Strawberry (Fragaria Spp.) belongs to family “Rosaceae”. Over six hundred varieties occurred which differ in taste, texture and size [1]. Strawberry is one of the important fruit among the berries. Various strawberry species grow wild all over the world, but the cultivated strawberry is based upon two species Fragarin Chiloensis and Fragarin Verginiana. Hybrids between these two species were the precursor of all the present strawberry cultivars [2]. Strawberry crop is recently introduced in Pakistan; its production in Pakistan is very low as compared to other countries of the world where strawberries were grown. In Pakistan it is grown inside definite places of Punjab, Islamabad and Khyber Pakhtunkhwa [3]. There are many varieties grown in the world but the main promising varieties grown in Pakistan are Mission, Corona, Tuft, Sweet Charlie, Super faction and Festival. During 2009-10 strawberry was grown on an area of about 193 acres and its production was 274 tons in Pakistan [4]. According to Food and Agriculture Organization (FAO), world production of strawberries has exceeds 4 million tons, in which United States contributes 28 percent [5]. Nutritionally, One cup of strawberry contains 1 g protein, 10.5 g carbohydrates, 0.6 gram fat, 1.6 gram fiber, 0.1 mg thiamin, 84.5 mg vitamin C, 26.4 mg folic acid, 0.1 mg riboflavin, 0.4 mg niacin, 0.2 mg zinc, 0.6 mg iron, 2 mg sodium, 21 mg calcium, 16 mg magnesium, 45 kcal calories [5]. Vitamin C level of Strawberry fruit is more than oranges. Strawberry fruit also give an exceptional supply of vitamin K, vitamin B5, vitamin B6, manganese, magnesium, copper, potassium and omega 3 fatty acids [6]. Malic and Citric acids are main organic acids which contribute to aroma. Strawberries also contain a higher percentage of antioxidants such as phenols and flavonoids as compared to other berry fruit [7]. Strawberry fruit have juicy, tiny, flavorful, nourishing, syrupy flavor, diuretic, demineralizing, tonic and astringent attributes [8]. The strawberry fruit can be used both in fresh form and preserved to Jellies, James and squashes that can be consumed in off season [9]. Osmotic dehydration (OD) is a technology that partially removes the water from fruits and vegetables, when it is dipped in a hypertonic solution of salt, sugar or others. This process is successfully applied to those fruit and vegetable, which is perishable and cannot be supplied to market in fresh form. However, OD is a pretreatment course of action so further is process is necessary for the stability of the product. Osmotic dehydration method (OD), also named dewatering and impregnation by immersion in concentrates or by immersion of fruits and vegetable in concentrated solutions or syrups of soluble solids, without phase change [10]. Alum is a salt that in chemistry is a combination of an alkali metal, such as sodium, potassium, or ammonium and a trivalent metal, such as aluminum, iron, or chromium. The most common form, potassium aluminum sulfate, or potash alum, is one form that has been used in food processing. The potassium-based alum has been used to produce crisp cucumber and watermelon-rind pickles as well as maraschino cherries, where the aluminum ions strengthen the fruits’ cell-wall pectins [11,12] Sugar provides sweet taste and flavor to the product; they also provide freshens and contributes to the product quality [13]. Different chemical preservatives are used in food products to extend their shelf life but the most important are sodium benzoate, potassium sorbate, potassium metabisulphite, sorbic acid, sulphur dioxide etc. Potassium sorbate, retard yeasts and molds growth in numerous food stuffs like yogurt, cheese, wine, dried meats and dehydrated commodities. Sodium benzoate is a becteriostatic and fungistatic preservative underneath acidic environment and is mostly used in acidic foods like salad dressings (vinegar), carbonated munchies (carbonic acid) and condiments [14]. Nonetheless, the future of strawberry production and processing might be very much bright in our country in general and Khyber Pakhtunkhwa in particular because this fruit fetches maximum economic returns for the farmer. Due to the perishability of strawberry fruit, this research work was designed to prepare a value added product from strawberry fruit to maintain is texture properties and physiochemical properties.

Sound, healthy and ripened strawberries were purchased from the orchard at Charsadda and were brought to the laboratory of Food Technology section, Agriculture Research Institute, Tarnab Peshawar where research work was conducted. Strawberry fruit without visible defects and color were selected. After washing with cold water the stalks of the fruits were removed with stainless steel knives. First two different concentration of potash alum solution was made (1.2% and 2%), strawberry fruit were dipped for 30 minutes [11]. At the same time three different concentrations of sucrose solutions (30°Brix, 40°Brix, 50°Brix) were prepared with supplementation of chemical preservative (Sodium benzoate) described by Khan [15] Furthermore, various treatments were prepared and named as; Table 1. Then the collected samples of strawberry fruit were immersed in 500 ml sterilized plastic jars filled with respective treatment solutions. For physiochemical and organoleptic analysis and shelf life study, the jars were completely sealed and stored at ambient temperature. The experiment was repeated with three replications.

Table 1: Treatment preparations.

Chemical used

Sodium Benzoate (Analytical grade-Merck Germany), Potash alum (Analytical grade-Merck), Sodium hydroxide (Analytical Grade- Sigma), Copper sulphate (Analytical Grade-Merk Germany), Oxalic Acid (Analytical Grade- Sigma), 2,6-dichloro indophenols (UK Chemical LTD), Potassium hydroxide (Analytical Grade-Sigma ), Methylene Blue (Sigma), Phenolphthalein (Analytical Grade-Merk). Ascorbic Acid (Merk Germany), Sodium Potassium tartrate (ChemPol England).

Chemical analysis

Acidity, pH, Total soluble solids, reducing and non-reducing sugar, Ascorbic acid were evaluated through standard methods of AOAC [16].

Organoleptic analysis

The samples of strawberry fruit preserve were sensory evaluated for color, texture, flavor and overall acceptability by 10 trained judge’s panel. Organoleptic study was carried out at each 15 days interval for 3 month storage. The evaluation was conceded out by using 9 points hedonic scale of Larmond [17]. The results are of scoring rate 1-9 awarded by judges of panel.

Statistical analysis

All the data concerning treatments and storage interval were statistically analyzed using factorial experiment in completely randomized design and the means were separated by applying least significant difference (LSD) Test at 5% possibility level as defined by Steel and Torrie [18].

Chemical analysis

pH: pH of the fruit is mainly associated to the flavor and perishability. The mean pH values decrease during storage from 3.50 to 3.33 (Table 2). The highest percent decreased was found in S₆ (5.71%) followed by S2 (5.20%) while the lowest percent decrease was observed in S₁ (3.99%) followed by S₅ (4.02%). A decrease in pH value during osmotic dehydration and rehydration was found as major characteristics of pineapple slices [19]. For sugar solution, pH showed a slight decrease from 5.4 to 5.1 while in sugar/salt solution it decreased from 5.1 to 4.9. The pH fluctuated with osmotic dehydration time but the changes were not significant, and these values have advantage of inhibiting microbial growth [10]. Similarly, strawberry syrup was also featured with decreasing trend of pH level with storage interval due to development of acidic compounds [20]. Similar results were also found by Khan [14] during storage of strawberry fruit in different concentration of sucrose solution and chemical preservatives. The Fluctuations in pH range was specifically observed in strawberry fruits while the syrup exhibited an unchanged pH levels.

Table 2: pH analysis of different treatments of strawberry fruit.

Total soluble solid (TSS): The mean Total Soluble Solids of the treatments increase from 18.20 to 21.07° brix during storage. Highest percent increase was found in S0 (21.25) followed by S2 (2.19) while lowest percent increase was observed in S5 (11.81) followed by S1 (17.48) as shown in Table 3. Increase in TSS was due to the movement of sucrose molecules from osmotic solution to strawberry fruit through the semipermeable wall of the fruit. Similar findings were recorded by Kumar [21]. Who observed an increase in total soluble solid during osmo dehydration of pineapple slices? Additionally, another study explained that the raise in TSS during storage interval of the pineapple slices was due to diffusion of molecule from dilute medium to concentrated solution (hypertonic solution) through a semi-permeable membrane until the concentration equilibrium was reached. The total soluble solid of the medium of strawberry fruits increased due to the movement of sugar content from higher concentration (sucrose solution ) to lower concentration (strawberry fruit) [19].

Table 3: Analysis of TSS (°brix) of different treatment of strawberry fruit.

Titratable acidity (%): Mean values for Titratable acidity increase from 0.38 to 0.53 during storage period (Table 4). The highest percent increase was observed in S₀ (31.63%), followed by S₃ (30.51%) while lowest percent increase was noted in S₁ (25.0%), followed by S₅ (27.27%) shown in Table 4. The results are in agreement with the conclusions of Ali [22] who observed an increase in titratable acidity during preservation of persimmon slice. Similar results were investigated by Kumar [23] in osmotically vacuumed dried mango slice. The increase in acidity might be due to development of acidic substances by the degradation of pectic bodies or breakdown and also attributed to hydrolysis of polysaccharides and non-reducing sugars through utilization of acids for converting them to hexose sugar [24]. High concentration of sucrose solution decreased the loss of acid from fruit to syrup. Sugar form a barrier on the surface of the fruit at high concentration, which makes it difficult to lose solids from the fruit. Such results are in treaty with those obtained by Heng [25] and Khan [15]. The titratable acidity of the blue berries also increased during osmodehration by Giovanelli [26]. Figure 1 shows various treatments on the strawberry fruit preserve, S₀, S₁, S₂, S₃, S₄, S₅ represents different treatments used during preservation procedure of strawberry fruit.

Table 4: Analysis of Titratable acidity of different treatment of strawberry fruit.

Figure 1: Various treatments on the strawberry fruit preserve, S_o, S₁, S₂, S₃, S₄, S₅ represents different treatments used during preservation procedure of strawberry fruit.

Ascorbic acid (mg/100 g): Ascorbic acid content decreased with the storage time. The mean values of ascorbic acid content were significantly decreased from 54.68 to 30.23 mg/100 g during storage. Maximum percent decrease was found in S₀ (63.17%) followed by S6 (49.10%), while minimum percent decrease was found in S₂ (32.93%) followed by S₃ (37.28%) shown in Table 5. These outcomes are in agreement with the conclusions of Kumar and Sangarand-lata [23] who observed decreased in ascorbic acid (AA) content of osmo-vac dehydrated mango slices. In all cases, the osmotic treatment caused significant losses in the AA content of samples, ranging from 24 to 43%. An additional decrease was observed during storage. Two independent mechanisms could be considered to explain these AA losses: losses by diffusion from the fruit tissue into the Osmotic Solution (OS) during dehydration and losses due to chemical degradation during processing and storage. The reaction mechanism of ascorbic acid decomposition in foods has been extensively studied. When oxygen is present, AA degradation occurs simultaneously by oxidative and anaerobic mechanisms, the latter pathway being slower than the oxidative one [27]. The oxidative degradation of AA is related to ascorbinase activity and by indirect degradation throught polyphenol oxidase, cytochrome oxidase and peroxidase activity [28]. During the first two weeks of storage, the predominant effect is that of the oxidation of the AA to L-dehydroascorbic acid (DHA). From that storage time, the anaerobic degradation becomes predominant [29].

Table 5: Analysis of Ascorbic acid content (mg/100 g) of different treatment of strawberry fruit.

Reducing sugar: During storage interval amount of reducing sugars gradually increased from 8.35 to 11.16. Maximum percent increase was found in S₂ (43.85%) followed by S₃ (32.18%) while minimum percent increase was found in S₁ (24.15%) followed by S₅ (26.31%) while control samples S0 showed decrease during storage as shown in Table 6. Results showed that reducing sugar increased with time but the control decreased with time. The reason for increasing the reducing sugar might be due to the presence of invertase enzymes but invertase enzymes works properly at 4.6 pH and 50ºC temperature And since the temperature was ambient in this condition, thus making it inadequate for activity of invertase enzyme.. The possibility of acid hydrolysis of sucrose will also increase. These results are shown by Wisal [30] during storage of strawberry juice with different chemical preservatives and sucrose content. Kumar [21] also show similar results during pineapple slices osmo dehydration.

Table 6: Analysis of reducing sugar (%) of different treatments of strawberry fruit.

Non reducing sugar: The mean values of non-reducing sugar of strawberry fruit samples (S₀ to S₆) were decreased from 11.89 to 11.04 during storage duration. Highest percent reduction in non-reducing was occurred in S₀ (89.29%) followed by S₄ (6.88%), while percent lowest reduction was recorded in S₁ (3.40%) followed by S₂ (4.84%) as shown in Table 7. The reason for decreasing in non-reducing content might be due to inversion by acid. [31] Zia and Ayub observed similar results in decreased in non-reducing sugar of sucrose and glucose preserved melon cubes. Wisal [30] Reviewed similar results in strawberry juice preservation in different chemical and sucrose contents.

Table 7: Analysis of non reducing sugar (%) of different treatment of strawberry fruit.

Sensory evaluation

Color: The average mean values for strawberry fruits preserved were considerably (P< 0.05) declined from 9.00 to 4.96 during storage period. Highest percent decreased in color occurred in S₀ (88.89%) followed by S₅ (47.78%) while percent lowest decreased in color occurred in S₂ (27.78%) followed by S₁ (32.22%) as presented in Table 8 [32]. Moreno observed color changes during ohmic heating, vacuum impregnation and osmodehydration of strawberry fruits. Mancilla [33] showed similar changes in color during osmo dehydration under hydrostatic pressure of strawberry fruits. The change in strawberry color is associated with chromatic coordinates which is affected by osmodeydration under hydrostatic pressure. Perera [34] also observed a decrease in color due to browning reaction (millard) that occurred throughout storage in pineapple juice. A decrease in color intensity was recorded in pineapple slices and mango chips due to osmodehydartion [21,35]. Red color of strawberry is due to presence of anthocyanin pigments. Reduction in anthocyanin pigments causes loss in color for strawberry fruits during the preservation due to osmosis or solubility of anthocyanin in free water present in osmotic solution. Many investigations revealed that fructose is one of the major carbohydrates in promoting color degradation during processing while sucrose stabilizes anthocyanin level [36].

Table 8: Sensory evaluation score for color of strawberry fruit.

Texture: The average mean values for strawberry fruits preserved were considerably (P< 0.05) declined from 9.0 to 5.14 during storage period. Highest percent decreased in texture occurred in S₀ (88.89%) followed by S₆ (34.44%) while percent lowest decreased in texture occurred in S₂ (26.56%) followed by S₁ and S₄ (26.67%) shown in Table 9. These results are similar to those reported by Iman [37] who preserved apple slices in sucrose/glucose solution. Maldonado [38] showed decline of the texture during rehydration of mangoes. Texture is usually related to the firmness of the fruit. Aday and Caner [39] observed changes in firmness of the strawberry fruit using an oxygen absorber in a bio based package. Change in texture might be due to cell rupture or conversion of starch to sugar [40]. Deng and Zhao observed decrease in firmness of osmodehydrated apple with storage period. Textural properties are intimately associated with the cellular structure and pectic composition. Fuchigami, Miyazaki and Hyakumoto [41] observed that the inferior texture of frozen tomatoes was accompanied by a decrease in the content of pectic compounds. Also the level of protopectins and pectins were significantly reduced by the freezing process itself and during subsequent frozen storage of tomato tissue.

Table 9: Sensory evaluation score for texture of strawberry fruit.

Flavor: The mean value of the strawberry fruit preserved for flavor considerably (P< 0.05) declined from 9.0 to 5.23 during the whole storage period. Highest percent decreased in flavor score was observed in S₀ (88.89%) followed by S₆ (45.56%) while lowest percent decreased was found in S₄ (25.56%) followed by S₂ (27.78%) as described in Table 10. Flavor is the blend of taste and smell perceptions noted when food is taken. The overall flavor impression is the result of taste perceived by the taste buds in the mouth and the aromatic compounds detected by the epithelium in the olfactory organ in the nose. Loss of aromatic substances occurred in osmo dehydration of mango and concluded that high osmodeydration level effect the volatile compound negatively while lower osmodehydration level give rise to enhancement of the volatile profile [42]. Similarly, Pani [43] also observed degradative event during osmodehydration and air dehydration of tomato slices. Another study also displayed a decline in flavor of mango fruit after 15 days of storage interval. The biochemical and physiological changes in mango fruits were slower and the conversion of complex organic compounds into esters, aldehydes, acids, alcohols, ketones and ethers that contributes significantly to the aroma/ flavor developed at the later stage of storage period [44] Osmotic treatments stimulates changes in volatile profile of strawberry fruits [45] and kiwi fruit [46] altering the overall aroma of the fruits that leads to flavor degradation.

Table 10: Sensory evaluation score for flavor of strawberry fruit.

Overall acceptability

The mean score for overall acceptability of the strawberry fruit preserve decreased from 9.00 to 5.40. Highest percent decreased in overall acceptability was founded in S₀ (88.89%) followed by S₆ (46.67%), while the lowest percent decreased was occurred in S₂ (25.56%) followed by S₄ (26.67%) as shown in Table 11. These results are in commerce with Sabrina [47] who observed decline in the overall acceptability of osmo dehydrated mango slices with inverted sugar syrups and sucrose. The overall acceptability showed that the sucrose solution retain the volatile component of the strawberry fruit as compared to way of preservation Khan [15]. Results investigated that treatment S₂ followed by S₄ showed better overall acceptability score as compared to treatments. Figure 2 Shows all different treatments and replications of the strawberry fruit preserve.

Table 11: Sensory evaluation of overall acceptability score of strawberry fruit.

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Figure 2: Shows all different treatments and replications of the strawberry fruit preserve.

The objective of the research work was to evaluate the effect of treatment on overall quality of strawberry fruit. The resulted data showed that changes in chemical nature of the strawberry affect the organoleptic properties of the strawberry fruit. The sample showed significant response to the treatment but the temperature dependent of color changes and vitamin C losses were observed after 15 days of storage interval. The changes in color was due to the water soluble nature of anthocyanin, continuously changes the color of osmotic medium. The potash alum shows significant effect on the texture property of the strawberry fruit, as compared to the treatment without potash alum. It is concluded from the present research study that all the treatment response well to the potash alum both during physiochemical and organoleptic evaluation. The potash alum showed better results in 30° and 40° brix sucrose solution concentration. Among all other treatment S₂ (Strawberry fruit + sucrose solution (30° brix) + sodium Benzoate 0.1% + Potash alum 2%) was found highly adequate during 90 days of storage interval both physiochemically and organleptically. The treated sample could be used for syrup, squash, salads and ready to eat is also possible.