Packing curd, dahi and buttermilk: cups, pouches and freshness

In most Indian homes, dahi is set overnight in a bowl that nobody is allowed to touch. Most of us learnt that rule as children.

A dairy making set curd follows the same rule.

Only the bowl is now a plastic cup, and there are thousands of them on trolleys in a warm room.

The short answer is this. If the curd sets inside the pack, you need a rigid cup with a sealed lid. If the curd is stirred after setting, as with stirred dahi, lassi and chaach, you can use cups, bottles or polyethylene pouches. In both cases the cold chain decides freshness more than the pack does.

One honest note before we start.

At Jagannath Polymers we make film, not cups. So the pouch film section is closest to our own work, and on cups I am giving you a buyer's checklist built from the official sources.

Set curd or stirred curd: the choice that decides the pack

Let us settle this first, because everything else follows from it.

For set curd, the cup is the vessel the curd sets in. I would choose it the way a cook chooses a vessel, and not the way a storekeeper chooses a box.

The reading material for curd making that NIFTEM prepared under the government's PMFME scheme, Curd (Dahi) Production (NIFTEM, uploaded August 2022), gives the industrial order of work:

1. Milk with culture is filled and sealed into cups, poly pouches or matkas.
2. The packs are incubated at 42 ± 2°C for 4 to 6 hours, until the pH reaches 4.7 ± 0.5.
3. They go straight into blast cooling, which brings them from 42°C to 10°C in 3.5 to 4 hours.
4. They are stored at 4 to 5°C and dispatched in vehicles below 8°C.

FSSAI's guidance document for milk and milk products (first edition, February 2018) shows the same sequence for plain curd: filling and sealing first, then incubation, then blast cooling.

So a set curd cup lives a hard life in its first day. It is filled warm, sealed, stacked, kept warm for hours, then chilled hard.

The lid seal has to survive all of it.

Stirred products are different. The curd sets in a tank and is broken up before filling, so the pack only has to hold a finished product. That makes pouches and bottles possible.

Cups: what they are made of, and what to check

Let us look at the cup itself. The NIFTEM material names injection moulded polystyrene (PS) and polypropylene (PP) or HIPS cups with aluminium foil based peelable lids as the industrial choice. It praises them for low weight, hygiene, good impact resistance and easy running on machines. FSSAI's Packaging Regulations, 2018 (compendium version IV, 9 September 2022) also list PP or PS cups with paper or peel-off lids, and thermoformed cups with peel-off lids, in their suggestive list for milk products.

When I look at a cup and lid pair, these are the checks I would make:

1. The rim. A lid seals only to a clean, flat rim. A drop of curd or milk on the rim gives a weak spot that may open in the crate. Watch the filling nozzle for drips.
2. The peel. The lid should come off in one piece with a firm, even pull. If it tears, customers fight with it. If it lifts too easily, it can lift in transit.
3. The stack. Warm cups stand on trolleys for hours during incubation. Check that the bottom cups in a stack do not lean or crack.
4. Light. NIFTEM's defect table lists oxidised flavour from fluorescent light and sunlight. A white, opaque cup and lid help.
5. The print. Inks must meet IS 15495, and the printed surface must not touch the curd, as the FSSAI Packaging Regulations require.
6. The certificate. Ask for a certificate of conformity from a NABL-accredited lab for the food-contact material, as regulation 3(14) of the same rules requires.

If you need a number for lid seal strength, I would send samples to a lab that tests by ASTM F88/F88M (ASTM International, 2023 edition), which covers seals between a flexible material and a rigid one.

What about the matka that many of us grew up with? FSSAI's list includes mud or clay pots. NIFTEM is honest about their problems: they are heavy, they break easily, and the curd develops shrinkage cracks because water seeps out through the clay. The pot works like a slow sponge. A plastic cup holds every drop of whey inside, so any whey that separates simply sits on top.

Pouches for dahi, lassi and chaach

This is the film side of the business, the part we make, so I will keep it practical.

Pouches are lighter to carry and use far less plastic than a rigid cup. In our view they suit chaach and lassi better than set curd. NIFTEM mentions LDPE pouches of 200 ml, 500 ml and more for dahi. FSSAI's dairy standard puts lassi, chaach and buttermilk under one heading, "drinks based on fermented milk", in its standards for dairy products (FSSAI, chapter 2.1 of the Food Products Standards and Food Additives Regulations, 2011, as amended).

The same chapter has one rule that is easy to miss. Flavoured dahi shall only be sold in pre-packaged form. Loose mango dahi from a counter is not allowed.

A buttermilk pouch is filled and sealed much like a milk pouch, so the same film rules apply. The Indian milk pouch standard, IS 11805, is written for milk, but its leak and drop tests are good checks for any liquid dairy pouch. I have explained both tests in our guide on why milk pouches leak. One extra point for masala chaach: pieces of spice or herb in the seal area can cause the same channel leaks that milk foam does.

The pouch has two downsides. It cannot be closed again once cut, and it gives set curd no protection from squeezing. On light, we would follow the same logic as for milk. The black-white film explained in our guide to milk pouch film protects chaach too.

Whey on top: is it the packaging or the process?

Mostly the process, and I say that as a packaging man.

NIFTEM's defect table gives three causes of syneresis, the technical word for whey separating:

1. Heat treatment of the milk below 85°C for 30 minutes.
2. Milk solids-not-fat that are too low. NIFTEM suggests raising them to at least 11% with skim milk powder.
3. Disturbing the cups during fermentation.

The third one is where packing and handling come in. NIFTEM's advice is short: do not disturb the cups during fermentation. That is the kitchen rule again, written for a factory. Trolleys that jerk, conveyors that knock and stacks that are moved during incubation all show up later as whey on the surface.

No cup, lid or film, ours included, can fix a weak recipe. But a careless incubation room can spoil a good one.

Swollen cups and puffed pouches

A domed lid or a tight, puffed pouch means something inside is making gas. That is contamination, often yeast. NIFTEM lists a yeasty or cheesy taste, from contaminating yeast, as a known dahi defect.

There is a useful detail in a 2015 study of swollen organic yogurt packs in Spain, published in the journal Microorganisms (Wrent and others, 2015). The yeast grew in plain and fruit yogurts alike, but only the yogurts with added fruit and jam sugars swelled with gas. In their lab trials, yogurts kept at 8°C showed no gas after 11 days. Two lessons follow for our Indian products. Sweetened curds and sweet lassi can carry more risk of swelling, and cold storage slows it down.

We cannot remove yeast with packaging once it is inside. What it can do is keep new contamination out, which is one more reason to check every lid seal. My rule is simple.

A domed pack is never sold. Pull it, and look for the cause in hygiene and temperature.

Cold chain: the part the pack cannot do

Let us come to the cold chain, where freshness is really decided. The FSSAI guidance table for cultured products such as dahi and chaach keeps them at 4 to 8°C at the plant, in refrigerated transport to the shops, and says retail storage should not exceed 4°C. NIFTEM's sheet asks for storage at 4 to 5°C and dispatch below 8°C.

Let us be clear that the pack cannot cool anything. But packing does affect how fast the curd gets cold. NIFTEM's flow mentions cups placed in corrugated boxes and pouches in crates. A closed carton slows cooling. If you blast cool cups inside cartons, probe a cup from the middle of the stack, not one from the edge. The middle cup is the one that stays warm longest.

Checks your QC team can run

We would start with these five, none of which needs a lab:

1. Turn a sealed cup upside down over a tray for a few minutes and look at the lid edge for wet spots.
2. Peel ten lids by hand from each shift and note any that tear or lift too easily.
3. Run the IS 11805 leak and drop tests on chaach and dahi pouches.
4. Probe the temperature of a middle cup after blast cooling and write it down with the time.
5. Keep retained samples at your normal storage temperature and check them daily against a fresh pack for whey, gas and taste, up to the date on the label.

If you pack chaach or dahi in pouches and want to test a film, write to us with the pack size and your filling machine type. We will suggest a film, and we can plan the trial together.