This includes external factors as appearance size, shape, colour, gloss, and consistency , texture, and flavour; factors such as federal grade standards e. Members of the public complain to trading standards professionals, [ specify ] who submit complaint samples and also samples used to routinely monitor the food marketplace to public analysts. Public analysts carry out scientific analysis on the samples to determine whether the quality is of sufficient standard.
Food quality is an important food manufacturing requirement, because food consumers are susceptible to any form of contamination that may occur during the manufacturing process . Many consumers also rely on manufacturing and processing standards, particularly to know what ingredients are present, due to dietary, nutritional requirements kosher , halal , vegetarian , or medical conditions e. Besides ingredient quality, there are also sanitation requirements. It is important to ensure that the food processing environment is as clean as possible in order to produce the safest possible food for the consumer.
A recent example of poor sanitation recently has been the North American E. Food quality also deals with product traceability , e. It also deals with labeling issues to ensure there is correct ingredient and nutritional information. There are many existing international quality institutes testing food products in order to indicate to all consumers which are higher quality products.
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Founded in in Brussels , The international Monde Selection quality award is the oldest  in evaluating food quality. In short, the judgements are based on the following areas: taste, health, convenience, labelling, packaging, environmental friendliness and innovation. From Wikipedia, the free encyclopedia. The examples and perspective in this article may not represent a worldwide view of the subject. You may improve this article , discuss the issue on the talk page , or create a new article , as appropriate. April Learn how and when to remove this template message.
Retrieved Retrieved 15 April During the active drying period when MW is on , moisture from the food surface is evaporated and carried away, while in the tempering period when MW is off the temperature and moisture are redistributed from the high- to low-concentration regions. Due to the moisture gradient, moisture still migrates from the interior region to the surface in the tempering period of the drying cycle.
This allows the high drying rate to be maintained for the next active cycle. This cycle of cooling and rewetting the sample during the tempering period prevents samples from overheating and quality deterioration.
Overheating can also be avoided by controlling the MW supply to the sample allowing the energy to be wisely ultilised, and therefore, the sample is heated within the safe region ensuring the food quality. Currently, despite the considerable potential of IMCD, the impact of IMCD on food quality has not been extensively examined 2 , 18 , 19 , particularly no study has investigated the effect of IMCD on the nutritional quality attributes of kiwifruit.
It is an abundant source of chlorophyll, actinidin, total polyphenol, vitamin E and vitamin C with a high level of antioxidant capacity while containing no fat or cholesterol Because of its abundant health benefits, a growing interest is being observed in the impact of thermal dehydration on the properties of these compounds and their relation to other quality characteristics. Kiwifruit is one of the highly perishable foods with seasonal availability; many preservation methods have been attempted to prolong their storage life and make this healthy product commercially available to consumers 21 — IMCD drying could be one potential dehydration method able to extend kiwifruits shelf life and add value by efficiently reducing moisture content, hence inhibiting microbial growth and deteriorative chemical and enzymatic reactions Kiwifruit has been investigated for convective drying by several researchers 21 , 27 — Similarly, changes in colour 25 , 31 , 32 and physical properties 33 , 34 of hot air convective dried kiwifruit have been published.
The influence of conventional drying conditions on bioactive compounds in fresh and dried kiwifruit has also been examined by Tian et al. However, no study exits of the impact of IMCD conditions on the drying characteristics and quality attributes of kiwifruit. It is especially challenging to produce high-quality dried kiwifruit as the fresh kiwi contains high amount of thermal-sensitive elements while intensive heating treatments generally have detrimental effects on the bioactive compounds, colour, water activity and microstructure of the product. Therefore, a comprehensive research is required to obtain an appropriate drying method to ensure the quality of nutrition-rich fruits like kiwifruit.
In this context, the primary aim of this study is to investigate the quality characteristic change of kiwifruits under different IMCD conditions. The retention of AAC and total polyphenol content, water activity, colour degradation and microstructure modification were investigated under different IMCD PRs, taking convective air-dried sample as a reference. They were carefully chosen to be identical in shape, size, firmness and ripeness.
The kiwifruits taken from the laboratory fridge were washed with distilled water to remove residue and dirt and then allowed to reach room temperature before conducting each drying experiment. The initial moisture content of the fresh kiwi slices was estimated to be approximately The kiwifruit skin was peeled, and then slices of 50 mm diameter and 7mm thickness were made by cutting perpendicularly to the main core fruit axis using a food slicer.
One slice of kiwifruit was dried in each IMCD condition and three replicates were carried out. The inverter MW oven provides constantly true power transmission at the setting values. The kiwifruit slice was placed in the centre of the MW cavity, for an even absorption of MW energy. The moisture loss was recorded at regular intervals at the end of power-off times by placing the sample slice on the digital balance. Once the kiwifruit had reached a moisture content of approximately Sample temperature was regularly checked during the drying process by aFlir E5 thermal imaging camera to record the maximum sample temperature.
Here t on was the MW on time in seconds and t off was the MW off time in seconds. An independent hot air convective drying experiment was also conducted to compare with the results from IMCD, which had the same experimental conditions as in IMCD method. The decrease in the moisture ratio MR with the drying time was used to examine the experimental data.
The MR denotes the remaining moisture in the kiwifruit samples in relation to the initial moisture content, which can be calculated by Equation 1. Where M is the moisture content of the material on a dry basis, M e is the equilibrium moisture content, and M o is the initial moisture content. The dried samples of different drying conditions were kept in a desiccant chamber until the sample temperature reach the room temperature before placing in the sample cup of the water activity meter. The water activity measurements of dried product were performed in triplicate.
The homogenised samples were vortexed, then filtered through a Whatman no. The reverse-phase separation was obtained using a Waters Symmetry C18 column 4. The flow rate was 0. Supernatants of the extracted samples were filtered through a 0. The retention time of AAC peak was archived at 2. A 5-point standard curve was established to calculate the ascorbic content of samples. All samples were tested in triplicate. The amount of total polyphenols in the samples was determined by Folin—Ciocalteau method 38 with some modification. The homogenised samples were shaken and then allowed to settle for 1 h at ambient temperature.
Extracts were centrifuged at 10, g for 15 min at 20 o C. The supernatant added with 5 mL extractant was filtered through a Whatman no. Extraction processes were repeated three times to get reliable data, and the extracts were diluted 10 times with distilled water. A standard curve with gallic acid standard solutions was established.
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Fresh samples were scanned under low vacuum conditions 40 Pa , LVSTD detector, at an accelerating voltage of 10 kV; while dried samples were scanned under high vacuum mode, 10kV voltage, SE detector. Digital images were obtained using 8. These images were stored in the bitmap graphic format. RGB-triplets for every pixel in the image represent the intensities of RGB colours in the range 0— Before processing of the sample images, the preprocessing was carried out based on a method proposed by Sharifian et al.
Colour measurement was performed in triplicate in each drying condition to determine hue angle and colour change 6. The hue angle h o is defined as:. Colour changes were defined as in Equation 4 :. The investigated characteristics were independently performed in triplicate. It can be seen that the processing times required to attain the same final moisture content were different at different drying conditions PRs.
The drying time required to reach the moisture content of The results demonstrate that IMCD can significantly reduce the drying time compared to convective drying. The drying rate was significantly high at the beginning of the IMCD process as the initial available moisture content in the samples was high. The drying rate was then gradually reduced towards the end of the drying process. It can be concluded that in the early drying stage, the MW energy was mainly absorbed by water near the sample surface, the free and loosely bound water of the sample, which was easily removed It is clear from Fig.
Because of the volumetric heating characteristics of MW, the MW energy directly penetrated inside the sample and excited water inside the sample. At the same time, the sample was gradually heated from the outer surface of the sample by the hot air Kumar et al. The moisture outward flux was eventually increased, and drying rate increased considerably. Another essential quality indicator measured in this study was water activity, which allowed determining the product stability and safety Water activity characteristics determine many chemical or enzyme reaction and biochemical processes, which are important for the control of food product safety and quality.
The results of water activity measurement in fresh and dried kiwifruits are presented in Table 1. The fresh material was characterised by the average water activity of 0. According to Sablani et al.
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Moreover, a change in water activity affects the relative speed of chemical, enzymatic and biological reaction 42 , This can be seen in Table 1 ; the water activity of all dried samples was less than 0. However, the highest a w value was obtained in convective drying 0. In drying pumpkin slices, Junqueira et al. Therefore, the results demonstrated that the intensity of MW heating takes a significant part in the rate of bound water removal from the sample, which is considered as the most time-consuming process and ineffective energy use in conventional hot air drying.
The result suggests that MW effectively reduced the sample water activity to the optimum stability level. Attaining low water activity at desired moisture level helped to extend product shelf life while maintaining its expected quality characteristics. Identical superscript letters indicate no significant difference.
Nutritional quality of heat-sensitive food materials in intermittent microwave convective drying
AAC is one of the most thermo-labile bioactive compounds in plant-based foods. Therefore, the thermal drying processes should be carried out in a way that ensures the highest preservation of AAC in the dried food products. It can be seen that drying conditions greatly affected the retention of AAC and its content decreased in all drying conditions in comparison with the fresh samples. This phenomenon could be due to the thermal destruction during drying and the exposure of the samples to the drying medium. The degradative chemical reactions of natural bioactive compounds are catalysed by heat and ascorbate oxidase enzyme released from disrupted cell membranes during the drying process.
The average total AAC found in fresh kiwifruit was The temperatures of samples were found fluctuated during each IMCD cycle, which was at peak point at the end of the MV heating period and lowest point at the end of the tempering period. The higher frequency of MV radiation PR increased the sample temperature during IMCD, which generally degrades the heat-sensitive bioactive compounds within the sample. This result affirms the thermal sensitivity of AAC at intensive heating conditions and lengthy drying process of the samples.
It is also noted that the total drying time is significantly longer in convective drying. Ascorbic acid content of fresh and dried sample slices under different PRs , , , and convective drying. Identical letters indicate no significant difference. The retention of total polyphenol content of the fresh and dried kiwi slices is shown in Fig. The average concentration of the total polyphenol in fresh kiwifruits was 4.
At the end of the drying process, the highest loss of In spite of the fact that the drying condition in PR was more heat-intensive than PR , it is interesting to observe the highest polyphenol retention at PR 3. Also, the MW radiation at PR might be sufficient to release the bound polyphenols by breaking the cellular food matrix for better extraction in chemical analysis. The discharge of the oxidative and hydrolytic enzymes from the collapsed food cells can decompose long-chain polyphenols into simple phenolic compounds.
Also, newly formed phenolic compounds were the result of the complex heat catalysed chemical reaction of the released enzymes and bioactive compounds. In addition, more phenolic products can be produced as the result of Maillard reaction under PR In contrast, thermal destruction by intensive MW heating in PR diminished polyphenols and longer drying time in PR and convective drying gained sufficient time to facilitate oxidative deterioration of polyphenols 47 — The obtained results also highlighted the advantages of combining hot air and intermittent MV to enhance extraction efficiency of the bioactive compounds due to its combined effect on the cell membrane.
Moreover, the elevated temperature induced by MW radiation at suitable PR can reduce the degradation of polyphenols by inactivating the polyphenol oxidase, lipoxygenase and peroxidase enzymes released from the collapsed tissue Many published articles 51 — 53 reported that MW drying retains high total polyphenol. Overall, compared to fresh samples, all the dried samples retained high level of phenolic contents.
The lowest phenolic retention was approximately a third from fresh sample observed in IMCD at PR as well as in the case of convective drying. Total polyphenol content of fresh and dried sample slices under different PRs , , , , and convective drying. As MW penetrating deep into the samples causes volumetric heating and rapid moisture evaporation, it is suspected that significant microstructural changes may take place during IMCD.
However, no investigation on microstructural changes in IMCD can be found in the literature. Illustration of the microstructure of fresh and dried samples derived from different drying conditions has been presented in Fig. A limited number of pores and cell opening can be observed. The hot air drying induced slow water migration with high turgor reduction, structural shrinkage and collapse, whereas hot air drying coupled with periodic MW heating provided a more porous structure compared to the purely convective drying method as depicted in Fig. Microstructure of sample slice in fresh condition and dried state under different drying conditions.
The MW reduces the drying time due to rapid moisture evaporation from inside the material. However, cell collapse can also be observed due to overheated regions resulted from uneven heating during MW heating at higher PR. This incident is observed from Fig. It is also noted that there is swelling, loss of stability and disappearance of many cell walls of kiwifruit structure in the convective dried sample as well as IMCD samples at high PRs. It can be explained that the kiwifruit cells are composed of a small amount of cellulose, and a high amount of pectic polysaccharides 54 , which were degraded significantly during severe thermal stresses in hot air convective and higher PRs in IMCD.
IMCD at PR obtained a porous structure resembling the structure of the fresh sample, less shrinkage and collapses Fig. Tian et al. The modification of sample microstructure might also affect the other sensor properties, for example, hardness and crispiness. Highly porous structure obtained at PR and PR tends to produce crunchy and crispy products, while dense and collapsed structure in dried products at PR and PR usually results in chewy and hard products.
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During drying, sample colour can be considerably affected by pigment degradation 55 , 56 , enzymatic activity 57 , and Maillard nonenzymatic browning reaction The visual appearance, average RGB values along with total colour changes altered by different drying conditions are presented in Table 1.
It is clearly seen that higher PR caused more colour degradation in the sample. Similar to the change of nutrient content, sample colour was also negatively influenced by higher PR because of the accelerated colour degradation reaction occurred within the sample due to higher MW irradiation. The application of MW heating brought about the rapid colour change of the samples, caused by intense thermal effect and browning reaction during drying.
It is also noted that extended drying time in convective drying significantly degraded the colour of the dried product. From Table 2 , it can be stated that a higher PR led to higher colour changes in the sample. In addition to colour changes, the hue angle of the dried samples in convective drying, as well as IMCD, was considerably different from the fresh sample as shown in Fig. Reducing the PR further may result in less degradation of colour closer to the hue angle of the fresh sample. However, it will increase the total drying time, which might also negatively affect colour as extended time exposure to drying environment facilitated browning reaction as in the case of convective drying.
Therefore, PR can be considered the best operating condition for the best colour, AAC retention and achievement of shorter drying time. Average hue angle of the dried sample under different PRs , , , , and convective drying. Identical superscript letters above the bars indicate no significant difference.
The product qualities, as described in the above sections, decrease with the increase of PR due to higher temperature induced by intensive MW heating. Although some reports in the literature claimed that decreasing PR might provide better food quality, it would prolong the drying time and increase the overall energy consumption 3 , 62 , Moreover, as demonstrated above, prolonged drying time can cause undesirable effects on the quality of the samples. In the experiments of IMCD, the tempering period supported the redistribution of the temperature and moisture, especially at lower PR as it allowed sufficient time to even out the temperature and moisture difference in the sample Effect of the operating variable e.
In the IMCD experiments, the drying time was significantly reduced in comparison with conventional hot air drying process. The water activity of all dried samples was attained at a safe level for extended storage. Thus, the microbial growth has been inhibited, and degradation of chemical reactions has been retarded.
In addition, water activity decreased with the increase in PR of IMCD resulting in safer products with a longer shelf life. It was found that the IMCD method was substantially more efficient than convective drying as it significantly decreased the drying time and enhanced the product nutrient and colour quality with a porous microstructure. In addition, the highly porous microstructure of IMCD samples suggests better texture compared to hot air dried product.
However, further lowering PR might not guarantee better quality product e. PR attain lower total polyphenol content than at PR and efficient drying performance due to prolonged exposure of the products to the drying environment. Therefore, the PR should be carefully chosen based on product quality, MW power and drying performance to achieve optimal drying conditions.
Authors also acknowledge the contribution of Zachary Welsh and Dr.
Chandan Kumar for their assistance with the manuscript proofreading. The authors have not received any funding or benefits from industry or elsewhere to conduct this study. Karim is the leader of the project, developed main research plan and contributed to drafting the paper. National Center for Biotechnology Information , U. Journal List Food Nutr Res v. Food Nutr Res. Published online Oct Nghia Duc Pham , 1, 2 W.
Martens , 1 M. Joardder 1. Author information Article notes Copyright and License information Disclaimer. Email: ua. Abstract Background The retention of health promoting components in nutrient-rich dried food is significantly affected by the dehydration method. Objective In this study, natural bioactive compounds ascorbic acid and total polyphenol , water activity, colour and microstructure modifications which can occur in IMCD were investigated, taking kiwifruit as a sample. Results and Discussion The microwave MW power ratio PR had significant impact on different quality attributes of dried samples.
Conclusion Overall, IMCD significantly improved the drying performance and product quality compared to traditional convective drying. Keywords: intermittent microwave convective drying, Kiwifruit, microstructure, ascorbic acid, Polyphenol, colour analysis, nutritional quality, drying characteristics. Open in a separate window.