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Heat Application Processing Condition

Carrot is a vegetable with significant nutritional and medical value as well as health care benefits (Gong et al., 2015). Carrots are rich in sugar, vitamin, biological activity materials, and protein. Various processing methods can be employed on carrots to make them reach a certain standard of quality desired by consumers. In this, thermal processing is done with the purpose of destroying enzyme and microbial activity and making certain chemical and physical changes in the food items. Moreover, processing helps in converting unprocessed foods into transportable and shelf-stable products. The primary purpose of food processing is to ensure the safety, quality, and availability of perishable food items (MacDonald & Reitmeier, 2017). The purpose of this review paper is to discuss how carrots would react (physically, chemically, and sensorily) to heat application processing conditions, heat removal processing conditions, and ambient temperature processing conditions. For heat application processing, blanching has been used as an appropriate technique while for heat removal chilling has been selected. Size reduction was done for ambient temperature processing while storage of carrots at low temperature was deemed to be the best method to preserve the food’s qualities for a longer period of time. The paper will also discuss various deteriorative mechanisms that might occur during the processing and the causes behind those mechanisms. Finally, the paper will include a discussion of how the identified deteriorative mechanisms can be prevented using various control measures.

Thermal processing of foods is done with the purpose of destroying or reducing microbial and enzyme activity and producing certain chemical or physical changes in the food to make it reach a certain quality standard (Yilmaz & Gökmen, 2016). Numerous heat processing techniques (blanching, pasteurization, and sterilization) can be employed for this. For carrots, low-temperature blanching is the most appropriate method to obtain firm texture in the ingredient as it reduces the degree of methoxylation. Blanching is used as a pre-treatment before freezing, drying, and canning. It helps to inactivate enzymes that cause deterioration during storage. The effectiveness of blanching in the case of carrots can be judged by the peroxidase (POD) enzyme since this enzyme has high thermal stability.

If the carrot is blanched with combined ultrasound treatment and hot water, there can be a substantial decrease in yeast and mold that grows on a carrot. Studies have also found that blanched carrots have a higher content of β-carotene, antioxidant, and carotenoids in comparison to unblanched carrots. β-carotene primarily determines the nutritional quality of carrots. Vitamin C retention in microwave-blanched carrots is higher than the carrots blanched using hot water. As per (Xiao et al., 2017), hot water blanching for 4 minutes at 88°C in combination with 0.21% citric acid blanching is the most appropriate for carrot pre-treatment method to inactivate Salmonella.

Heat Removal Processing Condition

However, hot water blanching is characterized by the waste of water and loss of nutrients because of diffusion or leaching. Hence, microwave blanching is an alternative. Here, the nutrient loss is reduced and retention of ascorbic acid is also higher. Although microwave blanching improved nutritional quality, it does not improve carrot’s texture. To mitigate the negative effects of microwaves on the microstructure and texture of carrots, mild blanching is the most appropriate. Studies show that raw carrots have well-organized cells and an intact cell structure, mild microwave blanched carrots (60°C, 40 minutes) have a similar microstructure as raw ones while strong microwave blanched carrots (90°C, 1 minute) cause the disappearance of cell walls and cells melt together (Jaeger et al., 2016).  

Cell Structure of carrot under different conditions

Figure 1: Cell Structure of carrot under different conditions

 Source: (Xiao et al., 2017)

The heat removal processing of carrot can be done through chilling in which the temperature of the product will be reduced to be between 1 to 8°C. This helps in extending the shelf life of carrots by reducing microbiological and biochemical changes. Chilling would help to maintain carrot colour and will also have a stabilizing effect on β carotene with a reduction in enzyme-catalysed oxidation reactions.

Studies have shown that the firmness of carrots is better in chilling in comparison to freezing or freeze-chilling. Additionally, centrifugal drip loss is also significant in freezing and freeze-chilling. The impact of all three methods (chilling, freezing, and freeze-chilling) on the colour and sensory acceptability of carrots were found to be similar (Zhan et al., 2018).

The ambient temperature processing of carrots can be done through size reduction or comminution. This can be done by chopping the carrots into medium to small pieces. It is done by using compression, shearing, and impact. Size reduction will increase the surface-area-to-volume ratio of carrot which will ultimately increase the rate of cooling, drying, and heating. It will improve the extraction rate of the liquid component of carrots. Size reduction will improve the eating quality of carrots. It will also have a positive impact on the suitability of carrots for further processing.

At high temperatures, lipoxygenase enzyme activity can lead to enzymatic carotene damage. Hence, to prevent unwanted changes, low-temperature storage is the most suitable for carrots as this will ensure the minimum reduction in β carotene levels. The decrease of β carotene in carrots can be attributed to transpiration. As per studies, carrots stored at 5°C, 10°C, and 15°C show the highest retainment of beta carotene (Asgar, 2020).

  • Deteriorative Mechanism: Blanching invariably cause sensory and nutritional changes in the food. Blanching can easily damage tissue cells and cause protein denaturation and loss of the natural colour of the food (Lee, 2021). During hot-water blanching, there can be a loss of nutrients due to leaching. Blanching can have adverse effects on the texture, water-soluble contents (B vitamins and potassium), heat-sensitive nutrients (vitamin C), bioactivity, and quality of the product (Jabbar et al., 2014). To inhibit the side-effects of blanching, the process should be conducted at low temperatures. However, this would lead to an increase in the processing time.

Ambient Temperature Processing

Additionally, blanching can lead to cell death and metabolic and physical changes in food cells. Heat can also damage cytoplasmic and other membranes. This results in cell turgor loss (figure) 

 cell turgor loss

Figure 2

Source: (Fellows, 2009)

Cause: Degradation of thermally sensitive compounds and leaching of water-soluble nutrients can happen if bleaching temperature and time are not appropriate. Excessive blanching can lead to excessive softening and flavour loss in carrots. Vitamins, minerals, contents like flavour compounds, sugar, and proteins diffuse from the food into the water and lower the food quality.

  • Deteriorative Mechanism: During the process of chilling, exposure of carrots to temperatures below a certain critical limit can lead to chilling injury. Carrots are moderately susceptible to chilling injury (Aguiar, 2012). This condition causes off-flavours development, failure to ripen, superficial spots, increased decay, and external browning. In carrots, the chilling injury might cause uneven length-wise cracks and a blistered appearance. Additionally, the interior of carrots might become water-soaked and might darken upon thawing. Freeze damage in carrots occurs at a temperature of -1.4°C however, this can vary based on the sugar content of carrots.

Cause: Vegetables that grow in tropical and subtropical regions are more vulnerable to chilling injury (James & James, 2014). This happens because the tissue weakens (due to the inability to carry out normal metabolic processes at low temperatures) and leads to cellular dysfunctions. A safe time period and a particularly suitable chilling temperature are hard to predict since most vegetables show the symptoms of chilling injury only after they have been removed from cold storage and transferred to warm temperatures. 

  • Deteriorative Mechanism: Size reduction in carrots can lead to their degradation. It leads to the reduction of moisture content and degradation of β-carotene. Degradation reactions negatively impact the texture, colour, nutrient properties, and flavour of the food. Hence, when the degradation of β-carotene occurs in carrots, it leads to a reduction in the nutritive quality and flavour of the carrot.

Cause: The primary reason behind this degradation can be attributed to the size reduction in carrots. When carrots are sliced, the increased surface-area-to-volume ratio increases the rate of drying. The drying temperature has a significant impact on the moisture content of the carrot. Figure 2 shows that as the drying temperature increases, the moisture content decreases. β-carotene degradation also increases with temperature. This degradation is also caused by enzymes that occur naturally because of the damaged tissue or by oxidation and microbial activity.  

oxidation and microbial activity

Figure 3

 Source: (Demiray & Tulek, 2016)

  1. Microwave Blanching: It is an effective alternative to hot water blanching. Microwave blanching is emerging as a clean technology to be used for heat application processing of food. It involves zero wastewater production and a shorter heating time that facilitates lesser nutrient loss (Behera et al., 2017). Moreover, there is no ascorbic acid loss involved in microwave blanching which will help in better nutrient quality and colour outcomes in carrots. Hence, microwave blanching of carrots at optimal temperature is an appropriate technique for heat application processing to avoid leaching loss.

Studies have shown that there is little to no loss of sucrose, dry matter, and carotene in foods blanched through a microwave. For carrots, microwave blanching can contribute to higher nutritional retention in comparison to water blanching (Taranto et al., 2017). Various findings show that after microwave blanching and a 3 month of frozen storage, a significantly higher content of sugar, vitamin C, minerals, and dry matter was detected in carrots. Sugar content is important for the flavour of raw carrots because it enhances the sweetness of the ingredient. Microwave blanching also results in slightly higher carotene retention than conventional blanching methods.

Microwave blanching has many benefits such as high heating rates, volumetric heating, and short processing time. It also involves straight interaction between the food material and electromagnetic field for heating generation. Additionally, microwave blanching is energy efficient, requires a short start-up time, is easy to install, and is easy to clean up.

  1. Prevention of Chilling Injury: To mitigate the risk of chilling injury in carrots, various control measures can be taken. Firstly, there is temperature manipulation under which cold storage is done with a temperature that is high enough to avoid chilling injuries. Carrots should ideally be cooled to a temperature below 5°C within 24 hours of harvesting and for long-term storage, a temperature of 0°C with 95% humidity is optimal (Department of Primary Industries and Regional Development, 2016). Pre-storage temperature conditioning can also be used wherein the food is preconditioned to a temperature that is slightly above the critical chilling range. The food can be briefly exposed to warm temperatures to cause an interruption in chilling. This process is called intermittent warming.

Deteriorative Mechanism and Causes

Apart from temperature manipulation, controlled atmosphere (CA) storage is also an effective way of alleviating chilling injury. A controlled or modified atmosphere has proven to be useful in maintaining the quality of crops (Bodbodak & Moshfeghifar, 2016). As per a study, an atmosphere of 10% CO2 and 0.5% O2 reduces the RR (respiration rate) of carrot sticks, shreds, and slices at all temperatures. Reduced weight loss and decay were noticed and there was a suitable influence on storage quality when CA was used. In the same study, it was also found that carrots benefited from CA only when stored at 5°C and not when stored at 10°C. furthermore, (Lepse et al., 2014) in their study concluded that best results were obtained when carrots were stored at 7-13% O2 levels and 3-5% CO2 levels.

  1. Prevention of Degradation: In order to prevent the degradation of sliced carrots, preservatives can be utilized. These preservatives treatments are required because size reduction has no preservative effect. For the prevention of degradation, different acidic solutions can be used. This can preserve the quality of carrots and control the whiteness index of their surface. The whiteness in shredded carrots is caused by enzymatic reactions that happen after removing natural protection during minimal processing. In a study conducted on shredded carrots during storage at two different temperatures (4°C and 7°C), it was found that carrots that were dipped in 1.5% citric acid showed no colour variation at either temperature. Moreover, these carrots showed the least microbial charge post-processing and during storage at temperature 4°C. Additionally, it was found that carrots that were dipped in 0.5% citric acid along with 0.05% ascorbic acid and 0.05% calcium chloride showed lower POD and PAL activities during storage (Piscopo et al., 2019). Hence, dipping carrots in a 1.5% citric acid solution can help to prolong its shelf life to up to 14 days with storage temperature at 4°C.


Thermal processing of food reduces enzyme activity and improves the quality of food. Low-temperature blanching is an appropriate heat application technique for carrots. Blanched carrots have higher β-carotene, carotenoids, and antioxidants. Blanching also decreases the yeast and mold grown on carrots. It gives the carrot a firm texture. However, hot water blanching can cause nutrient loss due to leaching. It also leads to water waste. Hence, microwave blanching is an effective alternative. Microwave blanched carrots (blanched at mild temperature) have a well-organized cell structure.

Heat removal in carrots can be done through chilling wherein temperature is reduced to 0 to 8°C. This process helps to reduce the microbiological and biochemical changes. It also helps to retain carrot colour and stabilizes β-carotene. Chilling reduces enzyme-catalysed oxidation reactions and improves the firmness of carrots. However, chilling at temperatures less than a critical level can lead to chilling injury characterized by discolouration, failure to ripen, increased decay, cracks, and a blistered appearance in carrots. this can be prevented by temperature manipulation and a controlled atmosphere.

Ambient temperature processing can be done through size reduction. Size reduction increases the surface-to-volume-area ratio which increases the rate of cooling, drying, and heating. It enhances the suitability of carrots for further processes. Storing carrots at a high temperature can cause enzymatic carotene damage. Hence, low-temperature storage is most appropriate for carrots as it prevents unwanted changes and ensures minimum reduction in β-carotene. This can be prevented by using preservatives such as dipping sliced carrots in 1.5% citric acid which will help to increase the storage life of carrots to 14 days at 4°C. 

Control Measures for Quality Changes


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