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Journal of Food Science and Technology logoLink to Journal of Food Science and Technology
. 2022 Jan 22;60(4):1209–1221. doi: 10.1007/s13197-022-05355-x

Khoa and khoa based traditional dairy products: preparation, spoilage and shelf life extension

Richa Badola 1, Writdhama Prasad 1,✉, Narender Raju Panjagari 1, R R B Singh 1, Ashish Kumar Singh 1, Shaik Abdul Hussain 1
PMCID: PMC10020399  PMID: 36936112

Abstract

Khoa and khoa based products (burfi, peda, kalakand, milk cake, etc.) are a category of traditional dairy products of Indian subcontinent. They are prepared by open pan desiccation along with stirring and scraping of milk to the desired consistency, followed by addition of sugar and / or colour and flavoring ingredients. The peculiar sensory attributes developed during their course of preparation makes them unique, but their short shelf-life is a major challenge faced by the dairy industries. They are spoiled mainly because of yeast and mold growth along with detrimental changes in the sensory attributes. This review describes various preservation techniques explored in the last two decades such as packaging interventions, modified atmospheric and active packaging, chemical preservation, water activity modification, natural preservation, thermal treatments, bio-preservation, etc. which can be used either singly or in combination (hurdle technology), to enhance the shelf life of these milk products.

Keywords: Khoa and khoa based products, Shelf life extension, Packaging, Preservation, Hurdle technology

Introduction

Milk plays a significant role as a source of animal protein and other precious milk solids (i.e., calcium, lactose, milk fat, bioactive substances etc.) in the Indian diet which predominantly consist of vegetarians. But owing to high ambient temperature conditions prevailing in India, a large proportion of the milk produced is converted into more stable products for conservation of its nutritional goodness. Hence, ethnic dairy foods (commonly termed as traditional or indigenous Indian milk products) were developed utilizing locally available equipments, utensils and manufacturing procedures. Traditional milk products have distinct advantages of being value added products, high mass appeal, high profit margins, high export potential and providing substantial employment opportunity especially in rural areas. Because of inherent milk nutrients and new nutrients generated during processing, these products provides therapeutic health benefits such as osteoporosis prevention, reduced heart diseases, immunity enhancement, etc. There is an increasing demand for these products in India as well as in global market (Shariati et al. 2020; Pahwa et al. 2020; Ghafarloo et al. 2020; Kumar et al. 2021; Jouki et al. 2021).

India boasts of a variety of traditional milk-based confections to pamper the sweet toothed. They have a well-established position in the Indian diet and during festive seasons owing to their delicacy and nutritional attributes. A whole array of milk-based delicacies tastefully overcomes the basic limitation of milk—its highly perishable nature. India’s milk production stands at 187.35 million tons in 2018–19 as per Press Information Bureau, Ministry of Animal Husbandry, Dairying and Fisheries, Govt. of India and about 55% of the milk produced is used for product manufacture and 45% is consumed as liquid milk (Anonymous 2019). Out of 55% milk used for different products preparation, about 40% is used for manufacture of traditional Indian dairy products such as ghee, khoa, burfi, peda, rasogolla, paneer, etc. As per the available information, about 1.5 million tonnes of khoa is produced annually in India, which is worth at least Rs 18,000 crore (Indiatimes 2021).

Khoa is an important traditional dairy product which is used as a base material for preparation of wide varieties of products, viz., peda, burfi, kalakand, milk cake, gulabjamun, pantua, etc. Commonly these products are termed as khoa based products. Khoa is prepared by continuous desiccation of milk in an open pan until a desirable consistency is achieved. It has been observed that about 1 kg of khoa is produced from 5 kg of milk (Aneja et al. 2002). However, easily available nutrients and high water activity render these products easily spoiled by microbial proliferation and enzymatic activity. They have a shelf life of about 3–5 days at ambient temperature and 7–10 days at refrigerated temperature. This limits the transportation of khoa to longer distances. Although sugar addition during different products preparation (like burfi, peda, etc.) decreases the water activity but this does not result into significant improvement in the shelf life of these products in comparison to khoa. This has led to research on increasing the shelf life of khoa and khoa based products by exploring different approaches, viz., application of chemical and natural preservatives, packaging interventions, post production thermal and irradiation treatment, combined factor preservation, etc. Increase in the shelf life of khoa and khoa based products has been reported upon application of these strategies. This review deals with all the technological interventions undertaken till date to increase the shelf life of khoa and khoa based products. This information will be useful to dairy industries and research scholars who are working on shelf life extension of these products.

Khoa

Khoa, also termed as mawa or khoya, is an important traditional dairy product prepared at atmospheric pressure by thermal desiccation of milk to desired total solids, preferably 55–65%, so that the final product has a semi-solid like consistency. Buffalo milk is preferably used for khoa preparation because of higher total solids content, white color and sweet taste. A superior quality khoa is characterized by uniform white with tinge of brown colour, a mildly cooked flavor and uniform granular texture. Since khoa is prepared by desiccation of moisture from milk, it has higher nutritive value (about 458 KCal/100 g). Further, concentration of milk solid also makes it a rich source of proteins, minerals (specifically calcium), fat and lactose.

Classification of khoa

Khoa is classified into three different types, viz., dhap, danedar and pindi. Each of the khoa types has different end use and has slight differences in the preparation method, composition and sensory attributes. Legal requirements for khoa are provided in Table 1, chemical composition of the three types of khoa as per Bureau of Indian Standards (BIS 1980) is provided in Tables 2 and 3 provides the microbiological requirements of khoa as per FSSAI (Food Safety and Standards Authority of India, an autonomous organization under the Indian government which deals with food quality and safety). Classification and utilization of khoa for different products preparation is provided in Fig. 1.

Table 1.

FSSR requirements for khoa (FSSR 2011)

Parameter Khoa
Total solids, minimum, %, (m/m) 55
Milk fat, minimum, %, (m/m), dry matter basis 30
Total ash, maximum, %, (m/m) 6
Titratable acidity (as % lactic acid), maximum, % 0.9
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Table 2.

BIS requirements for khoa (BIS: 4883, 1980)

Characteristics Requirement for type
Pindi Danedar Dhap
Total solids (percent by mass min.) 65 60 55
Fat (percent by mass (on dry matter) min.) 37 37 37
Total ash (percent by mass (on dry basis) max.) 6.0 6.0 6.0
Titratable acidity (as lactic acid, percent by mass (on dry matter) max.) 0.8 0.9 0.6
Coliform count (per gm max.) 90 90 90
Yeast and mould count (per gm max.) 50 50 50
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Table 3.

Microbiological requirements of khoa/ khoa based sweets – food safety criteria (FSSR 2011)

Microbiological parameters Sampling plan Limit (cfu)
N C M M
Aerobic Plate Count 5 3 2.5 × 104/g 7.5 × 104/g
Coliform Count 5 2 50/g 1 × 102/g
Yeast and mold count 5 3 10/g 50/g
Staphylococcus aureus (Coagulase positive) 5 3 10/g 1 × 102/g
Escherichia coli 5 0  < 10/g NA
Salmonella sp. 5 0 Absent/ 25 g NA
Listeria monocytogenes 5 0 Absent/ g NA
Sulphite Reducing Clostridia NA NA NA NA
Enterobactersakazakii NA NA NA NA
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Where, NA- Not Applicable n = Number of units comprising a sample c = Maximum allowable number of units having microbiological counts above m for 2-class sampling plan and between m and M for 3-class sampling plan. m = Microbiological limit that separates unsatisfactory from satisfactory in a 2- class sampling plan or acceptable from satisfactory in a 3-class sampling plan M = Microbiological limit that separates unsatisfactory from satisfactory in a 3-class sampling plan. m = represents an acceptable level and values above it are marginally acceptable in terms of the sampling plan

Fig. 1.

Fig. 1

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Classification of khoa and utilization for products preparation

Pindi type of khoa is prepared by desiccating higher amounts of moisture during its preparation and it contains the highest amount of total solids among all the khoa variants. It has very fine and dry grains. Presence of burnt particles or any sign of intense heat treatment is highly objectionable in this variety of khoa. It is primarily used for the preparation of burfi, peda and other sweets which have relatively lower moisture content. Danedar khoa has a distinctively intense granular texture and uneven body. It is prepared by adding a small amount of acidulant (usually citric acid at 0.02% level) solution during the initial stages of khoa preparation, when the milk starts to boil. Grain size depends upon the amount of coagulant used, type of milk and its quality. Higher amount of acid addition and milk having higher initial acidity will have larger grains because of the fact that higher acidity results into early onset of coagulation of milk proteins, which grows in size as the milk concentrates and takes the form of a consolidated mass (i.e., khoa). At similar initial acidity, buffalo milk khoa has larger grains as compared to cow milk khoa because of higher calcium content in the former, that tends to destabilize milk proteins during thermal processing because of its bridging / cross-linking activity. This type of khoa is suitable for the preparation of products which have granular texture, for example kalakand, milk cake, etc. Dhap (also called as Kaccha mawa) khoa has a loose and sticky body because of higher moisture content in it. Higher moisture content enables its easy and thorough mixing of other ingredients. This type of khoa is preferred for preparation of gulabjamun, kalajamun, pantua, carrot halwa etc. (Aneja et al. 2002).

A brief about preparation of khoa and khoa based products

Khoa is prepared by large dairies and small scale dairy sweet makers. In both the cases, the principle of khoa preparation remains the same (Fig. 2), but has end products of slightly different quality (Aneja et al. 2002). At the small scale, khoa is prepared by transferring milk (preferably buffalo milk) to a kettle. The milk is then heated by a non-smoky heating source located at the bottom of the pan. While the milk boils and concentrates, the contents are regularly stirred and scrapped to maintain uniform temperature of milk and prevent burning of particles by sticking at the kettle surface. After the milk reaches 2.5 to 3 times concentration, its consistency changes and starts to stick more frequently at the kettle surface. At this point, heating intensity is lowered and more vigorous scraping is done. This results in maintaining a uniform consistency of the concentrated milk solids and also breaks the fat globule membrane. Breaking of fat globule membrane results in release of fat from the globules and it starts to cover the entire milk solids content. After some time, the milk solids tend to fall off from the kettle surface which is because of the fat layering over them. This also indicates about an end of the heating process. The contents are then transferred and spread on vessels for rapid cooling. Vessels having wide surfaces are preferred because of the fact that increasing surface area results into higher heat transfer and subsequent cooling. Further, surface of these vessels are often greased using ghee (Indian counterpart of butter oil) in order to avoid sticking of khoa to the vessel surface. During the process of thermal concentration of milk solids, heat denaturation of milk proteins (particularly whey proteins) occur which results in development of characteristic cooked flavor in the product (Al-Attabi et al. 2009). Further, the vigorous stirring during later stages exerts an appreciable homogenizing effect and the contents become uniform in structure, i.e., the coagulated milk proteins particles attains similar size, fat is distributed throughout the mass and remaining moisture is dispersed as fine droplets.

Fig. 2.

Fig. 2

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Flow diagram for khoa preparation

At the industrial scale, equipments like inclined scraped surface heat exchanger (SSHE), multiple stage SSHE, conical process vat, etc. are used. Application of these equipment results in an improvement in the microbiological quality of khoa as the human contact is avoided but this also results in changes in the sensory quality of the product. The products prepared at industrial scale using different equipment have a mild pasty body and thus the product prepared at the small scale by skilled manpower is preferred over those prepared at large dairies using different equipment. However, the difference in sensory attributes is not much significant and the product prepared at industrial scale is preferred primarily because of their hygienic conditions of preparation with minimal human contact. For the same reason, khoa prepared at industrial scale has a higher shelf life as compared to the one prepared at small scale using human skills. Preparation of khoa based products follows the same protocol as that of khoa with minor modifications. During the product preparation, sugar is added @ 6–8% (of milk) or 30% (of khoa) along with coloring and flavoring ingredients during the later stage of boiling so as to minimize the effect of heat on the sensory characteristics of these ingredients (Aneja et al. 2002).

Microbiological quality of khoa and khoa based products

The microbiological quality of the khoa and khoa based products is well defined by FSSR (2011) as mentioned in Table 3. Studies conducted on microbiological attributes in khoa and khoa based products indicated about the presence of microorganisms such as Staphylococcus aureus, Bacillus cereus, Bacillus subtilis, Penicillium aeroginosa, Salmonella schottmuelleri, Streptococci species and coliforms (Godbole et al. 2013). Bhatnagar et al. (2007) studied the microbial load in the khoa samples sold in Madhya Pradesh (India) and reported about the presence of Staphylococcus spp. and Streptococcus spp. in the 1.3 × 104 to 2.1 × 106 cfu/g range. Also, the authors reported the presence of bacterial species like Salmonella, Staphylococcus, Proteus, Enterobacter, Klebsiella, Pseudomonas and Streptococcus. Karthikeyan and Pandiyan (2013) compared the microbiological load in khoa and khoa based products (burfi, gulabjamun, kalakand and peda) collected from different sources from Tamil Nadu (India) and reported that the microbial load was highest in the samples procured from local vendors. The authors also reported about the presence of Aspergillus flavus and Aspergillus fumigatus in the khoa samples. Aggarwal et al. (2019) studied the changes in microbiological attributes of laboratory prepared khoa samples from lactose hydrolyzed milk during refrigerated storage. The authors observed significantly not different (p > 0.05) results between the microbial load of control and experimental samples and reported that the total plate count at the end of study were 4.865 and 4.590 log10cfu/g for control and experimental sample, respectively. Similarly, the yeast and mold count at the end of storage study were 2.30 and 1.72 log10cfu/g for control and experimental sample, respectively). Singh et al. (2005) reported that fresh burfi samples had 2.33 × 105 and 106 (per gram) total plate, and yeast and mould count, respectively. Presence of Staphylococcus aureus, Escherichia coli and Klebsiella spp., was also reported in burfi samples collected from Pakistan (Farzana et al. 2009). Tambekar and Bhutda (2010) studied the microbial quality of 50 peda samples sold in Amarawati (India) and reported about the presence of Proteus vulgaris, P. aeruginosa, S. aureus, Salmonella typhi, E. coli, Ent. aerogenes and Shigella flexeneri. All these reports indicate about the short shelf life of khoa and khoa based products because of the high microbial load which results in rapid spoilage in the product’s quality and also poses a threat to safety of the consumers.

Spoilage and storage changes in khoa and khoa based products

Presence of easily available nutrients, high water activity and fat in free (not encapsulated) form are the major factors inherent to khoa which are responsible for the limited shelf life of khoa and khoa based products. At the small retailer, khoa remains stable for only 2–3 days at ambient temperature and for one week at refrigerated temperature. Acharya and Agrawal (2010) reported that shelf life of khoa depends primarily on the type and load of micro-organisms present in the raw milk, hygienic conditions maintained during khoa preparation, moisture content of khoa, type and method of packaging, and storage duration and temperature. Although, keeping khoa at low temperature (5–10 °C) decreases the microbiological proliferation and associated changes but the body and textural attributes of khoa are severely affected at low temperatures that make it less suitable for khoa based sweets preparation. Also, despite severe heat treatments given during manufacture of khoa based milk products, post-production microbial growth is one of the main reasons for their spoilage. Microbial counts of burfi have been reported to be less than that of khoa from which it is made due to the preservative action of sugar and heat treatment given during blending (Aneja et al. 2002).

Mold growth at the surface and hardening of the product are the most common defects encountered in spoilage during storage under ambient conditions besides lipid oxidation and staleness. Choudhary et al. (2019) studied the changes occurring in khoa during storage. The authors reported that with an increase in the storage period, titratable acidity, microbiological counts, lipid oxidation, browning and proteolysis increased in the samples. Increase in the microbial counts was attributed to the easy availability of nutrients along with growth supporting conditions like high water activity (0.97–0.99). Proliferation of microflora resulted into conversion of lactose into glucose, galactose and lactic acid. Lactic acid production resulted in a corresponding increase in the titratable acidity. Glucose and galactose, which were generated as a result of microbial growth, caused a decrease in the lightness value via Mallard’s browning by interacting with amino acids. Increase in oxidation was attributed to the presence of oxygen in the headspace of the product and higher level of iron (around 100 ppm) in the product. In addition, microbial growth also resulted into production of oxidative enzymes. Progression of oxidation resulted in the development of bitter flavor in the product. Bitterness was also attributed to the proteolysis and rancidity in the product, which occurred as a result of the activity of lipase and proteolytic enzymes. Although addition of sugar, during the preparation of different khoa based sweetmeats, results in an increase in the storage stability by decreasing the water activity, the spoilage in these products occurs in a similar manner. A brief about different spoilage pathways is provided in Fig. 3. At refrigerated temperatures, the microbial proliferation is decreased but the product becomes increasingly dry after 15 days of storage because of migration of moisture to the surrounding air (Prasad et al. 2017b; Kumbhare et al., 2021; Meena et al. 2021a).

Fig. 3.

Fig. 3

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Spoilage pathways in khoa and khoa based products

Strategies for shelf life extension of khoa and khoa based products

From the previous section, it could be observed that shelf life of khoa and khoa based products is less because of various physico-chemical and microbiological factors. Huge demand of these products, both in the Indian subcontinent and abroad, has resulted in exploitation of different approaches like chemical additives, packaging intervention, application of natural preservatives, modification in water activity, etc. for shelf life extension of khoa and khoa based products. Increasing the shelf life will facilitate long distance transportation; improve quality and functionality of these products. Various research works performed in the last two decades using these preservation techniques and their findings are provided hereunder.

Chemical preservatives and antioxidants

These are the compounds added to products with an intention to delay deteriorative changes, by decreasing the microbiological proliferation and inhibiting the activity of free radicals to cause oxidation. Their effect is associated with their ability to alter the permeability of cell membrane, cause cellular damage or reduce the free radicals. Various chemical preservatives have been used over the decades to increase the shelf life of khoa and khoa based products such as propyl gallate and dodecyl gallate, sorbic acid and its salts (potassium sorbate, calcium sorbate, etc.), benzoic acid and its salts (sodium benzoate and potassium benzoate), antioxidants etc. (Table 4). These could be either added during the last stage of their manufacture (Lodh et al. 2018) or sprinkled over the surface (particularly anti-fungal agents to delay the fungal growth) or added into the packaging material which will be in the contact of product during the entire storage period. Sarkar et al. (2002) reported addition of sodium and potassium metabisulphites (1000 ppm on the basis of milk) increased the shelf life of cow milk burfi at both ambient (30 °C) and refrigerated (7 °C) storage temperatures to 25 and 60 days, respectively. Palit and Pal (2005) observed that addition of cardamom and potassium sorbate, each at the rate of 0.1% of khoa (w/w) increased the shelf life of burfi up to 60 days at 30 ± 1 °C. Although studies are available during which positive effect of chemical preservatives and antioxidants on the shelf life of khoa and khoa based products has been reported, but recent research work on these chemicals have also reported about their detrimental effect on the health and well-being. This has made the researchers to search for other approaches to extend the shelf life of khoa and khoa based products.

Table 4.

Shelf life extension of khoa and khoa based products by chemical preservatives

Product Preservative Level studied Results Reference
Khoa Synthetic antioxidants such as butylated hydroxy anisole (BHA) and butylated hydroxy toluene (BHT) 0.02% Restricted the development of rancidity during storage Rehman and Salariya (2006)
Khoa Tocopherol acetate and sodium ascorbate 15 mg/Kg and 600 mg/Kg, individually and combined Sodium ascorbate had lesser oxidation than tocopherol acetate added samples, Extended the shelf-life to 30 days at 5 ± 2 °C Kumar et al. (2010)
Khoa Antifungal agents such as natamycin and potassium sorbate 0.5% and 0.3% Significantly (P < 0.01) lower yeast and mould count during storage at 30and 5 °C Rajarajan et al. (2010)
Cow milk burfi Sodium and potassium metabisulphites 1000 ppm on the basis of milk Increased shelf life at both ambient (30 °C) and refrigerated (7 °C) storage temperatures to 25 and 60 days, respectively Sarkar et al. (2002)
Burfi Cardamom and potassium sorbate each @ 0.1% of khoa (w/w) Shelf life increased upto 60 days at 30 ± 1 °C Palit and Pal (2005)
Peda Tocopherol acetate, sodium ascorbate and potassium sorbate 10 ppm, 400 ppm and 0.2% w/w Sample was stable for 60 days at −15 ± 2 °C Yadav and Beniwal (2009)
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Natural preservatives

In the recent years, increase in the consumers’ consciousness towards the detrimental health effects of synthetic additives has led to an increase in the global demand for foods which are preserved using natural preservatives. This has led to research work focused on exploiting the preservative activity of herbs and spices for increasing the shelf life of food product (Meena et al. 2021b), and traditional dairy products are no exception from this. Bactericidal activities of herbs and spices have been well documented and are reported to possess broad-spectrum activity against Gram-positive and Gram-negative bacteria.

A number of studies have been reported which deals with application of herbs and their extracts for preservation of food products because of their antimicrobial and antioxidative activity. Sivakumar et al. (2014) studied the effect of betel leaves (Piper betel Linn) extract addition on the quality attributes and storage stability of khoa. The authors added betel leaves aqueous extract at 0.5% level (of khoa on weight basis) and reported that storage stability of khoa increased without much effect on the sensory attribute of khoa. Gavhane et al. (2014) observed that the shelf life of control peda was up to seven days whereas shelf life of ginger peda increased to 2 weeks. Prasad et al. (2017a; 2018a) observed significant (p < 0.05) increase in antioxidative activity in burfi with an increase in levels of herbal essential oils of turmeric, ginger and cardamom. In continuation to this, Prasad et al. (2017b; 2018b), reported a synergistic effect of combination of essential oils (turmeric @ 100 ppm, ginger @ 200 ppm and cardamom @ 300 ppm) and packaging materials (cardboard or HDPE boxes). Panday et al. (2018) reported that the shelf life of control peda as 32 days and herbal peda, prepared by addition of 1% turmeric and 1% black pepper, as 48 days at 7 ± 1 °C and thus concluded that turmeric and black pepper powder worked as natural preservatives and increased shelf life of the product. Puri et al. (2018) studied the shelf life of peda prepared by incorporating 0.09% (w/v of milk) heartwood extract of Caesalpiniasappan, a medicinal plant, during khoa preparation. The authors reported that heartwood extract addition increased the shelf life of peda from 21 to 49 days and from 14 to 35 days at 4° and 30 °C storage temperature, respectively. Badola et al. (2018) reported that addition of curry leaf and clove bud essential oil at 0.10 ppm and 0.20 ppm level (on khoa basis) was optimum for increasing the storage stability of burfi without compromising its sensory acceptability. Pandey and Poonia (2020) prepared burfi using ber (Zizyphus mauritiana L) fruit powder at different levels (5, 10 and 15% w/w). It was observed that addition of ber powder increased the antioxidant activity and total phenolic content in burfi and highest sensory acceptability was obtained at 10% level of ber powder addition.

Biopreservatives

Preservation by biopreservatives approach involves utilization of antagonistic microorganisms or their metabolic products to inactivate the undesirable microorganisms present in the food products. Bacteriocins, a widely reported biopreservative, are antimicrobial peptides that are ribosomally synthesized and exert an inhibitory effect against the microorganisms which are closely related to the producer strain. Mahalingaiah et al. (2014b) reported that addition of nisin in kunda, an indigenous heat desiccated sweet milk product, resulted in slower quality changes during storage. Chawla et al. (2014) reported that functional doda burfi, prepared by incorporating artificial sweetener (stevia) and nutraceuticals, packaged in five-ply polyethylene pouch and preserved with biopreservative {(pediocin 0.12%) and microgard 100 (0.5%), potassium sorbate (0.1%) and sodium EDTA (20 mM)} was stable for 27 days as against the control sample which was stable only for 12 days at 30 °C. Bhardwaj and Kumar (2020) studied the effectiveness of natamycin (at 10 ppm level) and nisin (at 100 IU/g level) along with vacuum packaging on the shelf-life of khoa and reported that the samples were sensorially acceptable up to 5 months of storage under frozen conditions (−18 ± 2 °C).

Packaging interventions

Diverse packaging materials (tin cans, laminate pouches, etc.) and interventions (hot filling, MAP, vacuum packaging, Cryovac Shrink wrap pouches, etc.) have also been exploited for shelf life extension of khoa and khoa based products. Newer packaging concepts like modified atmosphere packaging (MAP), active packaging and edible films have also been reported to increase the shelf life of these products. Sharma et al. (2003) studied the shelf life of malai peda in flexible packaging material (Poster paper/Al foil/LDPE) and reported that malai peda remained stable for 6 days at room temperature (32 ± 3 °C) and for 31 days at refrigerated temperature (11 ± 1 °C). Mahalingaiah et al. (2014a) reported that kunda, a khoa based product popular in southern parts of India, packaged in tin cans and metalized polyester pouches had higher shelf life than the product packaged in LDPE packages, primarily because of the higher barrier properties of the former packaging material. Application of different packaging interventions, viz., vacuum and shrink packaging, modified atmosphere packaging, active packaging and edible films for shelf life extension of khoa and khoa based products is provided in Table 5.

Table 5.

Shelf life extension of khoa and khoa based products by packaging interventions

Packaging intervention Product Other experiment details Results Reference
Shrink packaging Khoa Three different packaging materials, viz., LDPE, aluminum foil/PVC and multi layered laminate (polyester/met. BOPP/LDPE) Higher stability was observed for the sample packaged in multi-layered laminate at 25 °C storage temperature Acharya and Agrawal (2010)
Vacuum packaging Danedar khoa 3-ply package which consisted of poster paper/ aluminum foil/ LDPE Shelf life extended upto 60 days at 11 °C Sharma et al. (2001)
Khoa Biopreservatives (nisin and natamycin) Stable upto 5 months at subzero (−18 ± 2 °C) storage temperature Bhardwaj and Kumar (2020)
Burfi Co-extruded film and addition of cardamom and potassium sorbate at the rate of 0.1% of khoa (w/w) Shelf life of increased upto 60 days at 30 °C Palit and Pal (2005)
Burfi Storage at 27 °C and 65% RH Retarded microbial growth but also adversely affected the textural and sensory quality Vijayalakshmi et al. (2005)
Coconut burfi Flexible pouches of multilayer films (polyester/ polyethylene and polyester/ aluminum foil/ polypropylene) Shelf life of 45 days as compared to less than 15 days for normal packing at 27 °C and 65% RH Gupta et al. (2010)
Brown peda Storage at room temperature (30 ± 1 °C) Product was stable upto 40 days without an appreciable quality loss Londhe et al. (2012)
Modified atmosphere packaging Khoa Headspace composition of 10% CO2 and 90% N2 and preservative (benzoic acid) Product was stable upto 12 and 8 days, respectively with and without preservative Chowdhury et al. (2017)
Kalakand Headspace composition of 50% N2: 50% CO2 at 10 °C Product was stable upto 60 days Jain et al. (2015)
Lalpeda Headspace composition of 70% N2: 30% CO2 Product was more stable as compared to the samples packaged with atmospheric air headspace Jha et al. (2015)
Milk cake Headspace composition of 70% N2: 30% CO2 Increased the stability to 28 days at refrigerated temperature (4 ± 2 °C) Chawla et al. (2021a)
Active packaging Burfi Free-oxygen absorber coupled with high-barrier materials like metalized films/foil laminates More than 45 days shelf life at 27 °C (65% RH) Vijayalakshmi et al. (2005)
Khoa-jalebi Oxygen scavengers Increased stability of the product from 10 to 42 days at 30 °C storage temperature and 65% RH Chaturvedi (2010)
Edible films Dodaburfi Synergistic action of nisin and natamycin incorporated edible antimicrobial packaging with MAP (headspace composition of N2:CO2:: 70:30) followed by refrigerated storage (4 ± 2 °C) Decreased microbial proliferation and the product was stable for 42 days Chawla et al. (2021b)
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Thermal treatments

Lowering the temperature of a food commodity has been effectively used to enhance shelf life for a long time; however, lowering the temperature to sub-zero values tends to alter the structural and textural attributes of khoa. Lactose in khoa gets into super saturated state and imparts sandiness in the product. Also, free fat is usually observed at the surface of frozen khoa samples once they are brought back to the ambient temperatures. Because of this, heat treatments post khoa and khoa based products preparations have been utilized to extend the shelf life of these products by lowering their initial microbial load.

Hot filling, steaming and in-package heat processing

Hot filling is usually done in parallel with the production process. During hot filling, khoa from the preparation equipment is directly packaged in rigid pre-sterilized containers and sealed immediately. Sealing the package while the product is still hot tends to avoid the post processing contamination in the product and also maintains partial vacuum conditions in the head space. On similar lines, subjecting khoa to steaming eliminates the micro-organisms (particularly molds) present on the surface of the product. However, steaming is done for shorter duration. Longer duration steaming of khoa may result into moisture accumulation at the surface of the product and alter the compositional and sensory attributes. This challenge is taken care of by subjecting the heat treatment to pre-packaged khoa. During this treatment, pre-packaged khoa is subjected to heat treatment either by dipping in hot water or direct steam. But, this treatment is also done for shorter duration as longer duration heat treatment affect the color attribute of khoa by surface browning because of Maillard’s browning. Gupta et al. (2010) observed that the coconut burfi packaged in flexible pouches of multilayer films had a shelf life of less than 15 and 75 days in normal packing and in-package heat processing (95 °C in a steam retort for 25 min), respectively at 27 °C and 65% RH.

Tyndallization

Tyndallization is a form of sterilization that involves boiling goods in cans or jars for about 20 min a day, for three consecutive days. Primary reason for heat treatment to the product for three consecutive days is based upon the fact that all the microorganisms are not inactivated by one time heat treatment and those present in spore form will germinate during the cooling phase (post heat treatment) at their respective optimum growth temperature. Heat treatment on the subsequent days will inactivate these micro-organisms and ultimately result into decreasing the microbial load to negligible level. A successful study has been carried out to enhance the shelf life of gulabjamun packed in glass containers by tyndallization process, in which the shelf life of the product extended to 28 days at room temperature (Banupriya et al. 2020).

Solar and microwave treatment

Microwave processing involves electromagnetic radiation for rapid processing of foods with high organoleptic, nutritional and keeping quality. The advantage of using microwaves is that these electromagnetic radiations generate heat within the food, by interacting with water molecules and mineral components of the food, rapidly raising the temperature to the desired extent. Chavan and Kulkarni (2006) reported that both solar energy and microwaves exhibited germicidal activity and restricted the microbial proliferation in khoa. Chavan and Kulkarni (2007) studied the influence of microwave heating (ranging from 10 to 100% power level for 60 to 80 s time duration) on the microbiological quality of khoa in polypropylene squats during storage for 7 days at room temperature (32–37 °C). The treatments ranging from 40% power (for 60 s) to 60% power (for 80 s) were found to be promising based on germicidal efficiency as well as overall sensory quality. Kumar et al. (2017) observed that shelf life of gulabjamun was extended by 7 days at room temperature (30 °C) and 10 days at refrigeration temperature (5–7 °C) by microwave treatment.

Water activity modification technique

Water activity could be defined as the ratio of the vapor pressure of water in the food matrix to the vapor pressure of pure water at the same temperature. Water activity of a food indicates the amount of water present in “free” form, which can act as solvent and participate in various chemical and biochemical reactions and support the growth of microorganisms. Because of these attributes, it is considered as an important factor affecting the stability of food with respect to various detrimental changes occurring in food as a result of microbial growth and physico-chemical changes. It is a well-known fact that addition of humectants (such as sugar, salts, polyhydric alcohols, hydrophilic colloids etc.) results in a decrease in the water activity of food because of their water binding activity. Attempts have been made by researchers to explore various humectants to decrease the water activity of traditional dairy products, which in turn will be useful to small scale units preparing these products. Ahmad et al. (2008) added maltodextrin {Dextrose Equivalent (DE) 16} at different levels (2, 5 and 10%, w/w) in khoa and reported addition of maltodextrin at > 2% level decreased the free moisture content (water activity) and resulted into an increase in the storage stability of khoa. Badola et al. (2017a) reported that five different humectants viz., polydextrose, maltodextrin, mannitol, sorbitol and corn syrup had a significant (P < 0.05) effect on the physical and sensory parameters of reconstituted khoa and among all, highest reduction in water activity was observed for sorbitol (0.931 at 25 °C).Chetana et al. (2005) prepared burfi using various sugar substitutes {sorbitol, sorbitol and mannitol (9:1), polydextrose, polydextrose and maltodextrin (5:5)}, along with aspartame.

Combined factor preservation technique

Most of the previous shelf life extension approaches of khoa and khoa based products were based upon utilization of one approach as primary approach, with or without other approaches as secondary approaches. Solely depending upon a single approach for food product preparation tends to affect the product quality and the shelf life is also rather less. For example, post production heat treatment of khoa to higher temperature decreases the microbial load but also results into undesirable changes in the color and other associated sensory attributes of khoa. Similarly, lowering the storage temperature to sub-zero (frozen) values of these products tends to cause lactose crystallization (leading to sandiness) and appearance of free fat in the final product. On the other hand, simultaneous application of multiple preservation approaches has a much higher preservative effect than the preservative effect obtained using these approaches individually. Also, lesser intensity of each preservation approach results into lesser changes in the sensory attributes of the product. This concept is known as combined preservation factor technique or hurdle technology or barrier technology, which emphasizes on combination of preservation techniques to establish a number of preservative factors (hurdles) that the microorganism has to overcome during multiplication and proliferation in the product. However, selection of preservation approach needs to be done meticulously in such a manner that they are compatible with the product characteristics. For example, preservation by low pH approach cannot be used for preservation of khoa and khoa based products as these products have a pH of about 6.2–6.4 and further decreasing the pH will change the sensory attributes of the product. Hence, selection of preservation approaches is very important during combined factor preservation of khoa and khoa based product. Application of hurdle technology for the preservation of milk cake was carried out by Kumar (2005). Badola et al. (2017b) reported increased shelf-life of burfi by combined effect of in-package microwave thermization and herbal essential oils.

Conclusion

Traditional Indian dairy products and sweets have been produced since decades, as a preservation intervention to keep the nutritional benefits of milk, which has comparatively low shelf life. This is primarily due to decreased water activity, greater stability of nutrients such as protein and fat, as well as the reduced possibility of microbial spoilage. Indian sweets provide high profit margins, mainly because of lower cost of sugar compared to the cost of milk solids. They are popular not only in India but have managed to have a strong foothold in the international market too. Various preservation methods have been explored to increase the shelf life of khoa and khoa based milk products, viz., chemical and natural preservatives, packaging interventions, post production thermal treatment, etc. Application of a single preservation technique can lead to varying level of changes in the sensory attributes and acceptability of the product. The growing consumer preference towards minimally processed and healthy foods emphasizes on the need to look for better alternatives. Hence, natural preservation, packaging interventions and combined factor preservation approaches are being increasingly explored for shelf life and functionality enhancement of traditional Indian dairy products.

Acknowledgements

The authors are thankful to Director, ICAR-National Dairy Research Institute, Karnal, India for providing the required facilities to prepare this MS.

Abbreviations

BIS

Bureau of Indian Standards

FSSAI

Food Safety and Standards Authority of India

SSHE

Scraped Surface Heat Exchanger

Kcal

Kilo Calorie

Kg

Kilogram

DE

Dextrose Equivalent

LDPE

Low Density Poly Ethylene

Al

Aluminum

MAP

Modified Atmospheric Packaging

Ppm

Parts per million

IU

International Unit

EDTA

Ethylene diamine tetra acetic acid

HDPE

High Density Poly Ethylene

Author contributions

Dr. Richa Badola: initial draft writing and modifications; Dr. Writdhama Prasad: initial draft writing and modifications; Dr. Narender Raju Panjagari: Supervision, Dr. R.R.B. Singh: conceptualization and supervision; Dr. Ashish Kumar Singh: supervision; Dr. Shaik Abdul Hussain: Supervision.

Funding

The work is based on the PhD program of the first author, who received an institutional fellowship during her study.

Declarations

Conflict of interest

The authors have no competing interests to disclose.

Consent for publication

Studies published in the public domain have been used to prepare this MS. Figures provided in the MS have been drawn by the authors themselves based upon the available literature.

Footnotes

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Richa Badola, Email: richa.ucals@gmail.com.

Writdhama Prasad, Email: writdhama_3993@rediffmail.com, Email: wg.Prasad@icar.gov.in.

Narender Raju Panjagari, Email: pnr.ndri@gmail.com.

R. R. B. Singh, Email: rrb_ndri@rediffmail.com

Ashish Kumar Singh, Email: aksndri@gmail.com.

Shaik Abdul Hussain, Email: abdulndri@gmail.com.

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