Respiratory behaviour and quality attributes of fresh apple slices in modified atmosphere systems

• Main Author: Dhanawansa, Ullusu Hewage
• Language: English
• Published: 2008
• Online Access: http://hdl.handle.net/2429/1347

Ascorbic acid (5%) - treated, peeled, cored and sliced apples in modified atmospheres were studied in a two part investigation. Firstly, respiratory behaviour of slices of New Zealand Granny Smith and British Columbia grown Newtown apples at different temperatures (1, 5, 10 & 15°C) was studied in sealed impermeable chambers with 21.5% O₂ + 78.5% N₂ as the initial gas mixture. Secondly, the respiratory behaviour and quality changes of sliced Newtown apples of the same stock were studied for six weeks, at 1°C, in sealed flexible polymer film pouches made of high gas barrier (CL804), medium high barrier (6 mil polyethylene, PE) and medium low barrier (4 mil PE) packaging films. The same initial gas mixture was used. Regardless of the storage temperature and the variety, CO₂ levels for apple slices in respiratory chambers increased linearly with time (r > 0.99). O₂ consumption curves for apple slices of both varieties were biphasic with a linear saturated region followed by a logarithmic unsaturated region. However, at 1°C, the O₂ consumption curve for Newtown apple slices followed only a logarithmic relationship. The crossover point between the linear and logarithmic portions of the biphasic O₂ consumption curve was recognized as the transition O₂ level. Depending on the temperature, transition O₂ levels for apple slices ranged from 0.81 to 4.5% O₂ for Granny Smith and 2.2 to 6% O₂ for Newtown apples. Both apple varieties had a O₂ consumption rate of 1.45 ml O₂/kg.hr at 1°C. A Q₁₀ of 4.2 and a Q₁₀ of 5 for the temperature range of 1 to 15°C was calculated for Granny Smith and Newtown apples, respectively. At aerobic O₂ levels, respiratory quotient (RQ) values were less than unity for both varieties. Activation energy for the Arrhenius relationship for the respiratory activity of Granny Smith and Newtown apple slices, at 1 to 15°C, were -64.15 kJ/mole and -71.76 kJ/mole, respectively. At the end of storage, apple slices in pouches made up of each different kind of packaging film established low O₂ and high CO₂ concentrations which reflected the gas permeability of the particular film. Final package headspace equilibrium O₂ levels were approximately 0.28%, 0.96% and 1.32% for HB, MHB and MLB films, respectively. CO₂ levels at the end of the six week storage period were 31%, 13.5% and 7.3% for the same films. Changes of pouch headspace O₂ and CO₂ concentrations during the study were significantly different for package film types (p < 0.001), storage time (p < 0.001) and for the package film and storage time interaction (p < 0.001). pH and percent soluble solids of apple slices changed significantly for different packaging films (p < 0.001 for both properties), for storage time (p < 0.001 for both properties) and for the storage and package film interaction (p < 0.001 & 0.002, respectively). Both properties initially decreased and then began to increase during storage. Percent titratable acidity (as malic acid) decreased as storage time proceeded (p < 0.001) and changed significantly for packaging films (p < 0.001) and for the storage and packaging film interaction (p < 0.001). The two lowest permeable films showed the largest decrease in acidity. The soluble solids/titratable acidity ratio increased until the third week then began to decrease during rest of the storage (p < 0.001). Regardless of the package film type, surface browning of apple puree exposed to air was rapid. Hunter L, "a" and b values measured at different time intervals (0, 30, 60, 90 &120 min) changed significantly but not with a definite relationship. The surface discolouration of MAP apple slices in storage was determined mostly by the cultivar. All five texture measurements, bioyield point force (p = 0.001), deformation to bioyield point (p < 0.001), firmness measured as the ratio of bioyield point force to deformation (p < 0.001), rupture point force (p < 0.001) and the ratio of rupture point force to deformation (p = 0.001) measured with Instron plunger test, decreased significantly during storage. Deformation to bioyield point was statistically significant for the three different packaging films (p = 0.005) and for the packaging film and storage time interaction (p = 0.032). Rupture point force was different for three different packaging films (p = 0.010) and was statistically significant for the packaging film and storage time interaction (p < 0.001). Deformation to rupture point did not show a significant (p > 0.05) change. Rupture point force to deformation ratio was different for three packaging films (p = 0.007) and was statistically significant for the packaging film and storage interaction (p < 0.001). Instron texture profile analysis parameters fracturability, Hardness-1 and hardness-2 were significantly different for packaging films (p = 0.010) and decreased in storage (p ≥ 0.001), hardness-1 (p ≥ 0.009) and hardness-2 (p ≥ 0.024) also decreased during storage. Anaerobic microatmospheres resulted in overall higher texture values apple slices than those packed in aerobic microatmospheres. Regardless of the film type, sensory quality of apple slices deteriorated during storage. Appearance (whiteness) was affected most. Following the fifth week of storage, apple slices from the two lower permeability film type (HB and MHB) pouches developed slime and off-flavours while apple slices from MLB film pouches developed a moldy smell. Apple slices showed no weight losses in storage. Ascorbic acid (5%) dipping for 3 min prior to packaging preserved surface colour of sliced Granny Smith apples. Even with a longer dipping time (8 to 10 min) surface colour of sliced Newtown apples became unstable when exposed to air. The cultivar and the O₂ permeability of the packaging film were most important factors determining the quality of apple slices in pouches. Therefore, it is proposed, low browning apple cultivars and the high barrier packaging materials which provide less than the optimal O₂ requirement which result partial anaerobiosis should be used for MAP apple slices.