COMPOSITIONAL CHANGES OF STRAWBERRY DUE TO DEHYDRATION, COLD STORAGE AND FREEZING–THAWING PROCESSES

Strawberry is an excellent source of food ingredients, although compositional changes might occur in improperly controlled processing, affecting product quality. In this article, changes in sugar composition (glucose, fructose and sucrose), citric acid, water and total soluble content, as induced by partial dehydration and freezing–thawing processes, were analyzed in strawberries (var. Camarosa). Osmotic dehydration (OD) with 65 °Brix sucrose solution, air drying (AD) at 45C, or combined treatments (OD–AD) were applied to reduce strawberries’ water content to 70–85%. Fresh and dehydrated samples were frozen (?40C, 24 h) and stored (?18C, 30 and 180 days). All samples processed by OD and OD–AD showed a significant sugar gain, and depending on the dehydration treatment, total or partial sucrose hydrolysis was observed. Dehydration treatments caused small losses of citric acid. During the freezing–thawing process, drip loss and enzymatic action also cause changes in sugar concentration, especially in OD‐treated samples.     

DIRECT ANALYSIS OF INDIVIDUAL ANALOGUES OF ALPHA-ACIDS,BETA-ACIDS AND 4-DESOXYHUMULONES IN HOPS USING GAS-LIQUID CHROMATOGRAPHY AND NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

The techniques of gas-liquid chromatography and nuclear magnetic resonance allow simultaneous analysis of the individual α-acids and β-acids and the corresponding 4-desoxyhumulones. The rates of appearance in the growing hop-cone of humulone, lupulone, 4-desoxyhumulone and their co-homologues and ad-isomers are reported. The results indicate a relatively enhanced initial formation of the 4-desoxy compounds and β-acids. The α-acids reach their maximum concentration later than the desoxyhumulones and β-acids but eventually become the predominant group. It appears that formation of the α-acids is associated with a decline in the relative proportion of both β-acids and 4-desoxyhumulones. For each class of compounds, the co-homologues increase their relative concentrations during the growth of the hop; adhumulone and adlupulone remain in almost constant proportions during this period.     

STUDY OF CONFORMATIONAL CHANGES OF EWE'S HOLO (NATIVE) AND APO-α-LACTALBUMIN BY SPECTROSCOPY AND TRYPSINOLYSIS

Conformational changes of ewe's α‐lactalbumin (ALA) upon removal of Ca ²⁺ were determined by surface hydrophobicity, calorimetry and circular dichroism. Native ewe's ALA resisted trypsinolysis, showing 4% maximum degradation after 20 h of hydrolysis. Removal of bound calcium by addition of either ethylenediaminetetraacetic acid or ethylene glycol bis β‐aminoethyl ether‐N,N,N,N‐tetraacetic acid induced major protein conformational changes, enhancing its susceptibility to trypsinolysis, and leading to complete degradation of the protein. Reversed‐phase high‐performance liquid chromatography profiles of tryptic hydrolysate of Ca ²⁺ ‐free ALA were nearly the same through the whole enzymatic incubation period (24 h) showing the absence of sequential hydrolytic mechanism. They were characterized by the presence of five main peaks representing hydrophobic large‐sized peptides. Cleaving the S‐S bonds in the resulting hydrolysates with 2‐mercaptoethanol gave rise to new peaks representing more hydrophilic and hydrophobic peptides.     

CHARACTERIZATION AND INHIBITION OF POLYPHENOL OXIDASE FROM PEARS (PYRUS COMMUNIS L. CV. BOSC AND RED)

Red pears had higher PPO activity, total phenolics and chlorogenic acid concentration than Bosc pears. PPO activity and phenolics both decreased in fruits held at room temperature. pH and temperature optima for Bosc and Red pears PPOs were 5.0 and 5.5, and 20 and 23C, respectively. 4-Methylcatechol, catechol and dopamine were good substrates for PPO from both pear cultivars; however, no activity was observed with any of the mono-hydroxy substrates studied. Higher K_m and lower V_max values were observed for Bosc pear PPO. Heating at 75C for 30 min completely inactivated the enzyme from both cultivars. Heating at 55 and 65C for the same duration resulted in partial inactivation (45–60%) of this enzyme. Ascorbic acid, L-cysteine, sodium metabisulfite and thiourea effectively inhibited browning due to pear PPOs.     

SELECTION OF CHEMICAL AND THERMAL PRETREATMENT COMBINATION TO REDUCE THE DEHYDRATION TIME OF SOUR CHERRY (Prunus cerasus L.)

Sour cherry (Prunus cerasus L.) is a good source of anthocyanins showing antioxidant and anti‐inflammatory activities. Contemporary drying processes of fruits and vegetables should be fast and at relatively low drying temperatures to minimize the energy consumption and the thermal degradation of their nutritional composition. Therefore, sour cherries placed into sample bags were dipped in one of eight different pretreatment combinations consisting of chemical solutions (2% ethyl oleate, 2% K₂CO₃, 1% citric acid or water) and dipping temperature (23 or 60C) for 1 min to accelerate the skin moisture diffusivity by breaking down the waxy cuticular surface of sour cherry. Pretreated sour cherry samples were dehydrated by three different methods (artificial drying, solar drying and open sun drying). Sour cherries treated with 2% ethyl oleate solution at room temperature (23C) were the fastest to reach the final weight loss percentage (75%). They dried in 24 h in the laboratory tray dryer providing 45.2C drying air. The untreated samples reached the same weight loss percentage of 75% at a drying time of 92 h. A high dipping temperature (60C) eliminated the positive effect of ethyl oleate. Solar drying and open sun drying of sour cherries treated with 2% ethyl oleate at room temperature finished approximately in 149 h and 372 h, respectively. The color of dried sour cherries was used as a quality indicator. The increase of dehydration time decreased redness values as seen in solar drying and open sun drying. The recommended pretreatment and drying processes for sour cherries are dipping them into 2% ethyl oleate solution at room temperature for 1 min and then drying them artificially at 45.2C air temperature to effectively retain their original color.     

CHANGES IN QUALITY OF RICE (ORYZA SATIVA L.) CV. KHAO DAWK MALI 105 DURING STORAGE

Rice has been the main staple in Thailand for many centuries. Rice is also the main export produce of Thailand, with many varieties exported each year. Recently, the main focus of research has been the rice variety known as Khao Dawk Mali 105 (KDML 105) because of its aroma, with evidence showing this aroma could hold the key to rice stability which could extend its shelf life for longer storage and preservation. Because of the volatile nature of this aroma, we investigated how storage affects the quality of this rice by monitoring the amount of 2‐acetyl‐1‐pyrroline (2AP) over a period of 8 months. The pasting properties of KDML 105 following storage of up to 8 months showed a significant time effect at P ≤ 0.05. Breakdown viscosity was 51% after 8 months storage while setback viscosity and hardness increased during storage. The amount of 2AP decreased to 7% at 8 months while hardness increased proportionately. The number of high molecular weight protein bands increased as rice was aged for 5 and 8 months. In vitro digestibility of starch in fresh and 1‐year aged rice were not significantly different.     

INFLUENCE OF CUT AND PACKAGING FILM ON SENSORY QUALITY OF FRESH-CUT BUTTERHEAD LETTUCE (LACTUCA SATIVA L., CV. WANG)

ABSTRACT

The purpose of the present work was to study the influence of cut (manual and using a sharp knife) and packaging film (bioriented polypropylene and polyolefin PD‐961) on the sensory quality of fresh‐cut butterhead lettuce stored under passive modified atmosphere at 5C. Results showed that lettuce cut by hand had a lower respiration rate and deterioration rate than those cut with a knife. This difference could be attributed to a difference in the level of wounding, which can be explained by the fact that cutting by hand caused the tear to follow the naturally weaker areas of the lettuce, leading to less tissue damage than that engendered by the use of a sharp knife.

PRACTICAL APPLICATIONS

Results suggest that lettuce leaves cut by hand showed a lower sensory deterioration rate than those cut with a sharp knife. While not industrially applicable, the cutting of lettuce by hand might be appealing to small ventures such as buffets, restaurants and catering services. The sensory analysis carried out suggests that the use of polyolefin PD‐961 is preferred over bioriented polypropylene for packaging fresh‐cut lettuce, in view of achieving a better quality of the produce.     

PARTIAL PURIFICATION AND CHARACTERIZATION OF PECTIN METHYLESTERASE FROM ORANGE (CITRUS SINENSIS) CV. PERA-RIO

The enzyme pectin methylesterase (PME) from orange was extracted and partially purified by filtration on Sephadex G‐100. The extraction buffer for orange PME was borate–acetate containing 0.4 M NaCl. Orange PME showed optimum pH at 8.0 and optimum temperature at 50C. The PME enzyme was completely inactivated after 1 min of incubation at 90C. The specific activity increased in the presence of 0.15 M NaCl or 0.025 M Na ₂ SO ₄ , 0.10 M KCl, 0.025 M K ₂ SO ₄ , 0.05 and 0.1 M NH ₄ Cl. Lithium chloride and Li ₂ SO ₄ inhibited the enzymatic activity at all concentrations studied. The K_m and V_maxvalue of PME were 0.36 mg/mL and 5.26 µmol/mL‐mg protein, respectively.     

CHANGES IN PROXIMATE COMPOSITION OF THORNBACK RAY (RAJA CLAVATA, L. 1758) SURIMI DURING WASHING AND FROZEN STORAGE

The effects of frozen storage of surimi produced from thornback ray and the washing of mince on the chemical composition were investigated. The crude ash content which was initially found as 1.38% in raw thornback ray decreased approximately 12 and 80% after the first and second washing, respectively. After the third washing, crude ash content increased to 207% of the amount in the second washing because of addition of salt to the last washing water. The crude protein content of mince also decreased approximately to 28 and 20% after the first and second washing, respectively. After the third washing, the decrease in the lipid levels was approximately 30%. At the end of 6 months of frozen storage at − 23.8  ±  2C, dry matter, crude ash and crude protein contents increased in a greater ratio in surimi containing 4% sorbitol, 4% sucrose and 0.3% Na-tripolyphosphate than surimi prepared with 8% sorbitol and 0.3% Na-tripolyphosphate.

PRACTICAL APPLICATIONS

In the present study, the effects of frozen storage of surimi produced from thornback ray and the washing of mince on the chemical composition were investigated. Washing procedure significantly decreased crude ash, crude protein and crude fat content of mince. There were significant differences in moisture, crude ash, and crude protein contents during the 6 months storage period of frozen surimi obtained by using different cryoprotectant mixtures. The folding test scores were highest in fresh surimi and during the first two months of storage. Thornback ray can be used for the production of surimi.     

PURIFICATION AND KINETIC CHARACTERIZATION OF POLYPHENOL OXIDASE FROM TOMATO FRUITS (LYCOPERSICON ESCULENTUM CV. MUCHAMIEL)

Two different polyphenol oxidase (PPO) fractions, soluble and particulate, were purified from unripe tomato fruits (Lycopersicon esculentum M. cv. Muchamiel). The PPO present in the soluble fraction was purified fivefold with a 43.5% yield after ammonium sulfate fractionation. PPO in the particulate was purified 4.56‐fold with a 23% yield using the nonionic detergent Triton X‐114. A strong correlation between tomato fruit PPO activity and the physiological disorder blossom‐end rot (BER) was found, with a large increase of the PPO activity in the particulate fraction. Kinetic characterization, including kinetic parameters, pH and temperature profiles, substrate specificity and inhibitors showed similarities in both the soluble and the particulate enzyme(s). However, thermal stability of the particulate enzyme was significantly higher than stability of the soluble PPO, thus indicating possible structural differences. Cupric ions were activators, probably because of their ability to reactivate PPO partly denatured during purification.     

CHANGES IN THE CALPAIN-CALPASTATIN AND CATHEPSIN (B, B+L, H AND D) DURING POSTMORTEM STORAGE OF GOAT MUSCLES

The objective of this study was to elucidate a relationship between some endogenous proteinases and their inhibitors in four different goat muscles during postmortem storage. Samples were taken from the longissimus dorsi (LD), biceps femoris (BF), semimembranosus (SM) and semitendinosus (ST) muscles stored up to 20 days postmortem at 5C. Activities of calpain-I, calpain-II, calpastatin, cathepsins (B, B+L, H and D) and cystatin(s) were determined. Decreases in calpain-I and calpastatin activities were significantly more than that of calpain-II activity. The cathepsin B, B+L, H and cystatin level were found to fall by 9–35 % after 20 days, whereas the cathepsin D showed 11–17% decline in all the muscles. Thus changes in muscle proteinases and their inhibitors during postmortem storage differ and the results may shed light on their role in myofibrillar proteolysis and goat meat tenderization.