an increase in the stability of sunflower oil than α-tocopherol. The present results indicate that during frying of spinach leaves, increase in the TPC occurred in the sunflower oil, which may be responsible for the increase in stability and decrease in the lipid peroxidation. Thus, spinach or spinach containing products may contribute to the increase in the stability of frying oils.

Sample collection and preparation

Indigenous fresh leaves of the Spinacia oleracea (oriental spinach) were collected from the local market in Chakdara, Khyber Pakhtunkhwa. The sample (1.0 kg) was randomly sampled from the open air marketed shelf. The latitude, longitude of sampling area is 34.65° and 72.033° and altitude of 2,288 ft. Fried and unfried spinach leaves were grinded during day light and control conditions to avoid further oxidation of carotenoid contents.

The leaves were washed and cut into several pieces with dimension of 1 cm in length from all sides for the equal effects of heat processing. Representative samples of spinach leaves (200 g) were fried in sunflower oil (500 g) heated to the temperature of 250°C continuously for 15, 30, 45, and 60 min in laboratory open fryer. The unheated sample was taken as control. The selection of timing was based on the level of frying leaves in terms of visual color changes, while temperature was the one commonly used in Pakistan for street food frying, and mimicking the high temperature processing (pre-heating) in food industries (Weber et al., 2008). Each fried leaves and oil samples were collected in caped glass tubes and stored in refrigerator at −20°C till analysis.

Carotenoids extraction was carried out using the method from the recent work (Zeb, 2017). Sample processing and extractions were carried out under daylight, the mild nitrogen environment in a closed fume hood at 25°C, and in the absence of pressurized air, sunlight and high temperature. Briefly, one gram of fried and unfried grinded spinach paste was mixed with ice cold acetone (5 mL) and vortex for 60 min. Then 10 mL of absolute ice cold ethanol containing 0.1% BHT was added and again vortex for 30 min. The extractions were repeated until discoloration of spinach leaves. The solvent was evaporated under vacuum at 35°C. The residue was dissolved in to HPLC solvent (2 mL) and filtered using Agilent PFTE syringe filters (0.45 μm) and transferred into HPLC vials. Standard calibration curves of α-tocopherol, β-carotene epoxide, violaxanthin, lutein, its Z-isomers, and chlorophyll a were prepared for quantitative analyses. The limit of detection (LOD) and limit of quantification (LOQ) calculated from the standard curves were 0.4, 0.7, 0.9, 0.8, 0.05, 0.08, 0.31, 0.08 ng, and 2.5, 1.3, 1.6, 2.1, 1.1, 1.3, 2.0, 2.2 ng, for tocopherol, β-carotene epoxide, violaxanthin, luteoxanthin lutein, 9-Z-lutein, 9′-Z-lutein, and chlorophyll a, respectively.

Carotenoids, chlorophylls and α-tocopherol were separated using a reversed phase HPLC system. The HPLC system (Agilent 1,260 Infinity Better) was equipped with quaternary pump, degasser, auto-sampler, and diode array detector. The column was an Agilent rapid resolution column (Agilent Zorbax Eclipse C18, Agilent Technologies, Waldbronn, Germany) with the specification of 4.6 × 100 mm, 3.5 μm maintained at 25°C. The tertiary gradient system consists of solvent A as methanol: deionized water (92: 8, v/v) with 10 mM ammonium acetate, solvent B was deionized water containing 0.01 mM and solvent C was MTBE (100%) (Zeb, 2017). The flow rate was fixed at 1 mL/min and injection volume was 50 μL. The gradient program was started with 80% A, 18% B, and 2% C. At 3 min the gradient was 80% A, 12% B and 8% C, which reached 65% A, 5% B with 30% C. The gradient then finally reached 60:0:40 (A:B:C)% at 40 min with post gradient elution of 10 min for recovery of the initial gradient. The spectra were recorded in the range of 190–750 nm and the chromatograms were obtained at 450 nm using OpenLab Chemstation software (Agilent Technologies, Germany). The identification of carotenoids, chlorophylls, and α-tocopherol were based on either the available standards, their retention times, or by comparing the absorption spectra reported in the literature. The identified compounds was quantified from the peak area using respective calibrations and represented as mg/100 g of the fresh weight basis.

Lipid peroxidation was studied using a validated spectrophotometric method described recently (Zeb and Ullah, 2016) as thiobarbituric acid reactive substances (TBARS). Briefly lipid peroxidation products were extracted from the oil and leaves (1 g) samples using glacial acetic acid. The extract (1 mL) was mixed with 1 mL of thiobarbituric acid (TBA) reagent. The mixture was shaken for 1 h at 95°C. The absorbance of the room cooled mixture was measured at 532 nm in triplicates. The TBARS was measured as μmol/g of the sample.

Mistakes People Make When Cooking Spinach

FAQ

Is frying spinach healthy?

Does pan frying spinach remove nutrients?

What happens if you cook spinach?

Should you boil or fry spinach?

What happens if spinach is overcooked?

Overcooked spinach can take on a bitter burnt flavor and have an unpleasant mushy texture – not a good way to get people to eat their greens. Direct heat methods like sautéing are most likely to end up with overcooked spinach and call for extra diligence. If you’re cooking on the stovetop, make sure you’re keeping a watchful eye on your pan.

What are the disadvantages of spinach?

Spinach is a very healthy vegetable that contains many nutrients, including vitamin A, vitamin C, vitamin K, iron, folate, and potassium. It also has a lot of fiber. The only disadvantages of spinach, is that if you eat high amounts, it can cause gas, cramping, and abdominal pain. Eating too much spinach can also interfere with the body’s ability to absorb nutrients.

Is frying spinach a good idea?

While there are various cooking methods for spinach, frying offers a unique taste and texture that can elevate your dining experience. Frying spinach allows it to release its natural flavors while adding a delightful crispy texture. Plus, frying requires minimal ingredients and preparation time, making it a fuss-free cooking method.

What happens if you cook spinach raw?

When spinach is raw, its natural levels of oxalic acid can sometimes get in the way of your body’s natural ability to absorb important nutrients like iron and calcium. But when it’s cooked with heat, that acid is broken down.